# OpenGL Thinking in OpenGL

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Hi folks, I'm a relatively experienced programmer and I'm trying to get my head around some opengl concepts. I have a moderate math background and I think I understand the basics of the principles at work. I'd really appreciate a short conversation with some of the opengl gurus here to clarify a few points. Briefly, I'm trying to understand how to implement relative movement. I've seen a number of tutorials that use the familiar sin and cos operations to calculate coordinates after a movement along a specified angle. However, it was my understanding that due to the way the coordinate system works, it is not necessary to do this manually in opengl. If the object is rotated around its own axes, then merely translating along the desired axis is all that is necessary. Is this correct or am I barking up the wrong tree here? I have been able to load a model, rotate on own axes and display. The problem comes when I try to move the object "forward" in the scene, followed by further rotations. 1. Rotate object to "point" a specified direction in the scene. [works] 2. Move the object "forward" [works] 3. Rotate again - this time the rotation occurs around the center of the screen and not the model space. I am thinking that somehow I have to translate to the center of the model before rotating, and the first rotations work by coincidence because before any movement: center of model == center of screen. However, regardless of the order I issue the transformations, I just can't seem to get it right. When the rotation is always around the model axes then the translation is not in the direction the model is facing and vice versa. Here's my draw function:
    //  this sets the model position for the new frame
model->Update();

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glPushMatrix();

//  rotate the model to orientate correctly
glRotatef(-90, 1, 0, 0);
glRotatef(180, 0, 0, 1);

//  rotations derived from user input
glRotatef(model->rotation.x, 1, 0, 0);
glRotatef(model->rotation.y, 0, 1, 0);
glRotatef(model->rotation.z, 0, 0, 1);

//  move the object to its current position
glTranslatef(model->position.x, model->position.y, model->position.z);

// draw the vertices
model->Draw();

glPopMatrix();
glutSwapBuffers();


Any pointers would be greatly appreciated! FYI the model is a spacecraft and I am trying to get it to fly in the direction it is pointing.

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Try this:

    //  this sets the model position for the new frame    model->Update();    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);    glPushMatrix();    glLoadIdentity();           //  move the object to its current position *been moved    glTranslatef(model->position.x, model->position.y, model->position.z);    //  rotate the model to orientate correctly    glRotatef(-90, 1, 0, 0);    glRotatef(180, 0, 0, 1);    //  rotations derived from user input    glRotatef(model->rotation.x, 1, 0, 0);    glRotatef(model->rotation.y, 0, 1, 0);    glRotatef(model->rotation.z, 0, 0, 1);    // draw the vertices    model->Draw();    glPopMatrix();    glutSwapBuffers();

The order of matrix multiplications matter. You want to rotate on the origin first, but by translating you moved it off the origin, and then rotated it.

The matrix multiplications are associative. e.g. (a*b)*c = a(b*c) or a*(b*(c*d)) = ((a*b)*c)*d

You should look it up if you don't know what it is, as it will help you understand the multiplications.

Edit:

To fly in the direction, it would be easier to have a vector pointing the direction, and just add that to the model->position.

Also think of the lowest matrix calls acting first, e.g. rotating or translating etc.

[Edited by - andrew111 on April 30, 2010 11:10:32 AM]

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Hey thanks for such a quick reply. That is exactly what I had tried. With the translation called first, yes the model always rotates correctly on its axes - but then it always moves "down" the screen regardless of its rotation! (I guess because the translation is now relative to the world coordinate system and not the model)

I feel a "d'oh!" moment coming on. I'm sure I must be doing something stupid!

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Here are a few bits of information that might be helpful...

First of all, the problems you're talking about aren't really OpenGL-specific. It's understandable why one might think they are, but this is really just math that we're talking about, which is essentially the same regardless of what API you're using (OpenGL, Direct3D, etc.).

There are some (sort of) API-specific things that can hang people up though, and one of these is the way in which OpenGL's 'transform' functions operate. Before I continue though, let me point out that the functions you're using are actually deprecated and are more or less irrelevant in modern OpenGL code (typically, you would instead build these transforms yourself and then pass them in as shader parameters).

In any case, one of the most common causes of confusion when working with OpenGL is transform order. As I'm sure you know, matrix multiplication is not commutative, and the order in which a sequence of transforms is applied matters. For example, a rotation followed by a translation will generally have a different result than a translation followed by a rotation. Anytime someone says that they're expecting a model to rotate about its own origin but it instead appears to be rotating about some other point, you can bet that they're applying the transforms in the wrong order, and looking at the bit of code you posted, that appears to be the case here.

I think if you move the glTranslatef() call to before the calls to glRotatef(), you'll find that the behavior is correct (or at least closer to what you're expecting).

I'm sure someone's already pointed that out by now (in fact, after refreshing, I see that they have :), but I'll go ahead and elaborate a bit on this.

You may already know this, but the order in which matrices must be multiplied to yield a given effect depends on whether row or column vectors are being used. In short, when row vectors are used, the matrix product A*B applies the transforms in the order A->B, and when column vectors are used, the transforms are applied in the order B->A.

OpenGL (or the OpenGL fixed-function pipeline at least) has traditionally been thought of as using column vectors, although I don't know whether this really has any practical significance. What is significant though is that the most recent transform command issued is 'closest to' the vector to which it is to be applied, so to speak. This means that transforms are actually applied in the reverse of the order in which they appear in your code, and is why the translation actually needs to come first in your example in order to have the effect of a rotation followed by a translation.

As mentioned previously though, this really isn't the way things are done anymore. In modern OpenGL, you would typically build these transforms yourself, and then simply upload them to OpenGL as shader parameters. On your side of things, you can use whatever convention you want (e.g. row or column vectors), so long as the matrix data ends up in a form that OpenGL can understand.

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As for your follow-up post, can you post your revised rendering code, along with the code you're using to update the object's position?

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Hi thanks for the info. I'm certainly no math expert but I do understand the concepts you are raising. And I understand that the gl functions are merely helpers to construct a matrix, which is then applied to the vertices. I had read that they are now depreciated and the modern approach is to construct your own transformation matrices, however I thought it could still be a good approach to learn the basics.

I altered my drawScene function precisely as suggested by the poster above, and my code to update the model position merely increases its "position.y". The reason for that is because in the local model coordinate system, it "points" along its y axis and therefore I concluded that "forward" for the model is along the y axis. This is probably my problem, right? Although I can't quite get my head around why.

I'm assuming then that I have to do something a little more sophisticated to the model position vector?

Many thanks for the help, greatly appreciated!

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Quote:
 Original post by higHi thanks for the info. I'm certainly no math expert but I do understand the concepts you are raising. And I understand that the gl functions are merely helpers to construct a matrix, which is then applied to the vertices. I had read that they are now depreciated and the modern approach is to construct your own transformation matrices, however I thought it could still be a good approach to learn the basics.
Oops - well I guess that post was mostly redundant then :)
Quote:
 I altered my drawScene function precisely as suggested by the poster above
I suggested that as well.
Quote:
 and my code to update the model position merely increases its "position.y". The reason for that is because in the local model coordinate system, it "points" along its y axis and therefore I concluded that "forward" for the model is along the y axis. This is probably my problem, right? Although I can't quite get my head around why.I'm assuming then that I have to do something a little more sophisticated to the model position vector?
Right, that won't do what you're expecting. Why it won't do what you're expecting is actually a little tricky to explain, but here goes.

To keep things simple, let's assume that your model transform only incorporates rotation and translation (which is fairly common, and appears to be the case here, judging from your code).

These two transforms (rotation and translation) are typically applied in the order rotation->translation. If you hold some object in your hand (e.g. a toy car, spaceship, or whatever), you can visualize this by starting at a 'default' position and orientation ('identity' in matrix terms), then rotating it to the orientation that you want, and then translating it to the position that you want.

Note that because the rotation occurs first, the translation is unaffected by the rotation. If the translation is +5 along the y axis, then the model will be first rotated to the desired orientation, and then moved 5 units along the (world) y axis.

In order to get the model positioned where you want, your position variable (model->position in your code) needs to represent the object's actual position in world space. If all you ever do is move this position vector along the y axis, then your model will always be positioned somewhere along the (world) y axis (it won't, in general, move 'forward' based on the object's orientation).

To get the kind of motion you want, you need to translate the position vector not along the y axis (or whatever), but rather along the object's current direction vector(s).

There are a few ways to get these direction vectors, but the easiest way is probably to build a transform matrix for the object and then extract the direction vectors from the first three rows or columns of the matrix. (This can be done using OpenGL functions, but it's more straightforward if you use your own math code or a third-party math library for this.)

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That helps a lot, thanks. My main misconception was that opengl inherently handles this "relative" movement internally, knowing the "local" orientation of the vertices being drawn.

Is it then analogous to the operations required in 2d space to translate at an angle? i.e sine and cosine calculations? I assume it must be, only performed directly with matrices?

Although you're right, and the discussion is moving further away from opengl and more towards independent math topics!

But I now know the specific area that I need to investigate more is the general math behind it and not specifically the opengl API.

Thanks again.

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Quote:
 My main misconception was that opengl inherently handles this "relative" movement internally, knowing the "local" orientation of the vertices being drawn.
Yup, no such luck :) OpenGL neither knows nor cares about that stuff - it has no concept of 'object motion', 'local orientation', or anything of that sort. In the fixed-function pipeline, it just applies a series of transforms in matrix form to the input data (the programmable pipeline doesn't even do that much - with the PP, transforming of the input data is entirely up to you).
Quote:
 Is it then analogous to the operations required in 2d space to translate at an angle? i.e sine and cosine calculations?
Yes, it can be. In 2-d, you typically see something like this (or a vector version thereof):
x += cos(angle) * speed * dt;y += sin(angle) * speed * dt;
If you're dealing with Euler or spherical angles, you can do something similar in 3-d as well.
Quote:
 I assume it must be, only performed directly with matrices?
You can use either matrices or angles in both 2-d and 3-d. In 3-d especially, using matrices is more straightforward, IMO, and is the approach I'd recommend. (What I'm referring to here is building a transform matrix for the object, and then extracting the direction vectors from the matrix. Note that once you have the transform matrix, it can be uploaded directly to OpenGL via glLoad/MultMatrix*(), allowing you to bypass convenience functions such as glRotate*() entirely.)

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Excellent summary, well explained. Cheers!

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Hi,

I would recommend that you use this phenomenon sparingly. In OpenGL (and as you know, if you've made games), any other graphics API, it's best to keep track of an object's position and orientation yourself.

So, while you could define an object's movement as a series of translations and rotations, it's much easier to translate to the location, and then rotate to the proper orientation.

I have, however, used this technique for simple things (e.g., cause an object to "orbit" a point with a rotation*translation*rotation, instead of a translation*rotation).

Evidently, this new OpenGL 3 thing doesn't do hardware matrices, so for learning OpenGL, I'd recommend just getting used to translating and then rotating.

Cheers,
-G

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I'm back to bend your ears again if I may. My queries really are more math related now though so perhaps a moderator might want to move this to the appropriate forum?

Anyway, I have updated my project to utilize transformation matrices manually in place of glRotate and glTranslate so I'd like to ask if I'm on the right track.

I wrote my own simple vector and matrix classes and I'm pretty sure they are correct. I know that's a bit of a leap of faith but I understand the way matrices work and that opengl matrices are column major etc... To calculate the transform matrix for my model I do this:

  Matrix4 xRot = MathUtil::rotateX(rotation.x*PIdiv180);  Matrix4 yRot = MathUtil::rotateY(rotation.y*PIdiv180);  Matrix4 zRot = MathUtil::rotateZ(rotation.z*PIdiv180);  Matrix4 trans = MathUtil::translate(position.x, position.y, position.z);  transform = (yRot * zRot * xRot) * trans;

The MathUtil functions return the appropriate rotation and translation matrices.

My render function now looks like this:
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);    glPushMatrix();    glLoadIdentity();    // move "camera" back to view the model    glTranslatef(0,0,-600);    //  this now updates the model position and calculates its transform matrix    craft->Update();    //  apply the model transform matrix    glMultMatrixf(craft->transform.elements);    // draw the vertices    craft->Draw();    glPopMatrix();    glutSwapBuffers();

This results in the rotation working correctly as long as the model has not been translated. Then the vertices are just warped all over the screen. I'm kind of confused as to how the translation should work.

If forward movement is applied to the model, I increment its position.y (since it faces along its own y-axis. Then this position vector is fed into the MathUtil to obtain the translation matrix as above.

Here is how I generate the translation matrix:
Matrix4 MathUtil::translate(const float x, const float y, const float z){  GLfloat res[] = {    1,0,0,x, // actually *column* 1 because opengl is column major    0,1,0,y,    0,0,1,z,    0,0,0,1  };    return Matrix4(res);  }

I know its not the most elegant way, but for now my matrix class simply encapsulates an array of GLfloats and overloads the operators.

Can you see anything drastically wrong with my approach?

Cheers!

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Quote:
 Can you see anything drastically wrong with my approach?
I see a few potential problems:
Quote:
 To calculate the transform matrix for my model I do this:*** Source Snippet Removed ***
This:
transform = (yRot * zRot * xRot) * trans;
Would be correct for row vectors. If you're using column vectors, the order will need to be reversed.
Quote:
 This results in the rotation working correctly as long as the model has not been translated. Then the vertices are just warped all over the screen. I'm kind of confused as to how the translation should work.
I think this is probably because your matrices are set up incorrectly (see below).
Quote:
 If forward movement is applied to the model, I increment its position.y (since it faces along its own y-axis. Then this position vector is fed into the MathUtil to obtain the translation matrix as above.
Quote:
 Here is how I generate the translation matrix:*** Source Snippet Removed ***
Your matrices appear to be set up incorrectly. What you've shown above would work for either a row-vector/column-major setup or a column-vector/row-major setup, but OpenGL expects either a row-vector/row-major setup or a column-vector/column-major setup (in other words, OpenGL expects the elements of a given basis vector of the matrix to be contiguous in memory).

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Ok I'm REALLY confused about the "forward" movement issue then. I thought that in model space I am always moving along the Y axis so this is the only component of the position vector that I change. But this is then converted into world coordinates via the transformation matrix - which is a translation matrix constructed based on the local position, then multiplied by the rotation matrices to produce an absolute transform matrix to be fed to opengl.

Considering your post, I know my above reasoning is wrong and I can't quite get my head around it even re-reading your previous advice. I'm against spoon feeding as much as the next person, but maybe in this case you could give me an example to help me understand the concept - if I want to move the model in the direction it is facing, how should I alter the member position vector?

My matrices are set up such that the first column is:
element[0], element[1], element[2], element[3]

and the first row is:
element[0], element[4], element[8], element[12]
and so on...

Therefore in a translation matrix, the xyz magnitude components would take the positions [3], [7], [11] respectively. Is this what you refer to when you say opengl expects column major matrices?

Your comment regarding vectors/matrices has thrown me off a little, as I don't carry out any manual vector manipulation. I simply feed the vector components into my matrix generation functions as needed, multiply the matrices to get my compound transform, then apply this via glMultMatrixf before drawing the vertices.

As I type this, I'm getting the idea that every time I update the position of the model, I don't merely increment the y component, but I must multiply the position vector by some other velocity vector... but how exactly to I construct that "other" vector... :)

Thanks a lot for your time and patience. I hope I'm not asking any completely stupid questions here! The more I ask, the more I'm feeling I'd be better moved to the beginners section!

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Quote:
 My matrices are set up such that the first column is:element[0], element[1], element[2], element[3]and the first row is:element[0], element[4], element[8], element[12]and so on...Therefore in a translation matrix, the xyz magnitude components would take the positions [3], [7], [11] respectively. Is this what you refer to when you say opengl expects column major matrices?
This setup is wrong (for the OpenGL fixed-function pipeline, at least). The translation should reside in elements 12, 13, and 14, not 3, 7, and 11.

Basically, your matrices should be the transpose of what they are currently.
Quote:
 Your comment regarding vectors/matrices has thrown me off a little, as I don't carry out any manual vector manipulation. I simply feed the vector components into my matrix generation functions as needed, multiply the matrices to get my compound transform, then apply this via glMultMatrixf before drawing the vertices.
Whether or not you're performing any matrix-vector multiplications explicitly in your code, you still need to get your conventions set up in a way that makes sense.

Firstly, it doesn't matter if you use row vectors or column vectors - you can use either. The only requirement is that the 'majorness' of your matrices match the vector notation convention; that is, if you're going to use row vectors your matrices need to be row major, and if you're going to use column vectors your matrices need to be column major. (Currently your matrices are swapped around - they're either row-basis matrices with column-major ordering or vice versa, depending on how you look at it.)

Once you've settled on a vector notation convention (row or column vectors), you need to make sure that your transform matrices and matrix multiplication order reflect this convention correctly.

With row vectors, transforms should be built with basis vectors in the rows of the matrix; with column vectors, the basis vectors should be in the columns of the matrix.

Furthermore, with row vectors, the matrix product A*B applies the associated transforms in the order A->B, while with column vectors, the transforms are applied in the order B->A. Any expressions involving matrix multiplication will need to be ordered accordingly.

These topics are a frequent source of confusion for many, so don't hesitate to ask for clarification if you need it. You might also try searching the forum archives and/or internet as a whole for, say, 'row column major vector', and see what you find. Unfortunately though, confusion regarding these topics is so widespread that a good deal of what you find will be wrong anyway, so it's probably best to ask for any needed clarification here as well :)
Quote:
 Ok I'm REALLY confused about the "forward" movement issue then. I thought that in model space I am always moving along the Y axis so this is the only component of the position vector that I change. But this is then converted into world coordinates via the transformation matrix - which is a translation matrix constructed based on the local position, then multiplied by the rotation matrices to produce an absolute transform matrix to be fed to opengl.
I think I understand the source of your confusion. When you build the transform for your object, the rotation and translation transforms are combined in the order rotation->translation. However, the translation transform is not modified when these two transforms are combined; in other words, it's not 'adjusted' to 'match' the rotation in any way. If the input translation is (1,2,3), then after the combined transform is built, the translation is still (1,2,3). So, the translation has to be correct and in world/parent space from the outset. Whatever you set the translation to, that is where the object will be in world space. (Sorry if I'm being a little redundant, but I'm just trying to provide a few different ways of looking at it :)
Quote:
 if I want to move the model in the direction it is facing, how should I alter the member position vector?
As follows:

1. Build the rotation transform matrix for the object.

2. The rows or columns (depending on whether you're using row or column vectors) of this matrix are the direction vectors for the object. Extract the forward direction vector (which sounds like it's the y axis in your case), and store it in vector form.

3. Add this vector (most likely scaled by speed and time step) to your position vector.
Quote:
 As I type this, I'm getting the idea that every time I update the position of the model, I don't merely increment the y component, but I must multiply the position vector by some other velocity vector... but how exactly to I construct that "other" vector... :)
You don't multiply the vectors, but rather add them (as described above). Here's some example pseudocode:
matrix33 m = get_rotation_matrix();vector3 forward(m(0,1), m(1,1), m(2,1));position += forward * speed * time_step;

[Edited by - jyk on May 4, 2010 10:42:46 PM]

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Once again thanks for a very well explained answer. I have managed to get it working (I admit through 80% understanding of what's going on and 20% trial and error) and for what it's worth it turns out my forward vector (y) ends up in matrix elements [4],[5] and [6]. As far as I can gather, this is what you would expect from a column major rotation matrix?

My next obstacle is to work out why my "pitch" is still relative to the world: if the craft is facing at right angles to the world "up", then the pitch control becomes roll, and facing -z pitch-up raises the nose whilst facing +z lowers the nose. Though yaw and forward movement is working correctly.

Interesting stuff. And thanks again, your continued help is much appreciated!

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Quote:
 I have managed to get it working (I admit through 80% understanding of what's going on and 20% trial and error) and for what it's worth it turns out my forward vector (y) ends up in matrix elements [4],[5] and [6]. As far as I can gather, this is what you would expect from a column major rotation matrix?
That is what you would expect for either a column-major matrix intended for use with column vectors, or a row-major matrix intended for use with row vectors. For the other two configurations (row major/column vector and column major/row vector), the y axis would be in elements [1], [5], and [9].

But yes, [4], [5], and [6] is what you want for OpenGL.
Quote:
 My next obstacle is to work out why my "pitch" is still relative to the world: if the craft is facing at right angles to the world "up", then the pitch control becomes roll, and facing -z pitch-up raises the nose whilst facing +z lowers the nose. Though yaw and forward movement is working correctly.
Not sure about that one (not without seeing the code at least), but it sounds like either a) you just have your Euler-angle order wrong, or b) you need to ditch Euler angles and store your orientation in matrix form instead (which is what you'll need to do if you're trying to implement full 6DOF motion).
Quote:
 And thanks again, your continued help is much appreciated!
No problem :)

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• This article uses material originally posted on Diligent Graphics web site.
Introduction
Graphics APIs have come a long way from small set of basic commands allowing limited control of configurable stages of early 3D accelerators to very low-level programming interfaces exposing almost every aspect of the underlying graphics hardware. Next-generation APIs, Direct3D12 by Microsoft and Vulkan by Khronos are relatively new and have only started getting widespread adoption and support from hardware vendors, while Direct3D11 and OpenGL are still considered industry standard. New APIs can provide substantial performance and functional improvements, but may not be supported by older hardware. An application targeting wide range of platforms needs to support Direct3D11 and OpenGL. New APIs will not give any advantage when used with old paradigms. It is totally possible to add Direct3D12 support to an existing renderer by implementing Direct3D11 interface through Direct3D12, but this will give zero benefits. Instead, new approaches and rendering architectures that leverage flexibility provided by the next-generation APIs are expected to be developed.
There are at least four APIs (Direct3D11, Direct3D12, OpenGL/GLES, Vulkan, plus Apple's Metal for iOS and osX platforms) that a cross-platform 3D application may need to support. Writing separate code paths for all APIs is clearly not an option for any real-world application and the need for a cross-platform graphics abstraction layer is evident. The following is the list of requirements that I believe such layer needs to satisfy:
Lightweight abstractions: the API should be as close to the underlying native APIs as possible to allow an application leverage all available low-level functionality. In many cases this requirement is difficult to achieve because specific features exposed by different APIs may vary considerably. Low performance overhead: the abstraction layer needs to be efficient from performance point of view. If it introduces considerable amount of overhead, there is no point in using it. Convenience: the API needs to be convenient to use. It needs to assist developers in achieving their goals not limiting their control of the graphics hardware. Multithreading: ability to efficiently parallelize work is in the core of Direct3D12 and Vulkan and one of the main selling points of the new APIs. Support for multithreading in a cross-platform layer is a must. Extensibility: no matter how well the API is designed, it still introduces some level of abstraction. In some cases the most efficient way to implement certain functionality is to directly use native API. The abstraction layer needs to provide seamless interoperability with the underlying native APIs to provide a way for the app to add features that may be missing. Diligent Engine is designed to solve these problems. Its main goal is to take advantages of the next-generation APIs such as Direct3D12 and Vulkan, but at the same time provide support for older platforms via Direct3D11, OpenGL and OpenGLES. Diligent Engine exposes common C++ front-end for all supported platforms and provides interoperability with underlying native APIs. It also supports integration with Unity and is designed to be used as graphics subsystem in a standalone game engine, Unity native plugin or any other 3D application. Full source code is available for download at GitHub and is free to use.
Overview
Diligent Engine API takes some features from Direct3D11 and Direct3D12 as well as introduces new concepts to hide certain platform-specific details and make the system easy to use. It contains the following main components:
Render device (IRenderDevice  interface) is responsible for creating all other objects (textures, buffers, shaders, pipeline states, etc.).
Device context (IDeviceContext interface) is the main interface for recording rendering commands. Similar to Direct3D11, there are immediate context and deferred contexts (which in Direct3D11 implementation map directly to the corresponding context types). Immediate context combines command queue and command list recording functionality. It records commands and submits the command list for execution when it contains sufficient number of commands. Deferred contexts are designed to only record command lists that can be submitted for execution through the immediate context.
An alternative way to design the API would be to expose command queue and command lists directly. This approach however does not map well to Direct3D11 and OpenGL. Besides, some functionality (such as dynamic descriptor allocation) can be much more efficiently implemented when it is known that a command list is recorded by a certain deferred context from some thread.
The approach taken in the engine does not limit scalability as the application is expected to create one deferred context per thread, and internally every deferred context records a command list in lock-free fashion. At the same time this approach maps well to older APIs.
In current implementation, only one immediate context that uses default graphics command queue is created. To support multiple GPUs or multiple command queue types (compute, copy, etc.), it is natural to have one immediate contexts per queue. Cross-context synchronization utilities will be necessary.
Swap Chain (ISwapChain interface). Swap chain interface represents a chain of back buffers and is responsible for showing the final rendered image on the screen.
Render device, device contexts and swap chain are created during the engine initialization.
Resources (ITexture and IBuffer interfaces). There are two types of resources - textures and buffers. There are many different texture types (2D textures, 3D textures, texture array, cubmepas, etc.) that can all be represented by ITexture interface.
Resources Views (ITextureView and IBufferView interfaces). While textures and buffers are mere data containers, texture views and buffer views describe how the data should be interpreted. For instance, a 2D texture can be used as a render target for rendering commands or as a shader resource.
Pipeline State (IPipelineState interface). GPU pipeline contains many configurable stages (depth-stencil, rasterizer and blend states, different shader stage, etc.). Direct3D11 uses coarse-grain objects to set all stage parameters at once (for instance, a rasterizer object encompasses all rasterizer attributes), while OpenGL contains myriad functions to fine-grain control every individual attribute of every stage. Both methods do not map very well to modern graphics hardware that combines all states into one monolithic state under the hood. Direct3D12 directly exposes pipeline state object in the API, and Diligent Engine uses the same approach.
Shader Resource Binding (IShaderResourceBinding interface). Shaders are programs that run on the GPU. Shaders may access various resources (textures and buffers), and setting correspondence between shader variables and actual resources is called resource binding. Resource binding implementation varies considerably between different API. Diligent Engine introduces a new object called shader resource binding that encompasses all resources needed by all shaders in a certain pipeline state.
API Basics
Creating Resources
Device resources are created by the render device. The two main resource types are buffers, which represent linear memory, and textures, which use memory layouts optimized for fast filtering. Graphics APIs usually have a native object that represents linear buffer. Diligent Engine uses IBuffer interface as an abstraction for a native buffer. To create a buffer, one needs to populate BufferDesc structure and call IRenderDevice::CreateBuffer() method as in the following example:
BufferDesc BuffDesc; BufferDesc.Name = "Uniform buffer"; BuffDesc.BindFlags = BIND_UNIFORM_BUFFER; BuffDesc.Usage = USAGE_DYNAMIC; BuffDesc.uiSizeInBytes = sizeof(ShaderConstants); BuffDesc.CPUAccessFlags = CPU_ACCESS_WRITE; m_pDevice->CreateBuffer( BuffDesc, BufferData(), &m_pConstantBuffer ); While there is usually just one buffer object, different APIs use very different approaches to represent textures. For instance, in Direct3D11, there are ID3D11Texture1D, ID3D11Texture2D, and ID3D11Texture3D objects. In OpenGL, there is individual object for every texture dimension (1D, 2D, 3D, Cube), which may be a texture array, which may also be multisampled (i.e. GL_TEXTURE_2D_MULTISAMPLE_ARRAY). As a result there are nine different GL texture types that Diligent Engine may create under the hood. In Direct3D12, there is only one resource interface. Diligent Engine hides all these details in ITexture interface. There is only one  IRenderDevice::CreateTexture() method that is capable of creating all texture types. Dimension, format, array size and all other parameters are specified by the members of the TextureDesc structure:
TextureDesc TexDesc; TexDesc.Name = "My texture 2D"; TexDesc.Type = TEXTURE_TYPE_2D; TexDesc.Width = 1024; TexDesc.Height = 1024; TexDesc.Format = TEX_FORMAT_RGBA8_UNORM; TexDesc.Usage = USAGE_DEFAULT; TexDesc.BindFlags = BIND_SHADER_RESOURCE | BIND_RENDER_TARGET | BIND_UNORDERED_ACCESS; TexDesc.Name = "Sample 2D Texture"; m_pRenderDevice->CreateTexture( TexDesc, TextureData(), &m_pTestTex ); If native API supports multithreaded resource creation, textures and buffers can be created by multiple threads simultaneously.
Interoperability with native API provides access to the native buffer/texture objects and also allows creating Diligent Engine objects from native handles. It allows applications seamlessly integrate native API-specific code with Diligent Engine.
Next-generation APIs allow fine level-control over how resources are allocated. Diligent Engine does not currently expose this functionality, but it can be added by implementing IResourceAllocator interface that encapsulates specifics of resource allocation and providing this interface to CreateBuffer() or CreateTexture() methods. If null is provided, default allocator should be used.
Initializing the Pipeline State
As it was mentioned earlier, Diligent Engine follows next-gen APIs to configure the graphics/compute pipeline. One big Pipelines State Object (PSO) encompasses all required states (all shader stages, input layout description, depth stencil, rasterizer and blend state descriptions etc.). This approach maps directly to Direct3D12/Vulkan, but is also beneficial for older APIs as it eliminates pipeline misconfiguration errors. With many individual calls tweaking various GPU pipeline settings it is very easy to forget to set one of the states or assume the stage is already properly configured when in fact it is not. Using pipeline state object helps avoid these problems as all stages are configured at once.
While in earlier APIs shaders were bound separately, in the next-generation APIs as well as in Diligent Engine shaders are part of the pipeline state object. The biggest challenge when authoring shaders is that Direct3D and OpenGL/Vulkan use different shader languages (while Apple uses yet another language in their Metal API). Maintaining two versions of every shader is not an option for real applications and Diligent Engine implements shader source code converter that allows shaders authored in HLSL to be translated to GLSL. To create a shader, one needs to populate ShaderCreationAttribs structure. SourceLanguage member of this structure tells the system which language the shader is authored in:
When sampling a texture in a shader, the texture sampler was traditionally specified as separate object that was bound to the pipeline at run time or set as part of the texture object itself. However, in most cases it is known beforehand what kind of sampler will be used in the shader. Next-generation APIs expose new type of sampler called static sampler that can be initialized directly in the pipeline state. Diligent Engine exposes this functionality: when creating a shader, textures can be assigned static samplers. If static sampler is assigned, it will always be used instead of the one initialized in the texture shader resource view. To initialize static samplers, prepare an array of StaticSamplerDesc structures and initialize StaticSamplers and NumStaticSamplers members. Static samplers are more efficient and it is highly recommended to use them whenever possible. On older APIs, static samplers are emulated via generic sampler objects.
The following is an example of shader initialization:
Creating the Pipeline State Object
After all required shaders are created, the rest of the fields of the PipelineStateDesc structure provide depth-stencil, rasterizer, and blend state descriptions, the number and format of render targets, input layout format, etc. For instance, rasterizer state can be described as follows:
PipelineStateDesc PSODesc; RasterizerStateDesc &RasterizerDesc = PSODesc.GraphicsPipeline.RasterizerDesc; RasterizerDesc.FillMode = FILL_MODE_SOLID; RasterizerDesc.CullMode = CULL_MODE_NONE; RasterizerDesc.FrontCounterClockwise = True; RasterizerDesc.ScissorEnable = True; RasterizerDesc.AntialiasedLineEnable = False; Depth-stencil and blend states are defined in a similar fashion.
Another important thing that pipeline state object encompasses is the input layout description that defines how inputs to the vertex shader, which is the very first shader stage, should be read from the memory. Input layout may define several vertex streams that contain values of different formats and sizes:
// Define input layout InputLayoutDesc &Layout = PSODesc.GraphicsPipeline.InputLayout; LayoutElement TextLayoutElems[] = {     LayoutElement( 0, 0, 3, VT_FLOAT32, False ),     LayoutElement( 1, 0, 4, VT_UINT8, True ),     LayoutElement( 2, 0, 2, VT_FLOAT32, False ), }; Layout.LayoutElements = TextLayoutElems; Layout.NumElements = _countof( TextLayoutElems ); Finally, pipeline state defines primitive topology type. When all required members are initialized, a pipeline state object can be created by IRenderDevice::CreatePipelineState() method:
// Define shader and primitive topology PSODesc.GraphicsPipeline.PrimitiveTopologyType = PRIMITIVE_TOPOLOGY_TYPE_TRIANGLE; PSODesc.GraphicsPipeline.pVS = pVertexShader; PSODesc.GraphicsPipeline.pPS = pPixelShader; PSODesc.Name = "My pipeline state"; m_pDev->CreatePipelineState(PSODesc, &m_pPSO); When PSO object is bound to the pipeline, the engine invokes all API-specific commands to set all states specified by the object. In case of Direct3D12 this maps directly to setting the D3D12 PSO object. In case of Direct3D11, this involves setting individual state objects (such as rasterizer and blend states), shaders, input layout etc. In case of OpenGL, this requires a number of fine-grain state tweaking calls. Diligent Engine keeps track of currently bound states and only calls functions to update these states that have actually changed.
Direct3D11 and OpenGL utilize fine-grain resource binding models, where an application binds individual buffers and textures to certain shader or program resource binding slots. Direct3D12 uses a very different approach, where resource descriptors are grouped into tables, and an application can bind all resources in the table at once by setting the table in the command list. Resource binding model in Diligent Engine is designed to leverage this new method. It introduces a new object called shader resource binding that encapsulates all resource bindings required for all shaders in a certain pipeline state. It also introduces the classification of shader variables based on the frequency of expected change that helps the engine group them into tables under the hood:
Static variables (SHADER_VARIABLE_TYPE_STATIC) are variables that are expected to be set only once. They may not be changed once a resource is bound to the variable. Such variables are intended to hold global constants such as camera attributes or global light attributes constant buffers. Mutable variables (SHADER_VARIABLE_TYPE_MUTABLE) define resources that are expected to change on a per-material frequency. Examples may include diffuse textures, normal maps etc. Dynamic variables (SHADER_VARIABLE_TYPE_DYNAMIC) are expected to change frequently and randomly. Shader variable type must be specified during shader creation by populating an array of ShaderVariableDesc structures and initializing ShaderCreationAttribs::Desc::VariableDesc and ShaderCreationAttribs::Desc::NumVariables members (see example of shader creation above).
Static variables cannot be changed once a resource is bound to the variable. They are bound directly to the shader object. For instance, a shadow map texture is not expected to change after it is created, so it can be bound directly to the shader:
m_pPSO->CreateShaderResourceBinding(&m_pSRB); Note that an SRB is only compatible with the pipeline state it was created from. SRB object inherits all static bindings from shaders in the pipeline, but is not allowed to change them.
Mutable resources can only be set once for every instance of a shader resource binding. Such resources are intended to define specific material properties. For instance, a diffuse texture for a specific material is not expected to change once the material is defined and can be set right after the SRB object has been created:
m_pSRB->GetVariable(SHADER_TYPE_PIXEL, "tex2DDiffuse")->Set(pDiffuseTexSRV); In some cases it is necessary to bind a new resource to a variable every time a draw command is invoked. Such variables should be labeled as dynamic, which will allow setting them multiple times through the same SRB object:
m_pSRB->GetVariable(SHADER_TYPE_VERTEX, "cbRandomAttribs")->Set(pRandomAttrsCB); Under the hood, the engine pre-allocates descriptor tables for static and mutable resources when an SRB objcet is created. Space for dynamic resources is dynamically allocated at run time. Static and mutable resources are thus more efficient and should be used whenever possible.
As you can see, Diligent Engine does not expose low-level details of how resources are bound to shader variables. One reason for this is that these details are very different for various APIs. The other reason is that using low-level binding methods is extremely error-prone: it is very easy to forget to bind some resource, or bind incorrect resource such as bind a buffer to the variable that is in fact a texture, especially during shader development when everything changes fast. Diligent Engine instead relies on shader reflection system to automatically query the list of all shader variables. Grouping variables based on three types mentioned above allows the engine to create optimized layout and take heavy lifting of matching resources to API-specific resource location, register or descriptor in the table.
This post gives more details about the resource binding model in Diligent Engine.
Setting the Pipeline State and Committing Shader Resources
Before any draw or compute command can be invoked, the pipeline state needs to be bound to the context:
m_pContext->SetPipelineState(m_pPSO); Under the hood, the engine sets the internal PSO object in the command list or calls all the required native API functions to properly configure all pipeline stages.
The next step is to bind all required shader resources to the GPU pipeline, which is accomplished by IDeviceContext::CommitShaderResources() method:
m_pContext->CommitShaderResources(m_pSRB, COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES); The method takes a pointer to the shader resource binding object and makes all resources the object holds available for the shaders. In the case of D3D12, this only requires setting appropriate descriptor tables in the command list. For older APIs, this typically requires setting all resources individually.
Next-generation APIs require the application to track the state of every resource and explicitly inform the system about all state transitions. For instance, if a texture was used as render target before, while the next draw command is going to use it as shader resource, a transition barrier needs to be executed. Diligent Engine does the heavy lifting of state tracking.  When CommitShaderResources() method is called with COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES flag, the engine commits and transitions resources to correct states at the same time. Note that transitioning resources does introduce some overhead. The engine tracks state of every resource and it will not issue the barrier if the state is already correct. But checking resource state is an overhead that can sometimes be avoided. The engine provides IDeviceContext::TransitionShaderResources() method that only transitions resources:
m_pContext->TransitionShaderResources(m_pPSO, m_pSRB); In some scenarios it is more efficient to transition resources once and then only commit them.
Invoking Draw Command
The final step is to set states that are not part of the PSO, such as render targets, vertex and index buffers. Diligent Engine uses Direct3D11-syle API that is translated to other native API calls under the hood:
ITextureView *pRTVs[] = {m_pRTV}; m_pContext->SetRenderTargets(_countof( pRTVs ), pRTVs, m_pDSV); // Clear render target and depth buffer const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); m_pContext->ClearDepthStencil(nullptr, CLEAR_DEPTH_FLAG, 1.f); // Set vertex and index buffers IBuffer *buffer[] = {m_pVertexBuffer}; Uint32 offsets[] = {0}; Uint32 strides[] = {sizeof(MyVertex)}; m_pContext->SetVertexBuffers(0, 1, buffer, strides, offsets, SET_VERTEX_BUFFERS_FLAG_RESET); m_pContext->SetIndexBuffer(m_pIndexBuffer, 0); Different native APIs use various set of function to execute draw commands depending on command details (if the command is indexed, instanced or both, what offsets in the source buffers are used etc.). For instance, there are 5 draw commands in Direct3D11 and more than 9 commands in OpenGL with something like glDrawElementsInstancedBaseVertexBaseInstance not uncommon. Diligent Engine hides all details with single IDeviceContext::Draw() method that takes takes DrawAttribs structure as an argument. The structure members define all attributes required to perform the command (primitive topology, number of vertices or indices, if draw call is indexed or not, if draw call is instanced or not, if draw call is indirect or not, etc.). For example:
DrawAttribs attrs; attrs.IsIndexed = true; attrs.IndexType = VT_UINT16; attrs.NumIndices = 36; attrs.Topology = PRIMITIVE_TOPOLOGY_TRIANGLE_LIST; pContext->Draw(attrs); For compute commands, there is IDeviceContext::DispatchCompute() method that takes DispatchComputeAttribs structure that defines compute grid dimension.
Source Code
Full engine source code is available on GitHub and is free to use. The repository contains tutorials, sample applications, asteroids performance benchmark and an example Unity project that uses Diligent Engine in native plugin.
Atmospheric scattering sample demonstrates how Diligent Engine can be used to implement various rendering tasks: loading textures from files, using complex shaders, rendering to multiple render targets, using compute shaders and unordered access views, etc.

Asteroids performance benchmark is based on this demo developed by Intel. It renders 50,000 unique textured asteroids and allows comparing performance of Direct3D11 and Direct3D12 implementations. Every asteroid is a combination of one of 1000 unique meshes and one of 10 unique textures.

Finally, there is an example project that shows how Diligent Engine can be integrated with Unity.

Future Work
The engine is under active development. It currently supports Windows desktop, Universal Windows, Linux, Android, MacOS, and iOS platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and Metal backend is in the plan.

• Good Evening,
I want to make a 2D game which involves displaying some debug information. Especially for collision, enemy sights and so on ...
First of I was thinking about all those shapes which I need will need for debugging purposes: circles, rectangles, lines, polygons.
I am really stucked right now because of the fundamental question:
Where do I store my vertices positions for each line (object)? Currently I am not using a model matrix because I am using orthographic projection and set the final position within the VBO. That means that if I add a new line I would have to expand the "points" array and re-upload (recall glBufferData) it every time. The other method would be to use a model matrix and a fixed vbo for a line but it would be also messy to exactly create a line from (0,0) to (100,20) calculating the rotation and scale to make it fit.
If I proceed with option 1 "updating the array each frame" I was thinking of having 4 draw calls every frame for the lines vao, polygons vao and so on.
In addition to that I am planning to use some sort of ECS based architecture. So the other question would be:
Should I treat those debug objects as entities/components?
For me it would make sense to treat them as entities but that's creates a new issue with the previous array approach because it would have for example a transform and render component. A special render component for debug objects (no texture etc) ... For me the transform component is also just a matrix but how would I then define a line?
Treating them as components would'nt be a good idea in my eyes because then I would always need an entity. Well entity is just an id !? So maybe its a component?
Regards,
LifeArtist
• By QQemka
Hello. I am coding a small thingy in my spare time. All i want to achieve is to load a heightmap (as the lowest possible walking terrain), some static meshes (elements of the environment) and a dynamic character (meaning i can move, collide with heightmap/static meshes and hold a varying item in a hand ). Got a bunch of questions, or rather problems i can't find solution to myself. Nearly all are deal with graphics/gpu, not the coding part. My c++ is on high enough level.
Let's go:
Heightmap - i obviously want it to be textured, size is hardcoded to 256x256 squares. I can't have one huge texture stretched over entire terrain cause every pixel would be enormous. Thats why i decided to use 2 specified textures. First will be a tileset consisting of 16 square tiles (u v range from 0 to 0.25 for first tile and so on) and second a 256x256 buffer with 0-15 value representing index of the tile from tileset for every heigtmap square. Problem is, how do i blend the edges nicely and make some computationally cheap changes so its not obvious there are only 16 tiles? Is it possible to generate such terrain with some existing program?
Collisions - i want to use bounding sphere and aabb. But should i store them for a model or entity instance? Meaning i have 20 same trees spawned using the same tree model, but every entity got its own transformation (position, scale etc). Storing collision component per instance grats faster access + is precalculated and transformed (takes additional memory, but who cares?), so i stick with this, right? What should i do if object is dynamically rotated? The aabb is no longer aligned and calculating per vertex min/max everytime object rotates/scales is pretty expensive, right?
Drawing aabb - problem similar to above (storing aabb data per instance or model). This time in my opinion per model is enough since every instance also does not have own vertex buffer but uses the shared one (so 20 trees share reference to one tree model). So rendering aabb is about taking the model's aabb, transforming with instance matrix and voila. What about aabb vertex buffer (this is more of a cosmetic question, just curious, bumped onto it in time of writing this). Is it better to make it as 8 points and index buffer (12 lines), or only 2 vertices with min/max x/y/z and having the shaders dynamically generate 6 other vertices and draw the box? Or maybe there should be just ONE 1x1x1 cube box template moved/scaled per entity?
What if one model got a diffuse texture and a normal map, and other has only diffuse? Should i pass some bool flag to shader with that info, or just assume that my game supports only diffuse maps without fancy stuff?
There were several more but i forgot/solved them at time of writing
• By RenanRR
Hi All,
I'm reading the tutorials from learnOpengl site (nice site) and I'm having a question on the camera (https://learnopengl.com/Getting-started/Camera).
I always saw the camera being manipulated with the lookat, but in tutorial I saw the camera being changed through the MVP arrays, which do not seem to be camera, but rather the scene that changes:
#version 330 core layout (location = 0) in vec3 aPos; layout (location = 1) in vec2 aTexCoord; out vec2 TexCoord; uniform mat4 model; uniform mat4 view; uniform mat4 projection; void main() { gl_Position = projection * view * model * vec4(aPos, 1.0f); TexCoord = vec2(aTexCoord.x, aTexCoord.y); } then, the matrix manipulated:
..... glm::mat4 projection = glm::perspective(glm::radians(fov), (float)SCR_WIDTH / (float)SCR_HEIGHT, 0.1f, 100.0f); ourShader.setMat4("projection", projection); .... glm::mat4 view = glm::lookAt(cameraPos, cameraPos + cameraFront, cameraUp); ourShader.setMat4("view", view); .... model = glm::rotate(model, glm::radians(angle), glm::vec3(1.0f, 0.3f, 0.5f)); ourShader.setMat4("model", model);
So, some doubts:
- Why use it like that?
- Is it okay to manipulate the camera that way?
-in this way, are not the vertex's positions that changes instead of the camera?
- I need to pass MVP to all shaders of object in my scenes ?

What it seems, is that the camera stands still and the scenery that changes...
it's right?

Thank you

• Sampling a floating point texture where the alpha channel holds 4-bytes of packed data into the float. I don't know how to cast the raw memory to treat it as an integer so I can perform bit-shifting operations.

int rgbValue = int(textureSample.w);//4 bytes of data packed as color
// algorithm might not be correct and endianness might need switching.
vec3 extractedData = vec3(  rgbValue & 0xFF000000,  (rgbValue << 8) & 0xFF000000, (rgbValue << 16) & 0xFF000000);
extractedData /= 255.0f;