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OpenGL Problems with camera movement and rotation.

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Hi guys. I am trying to learn how to operate my camera in OpenGL. However I think I have problems with understaind the translations in local \ global coordinate system. I want to move my camera freearly around a cube which is located at (0,0,-5). However instead of moving my camera the sceene looks like the cube is moved to the orgin of global coordinate system. Also the rotation of my camera doesn't look "natural" to me. Something is wrong with that also. Here is a crucial part of my code: procedure ReSizeGLScene(Width, Height: Integer); cdecl; begin if Height = 0 then Height := 1; glViewport(0, 0, Width, Height); glMatrixMode(GL_PROJECTION); glLoadIdentity; gluPerspective(45, Width / Height, 0.1, 1000); glMatrixMode(GL_MODELVIEW); glLoadIdentity; end; procedure GLKeyboard(Key: Byte; X, Y: Longint); cdecl; begin if Key = 27 then Halt(0); case Key of 97: CameraPosition.Z := CameraPosition.Z + 1; 122: CameraPosition.Z := CameraPosition.Z - 1; end; end; procedure GLSpecialKeyboard(Key: Longint; X, Y: Longint); cdecl; begin case Key of GLUT_KEY_LEFT: CameraAngle.Y := CameraAngle.Y - 1; GLUT_KEY_RIGHT: CameraAngle.Y := CameraAngle.Y + 1; GLUT_KEY_DOWN: CameraAngle.X := CameraAngle.X + 1; GLUT_KEY_UP: CameraAngle.X := CameraAngle.X - 1; end; end; procedure DrawGLScene; cdecl; begin glClear(GL_COLOR_BUFFER_BIT or GL_DEPTH_BUFFER_BIT); glLoadIdentity; glRotatef(CameraAngle.Y, 0, 1, 0); glRotatef(CameraAngle.X, 1, 0, 0); glTranslatef(CameraPosition.X, CameraPosition.Y, CameraPosition.Z); glutWireCube(1); glutSwapBuffers; end; Also, the full working program can be downloaded here: http://www.speedyshare.com/files/21386187/MyCamera.zip The effect which I want to achive is to point my camera towards some point in scene and move camera towards that point. Thanks for your time.

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If you want to look at a specific object/position, you can just use the glLookAt() at function. However, if you're trying to freely rotate your camera, the rotation matrix based on the rotation of the camera won't do. You must use the inverted matrix of the camera's rotation.

EDIT: If the inverted matrix is your solution, let me know. It's a bit of a pain in the ass and I've got a working implementation here somewhere that I can post.

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Thank you for your reply.

I want to move my camera freerly like in a 3D Space game.

Could yu give me an example how to use such inverted matrix?

Firstly I need to create a rotation matrix umm...from my angles?
Than I need to converted it to the inverted matrix?
And than I need to multiply that matrix by the current view matrix?

EDIT: Yes, that is exactly what I need, thank you for your help. But will I be able to understand your implementation? I mean I need to learn how to use such matrix to achieve a given result.

[Edited by - Wodzu on March 12, 2010 8:40:14 AM]

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I'm probably jumping ahead and should be careful not to make this more confusing than it really is. It's been a while since I've had to delve into OpenGL so I need to get my bearings.

Okay, I see you're using a glRotatef() call for each axis. This is a pretty straightforward method so you can scratch my earlier comment about inverted matrices. This method (for me anyway) tends to require some trial and error. For instance, (and this is going on memory alone) you may need to negate the coordinates before translating:
glTranslatef(-CameraPosition.X, -CameraPosition.Y, -CameraPosition.Z);

instead of:
glTranslatef(CameraPosition.X, CameraPosition.Y, CameraPosition.Z);

If you think about this logically, the objects in your screen space appear to move in the opposite direction of the camera movement. Looking out of a train window (left) as the train moves forward (right), the train station appears to move opposite (left).

Also, I don't see where you're cube's position is being set and it appears that the cube should be at origin. To shift the position:

glPushMatrix (); //preserve the camera rotation matrix
glTranslatef (0.0, 0.0, -5.0); //shift the cube position
glMultMatrixf (Obj->GetRotation()); //if you need to rotate the cube, do it here
glutWireCube (1); //same as before
glPopMatrix (); //retrieve the camera rotation matrix

Hope this helps!

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In case you need it at any time in the future, here's an implementation of the invert matrix function in BASIC:


SUB TPLInvertMatrix (Dest AS SINGLE PTR, Source AS SINGLE PTR)
DIM X AS INTEGER
DIM Y AS INTEGER
DIM Index AS INTEGER
DIM Minor(11) AS SINGLE
DIM Adjoint(11) AS SINGLE
DIM AS SINGLE Determinant = Source[0] * (Source[5] * Source[10] - Source[9] * Source[6]) - _
Source[4] * (Source[1] * Source[10] - Source[9] * Source[2]) + _
Source[8] * (Source[1] * Source[6] - Source[5] * Source[2])
DIM AS SINGLE DetRec = 1.0 / Determinant 'Determinant reciprocal

'Calculate minors of source matrix
Minor(0) = Source[5] * Source[10] - Source[9] * Source[6]
Minor(1) = Source[4] * Source[10] - Source[8] * Source[6]
Minor(2) = Source[4] * Source[9] - Source[8] * Source[5]
Minor(4) = Source[1] * Source[10] - Source[9] * Source[2]
Minor(5) = Source[0] * Source[10] - Source[8] * Source[2]
Minor(6) = Source[0] * Source[9] - Source[8] * Source[1]
Minor(8) = Source[1] * Source[6] - Source[5] * Source[2]
Minor(9) = Source[0] * Source[6] - Source[4] * Source[2]
Minor(10) = Source[0] * Source[5] - Source[4] * Source[1]

'Calculate cofactors and adjoint in one shot
Adjoint(0) = Minor(0)
Adjoint(1) = -Minor(4)
Adjoint(2) = Minor(8)
Adjoint(4) = -Minor(1)
Adjoint(5) = Minor(5)
Adjoint(6) = -Minor(9)
Adjoint(8) = Minor(2)
Adjoint(9) = -Minor(6)
Adjoint(10) = Minor(10)

'Finally, we find the inverse by dividing the adjoint by
'the determinant |A|. Since you can't divide a matrix,
'we simply multiply each value by the reciprocal.
FOR Y = 0 TO 2
FOR X = 0 TO 2
Index = Y * 4 + X
Dest[Index] = DetRec * Adjoint(Index)
NEXT X
NEXT Y

'Last column can simply be copied
Dest[3] = Source[3]
Dest[7] = Source[7]
Dest[11] = Source[11]
END SUB




This inverts ODE (physics) matrices so the format is different than OpenGL. Here's another function that performs the conversion:


SUB RenderConvertODEMatrix (Source AS dReal PTR, Dest AS GLFloat PTR)
Dest[0] = Source[0]:Dest[1] = Source[4]:Dest[2] = Source[8]:Dest[3] = 0
Dest[4] = Source[1]:Dest[5] = Source[5]:Dest[6] = Source[9]:Dest[7] = 0
Dest[8] = Source[2]:Dest[9] = Source[6]:Dest[10] = Source[10]:Dest[11] = 0
Dest[12] = 0:Dest[13] = 0:Dest[14] = 0:Dest[15] = 1
END SUB


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Thank you codrex75 for the answers.

However you said that I do not need the inversion matrix but then you propose me to use glMultMatrixf(). So I am totaly lost now...

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A little primer to help your understanding:

So when dealing with cameras and objects, you have two matrices that you need to worry about. One is the Model Matrix, and the other is the View matrix. These are often combined together to get the "ModelView" matrix that you will often hear of.

When you define a mesh, all of the coordinates will typically be in "Object Space". This defines the location of vertices relative to the origin of your object. The object space coordinates have no information about where the object is in the "World" (or Global Coordinate System). If you have a cube at 0,0,0 and you translate the cube to another position, the object space coordinates are always the same, because they only define vertices relative to the object.

So if we want to move our cube to a new location, we need to find a way to move each vertex from "Object Space" into "World Space". This is done via a Model Matrix. If you want to translate your object 5 units to the right, then you define a Transformation Matrix that defines a translation to the right by five units. This matrix can be 'M', or our model matrix. So now if we have a point 'p' in object space, and we want to move it into world space 'P', we transform it with the Model matrix like so: P = Mp. Your model matrix can contain as many translations, rotations, scalings as you need. If you want to Translate, then rotate, then scale your model, via matrices To (T object), Ro, So, then your modelview matrix is constructed M = To*Ro*So.

But what about the view matrix? Because there is no camera construct in OpenGL, we must transform our vertices again into a new space called "Eye Space". The viewport in opengl always looks out from 0,0,0 in the negative z direction, so we must "transform the entire world" so that it looks accurate from that space.

As coderx was describing with the train analogy, you move your view in openGL by moving the entire world in the opposite direction. When you turn your head to the right, this is exactly the same thing as if the entire world is rotating to the left. When you move your eyes up, it is also as if the entire world is moving down. So however we want to move our "camera", we must transform the world by the inverse of this movement.

So we need to come up with a matrix V (view matrix), that transforms our world coordinates into eye coordinates. Using our original point p and model matrix M, the equation now looks like this: P = VMp, where P is now in eye space.

Now remember that V is supposed to be the inverse of the camera movement that we want. Lets say what we really want to do is move our camera up and then rotate it 45 degrees downward to get a birds eye view of our world. If we treat the camera like an object, then we want to transform our camera by a translation (Tc) and a rotation (Rc): C = Tc*Rc.

Now transform matrix C will take an object and translate it up and rotate it. But what we really want is the inverse of C, which will be our view matrix. inverse(C) = C' = V

Now to find C' (also known as V), you can either invert the matrix C using matrix inversion methods, or you can just compute it from the original transformations. Because of the properties of matrices, this holds true:

C' = (TcRc)' = Rc'Tc'

Where Tc and Rc are our camera transforms. However Tc' and Rc' are very easy to calculate. The inverse of a translation is just a translation in the opposite direction, and the inverse of a rotation is just a rotation in the opposite direction.

So now you can transform your original vertex into eye space like so:

P = Rc' * Tc' * To * Ro * So * p

or

P = VMp

You can either construct this by building the "VM" matrix yourself, or you can build it with opengl transform functions. For example:


glLoadIdentity(); //Modelview matrix now identity
//Setup the camera
glRotatef(negative camera rotation) //Modelview matrix now = Rc'
glTranslatef(negative camera movement) //Rc' * Tc'
//Setup the model matrix
glTranslatef(object translation) //Rc' * Tc' * To
glRotatef(object rotation) //Rc' * Tc' * To * Ro
glScalef(object scale)//Rc' * Tc' * To * Ro * So

//Now send your object space vercices, which are translated into eye space
glVertex(p) // P = Rc' * Tc' * To * Ro * So * p


And that concludes the basics of opengl cameras :)

I know it is confusing at first, but after you work at it for a while it will make more sense.

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Thank you karwosts for you explanation and your time.

You expleined it very nicely even nicer the the OpenGL book itself ;)

There is one thing strictly mathematical which I do not know how to calculate.
I have some ideas but I do not want to reinvent the wheel.

Lets assume that I've rotated mine camera view about three angles. So I have now new vector pointing in space. I would like to move my camera along this vector by some unit distance. So I need to know how much I must translate in X,Y,Z-plane.

I know how to do this in 2D-space but I do not know how to do this in 3D, atleast in easy way.

The idea which I have is that:

I have old vector and 3 angles. I need to rotate this vector and calculate it's new coordinates. When I have new coordinates I normalize the vector and multiply by the unit distance. Then I add this value to the calculated vector.

But this is a lot of work and I am redoing some thing which is already done by the OpenGL.

How to do it in a simpler way? The ideal way would be to know only how much I need to translate without calculating the new vector by myself.

Regards.

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Actually this information is stored inside the model matrix for you and easy to pull out. When you look at the actual elements of the matrix, this is what they represent:


0 4 8 12
1 5 9 13
2 6 10 14
3 7 11 15

Rx Ux Ox Px
Ry Uy Oy Py
Rz Uz Oz Pz
0 0 0 1



So elements 0,1,2 are the Right Vector (Rx, Ry, Rz), 4,5,6 is the Up Vector, and 8, 9, 10 is the Out Vector (or direction). 12,13,14 contain your translation. So whenever you perform the matrix math you already have your out vector. If you want to use the OpenGL transformations instead of performing the matrix op's yourself (preferred way is to do it yourself, but that is more advanced), you can download the matrix with glGetFloatfv and examine it's elements.


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Thank you karwosts :)

I have now a working camera however it is made after trials and errors and I feel that i know only in half how it works :|

I know that the order of command isue is crucial due to the matrix multiplication.
Howeve I can not find a logic in it. I wanted to compare two thhe most simple cases to observe how to rotation occurs.

Here are the cases:

CASE 1:

glLoadIdentity;
gluLookAt(0,0,5, 0, 0, 0, 0, 1, 0);
glRotatef(90, 1, 0, 0);
glRotatef(90, 0, 1, 0);
glutWireCube(1);

CASE 2:

glLoadIdentity;
gluLookAt(0,0,5, 0, 0, 0, 0, 1, 0);
glRotatef(90, 0, 1, 0);
glRotatef(90, 1, 0, 0);
glutWireCube(1);

So I only switched the order of rotation commands.

What I find illogical and impossible to understand (after rotating this damn cube for hours;)) is that:

In first case when I am thinking in terms of grand fixed orgin the command are issued in the reversed order, so:

1. Cube is drawn
2. Cube is rotated around OY counterclockwise by 90 degrees.
3. Cube is rotated around OX counterclockwise by 90 degrees.
4. Cube is translated -5 units in Z direction from the orgin.

Am I thinking correct?

But the same thinking in CASE two fails me, here it is:

1. Cube is drawn.
2. Cube is rotated around OX counterclockwise by 90 degrees.
3. Cube is rotated around OY counterclockwise by 90 degrees.
However the effect is different from expected! It looks like the step 2 (rotation around OX) also rotated the OY axis by 90 degrees! But in first case rotation around OY did not rotate the OX axis. This is the thing which I do not udenrstand.

Why in the first case the coordinate system has not been rotated with object and in the second case coordinate system has been rotated.

I can not see the logic here. Eiter in both cases the coordinate systems should be rotated with an object or they should stay fixed.

I am lost... :|

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Quote:

In first case when I am thinking in terms of grand fixed orgin the command are issued in the reversed order, so:

1. Cube is drawn
2. Cube is rotated around OY counterclockwise by 90 degrees.
3. Cube is rotated around OX counterclockwise by 90 degrees.
4. Cube is translated -5 units in Z direction from the orgin.

Am I thinking correct?


Sounds right to me. Your second case should work equally well, and I'm not sure I really understand what is wrong.

Forgive me if I'm missing something, but how can you tell what is happening by rotating a cube by 90 degrees? If I take a cube and rotate it by 90 degrees doesn't that look exactly the same? I think you need to render some kind of object that is visually unique from all sides so you can tell what is happening. Either that you can just draw some axis on your cube:


glLoadIdentity;
gluLookAt(0,0,5, 0, 0, 0, 0, 1, 0);
glRotatef(90, 1, 0, 0);
glRotatef(90, 0, 1, 0);
glutWireCube(1);

glBegin(GL_LINES);
glColor3f(1,0,0);
glVertex3f(0,0,0); glVertex3f(2,0,0);
glColor3f(0,1,0);
glVertex3f(0,0,0); glVertex3f(0,2,0);
glColor3f(0,0,1);
glVertex3f(0,0,0); glVertex3f(0,0,2);
glEnd();



If that's still not working you can post some images and I can maybe understand better.

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I had more or less the same problem. Here was my solution (in C++):

glLoadIdentity();
glRotatef( -camPitch, 1, 0, 0 );
glRotatef( -camYaw, 0, 1, 0 );
glTranslatef( -camX, -camY, -camZ );


The cam values are handled in a Lua script in my implementation, which has dodgy angle calculations so I'm not sure if camPitch and camYaw follow standard form, but a little tinkering with - signs should fix it.

Edit:
You'd then render all your stuff in world coords (so your cube at 0, 0, 5) after that.

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Firs of all, thank you karwosts for your time and helping me out.

Yes you have absolutely right, one the example which I have given I could not say how to cube was rotated. I just modified my working example, maybe not neceserly and I confused you.

I am drawing the axis as you suggested, so we have right now:

RED: X - axis
GREEN: Y - axis
BLUE: Z - axis

In case 1 I am rotating cube firstly 90-degrees around OY and after that the Z-axis is on the place of X-axis and the X-axis is on the place of Z-axis. So the X-axis has been rotated as weel. Then I am rotating around X-axis, hovewer X-axis is now on the position of Z-axis but the cube is rotating like the X-axis would be on the oryginal position!

In 2 case I am also performing the same rotation around OY, and the axises has been rotated in the same way(X-axis is on the Z-axis position). But then when I am rotating around X-axis the rotation occurs in a different way! Now the cube rotating around X-axis like it would been on the Z-axis!

I can not understand that.

Here are images with my commentary, hope this will be now easier to udenrstand.

Here is the starting position for both cases:

Starting position

Rotation 90 counterclockwise around OY gives this result for both cases:


OY 90 degrees rotation

As we see now the X-axis is on the place of Z-axis. Also the Z-axis should be on the left side of the cube, I don't understand why it is on the right side.

Now I am performing rotation around X-axis which is hidden (it is on the place of Z-axis).

Case 1 OX 90 deegre rotation

But instead of local coordinate system rotation (X-has been rotated) now the image is rotated about the world coordinate system around BLUE axis.
So the question is, why the cube has been rotated around BLUE axis?

Now lets compare it with case two rotation:

Case 2 OX 90 deegre rotation

Now the cube has been rotated aroud the RED-axis instead of the BLUE axis (like in case 1).

Why there is adifference? I can not understand the inconsistency in this rotations. Either both examples shoudl rotate around local coordinate system or around a world coordinate system but they behave differently and ONLY the order of rotations has been changed.

Here is the link to the working examples:

http://www.speedyshare.com/files/21431956/Rotation.zip

thomasfn1: Yes this solves the problem (I've found that solution on the NeHe tutorials page) but I would like to understand this rotation thing.

Thank you for your time.







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I think I must have confused you talking about applying operations in reverse order.

Quote:

In case 1 I am rotating cube firstly 90-degrees around OY and after that the Z-axis is on the place of X-axis and the X-axis is on the place of Z-axis. So the X-axis has been rotated as weel. Then I am rotating around X-axis, hovewer X-axis is now on the position of Z-axis but the cube is rotating like the X-axis would be on the oryginal position!

Quote:

CASE 1:
glLoadIdentity;
gluLookAt(0,0,5, 0, 0, 0, 0, 1, 0);
glRotatef(90, 1, 0, 0);
glRotatef(90, 0, 1, 0);
glutWireCube(1);


Everytime you call a "gl{MatrixOp}f" command, this happens on the coordinate axis that has already been transformed by all of the previous operations.

So when you call gluLookAt (essentially glTranslate), you first translate the cube on its local coordinate system. Then when you call glRotate on X, you rotate the cube on its local X axis. Finally calling glRotate on the Y axis rotates it on the local Y axis (which is now parallel to the global Z axis as you already rotated the object around X).


Quote:

Rotation 90 counterclockwise around OY gives this result for both cases.
...
Also the Z-axis should be on the left side of the cube, I don't understand why it is on the right side.

No this is correct. The +Z is towards the camera by definition. So if you rotate Y 90 degrees CCW then it will be pointed to the right, while X is pointed out.


I think I would suggest that you just spend some more time playing with it, possibly searching the internet for articles and more explanations.

I don't mind helping, it helps me too to verbalize these concepts and think about them, even after I think I understand it can still be confusing sometimes. I'm just afraid I've reached the limit of how I can explain it.

Best of luck!

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I think I've finally understand this. Somehow during the time I confused the thing whish is drawn on the screen with actual code execution and this gave me all the trobule.

Thank you for devoting your time in helping me on this. :)

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      Im pretty sure it has something to do with the rendering context (or maybe the window DC?). Here a snippet of my OpenGL class:
      bool CGLEngine::Initialize(HWND hWnd) { hDC = GetDC(hWnd); if(!SetupPixelFormatDescriptor(hDC)){ ReleaseDC(hWnd, hDC); return false; } hRC = wglCreateContext(hDC); wglMakeCurrent(hDC, hRC); // more code ... return true; } void CGLEngine::Shutdown() { // some code... if(hRC){wglDeleteContext(hRC);} if(hDC){ReleaseDC(hWnd, hDC);} hDC = hRC = NULL; }  
      The full source code is available here. The most relevant files are:
      -OpenGL class (header / source)
      -Main code (header / source)
       
      Thx in advance if anyone can help me.
    • By DiligentDev
      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.
      Creating Shaders
      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:
      SHADER_SOURCE_LANGUAGE_DEFAULT - The shader source language matches the underlying graphics API: HLSL for Direct3D11/Direct3D12 mode, and GLSL for OpenGL and OpenGLES modes. SHADER_SOURCE_LANGUAGE_HLSL - The shader source is in HLSL. For OpenGL and OpenGLES modes, the source code will be converted to GLSL. SHADER_SOURCE_LANGUAGE_GLSL - The shader source is in GLSL. There is currently no GLSL to HLSL converter, so this value should only be used for OpenGL and OpenGLES modes. There are two ways to provide the shader source code. The first way is to use Source member. The second way is to provide a file path in FilePath member. Since the engine is entirely decoupled from the platform and the host file system is platform-dependent, the structure exposes pShaderSourceStreamFactory member that is intended to provide the engine access to the file system. If FilePath is provided, shader source factory must also be provided. If the shader source contains any #include directives, the source stream factory will also be used to load these files. The engine provides default implementation for every supported platform that should be sufficient in most cases. Custom implementation can be provided when needed.
      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:
      ShaderCreationAttribs Attrs; Attrs.Desc.Name = "MyPixelShader"; Attrs.FilePath = "MyShaderFile.fx"; Attrs.SearchDirectories = "shaders;shaders\\inc;"; Attrs.EntryPoint = "MyPixelShader"; Attrs.Desc.ShaderType = SHADER_TYPE_PIXEL; Attrs.SourceLanguage = SHADER_SOURCE_LANGUAGE_HLSL; BasicShaderSourceStreamFactory BasicSSSFactory(Attrs.SearchDirectories); Attrs.pShaderSourceStreamFactory = &BasicSSSFactory; ShaderVariableDesc ShaderVars[] = {     {"g_StaticTexture", SHADER_VARIABLE_TYPE_STATIC},     {"g_MutableTexture", SHADER_VARIABLE_TYPE_MUTABLE},     {"g_DynamicTexture", SHADER_VARIABLE_TYPE_DYNAMIC} }; Attrs.Desc.VariableDesc = ShaderVars; Attrs.Desc.NumVariables = _countof(ShaderVars); Attrs.Desc.DefaultVariableType = SHADER_VARIABLE_TYPE_STATIC; StaticSamplerDesc StaticSampler; StaticSampler.Desc.MinFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MagFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MipFilter = FILTER_TYPE_LINEAR; StaticSampler.TextureName = "g_MutableTexture"; Attrs.Desc.NumStaticSamplers = 1; Attrs.Desc.StaticSamplers = &StaticSampler; ShaderMacroHelper Macros; Macros.AddShaderMacro("USE_SHADOWS", 1); Macros.AddShaderMacro("NUM_SHADOW_SAMPLES", 4); Macros.Finalize(); Attrs.Macros = Macros; RefCntAutoPtr<IShader> pShader; m_pDevice->CreateShader( Attrs, &pShader );
      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.
      Binding Shader Resources
      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:
      PixelShader->GetShaderVariable( "g_tex2DShadowMap" )->Set( pShadowMapSRV ); Mutable and dynamic variables are bound via a new Shader Resource Binding object (SRB) that is created by the pipeline state (IPipelineState::CreateShaderResourceBinding()):
      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 two samples, asteroids performance benchmark and example Unity project that uses Diligent Engine in native plugin.
      AntTweakBar sample is Diligent Engine’s “Hello World” example.

       
      Atmospheric scattering sample is a more advanced example. It 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 and Android platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and support for more platforms is planned.
    • By michaeldodis
      I've started building a small library, that can render pie menu GUI in legacy opengl, planning to add some traditional elements of course.
      It's interface is similar to something you'd see in IMGUI. It's written in C.
      Early version of the library
      I'd really love to hear anyone's thoughts on this, any suggestions on what features you'd want to see in a library like this? 
      Thanks in advance!
    • By Michael Aganier
      I have this 2D game which currently eats up to 200k draw calls per frame. The performance is acceptable, but I want a lot more than that. I need to batch my sprite drawing, but I'm not sure what's the best way in OpenGL 3.3 (to keep compatibility with older machines).
      Each individual sprite move independently almost every frame and their is a variety of textures and animations. What's the fastest way to render a lot of dynamic sprites? Should I map all my data to the GPU and update it all the time? Should I setup my data in the RAM and send it to the GPU all at once? Should I use one draw call per sprite and let the matrices apply the transformations or should I compute the transformations in a world vbo on the CPU so that they can be rendered by a single draw call?
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