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OpenGL Quaternion camera, _translation_ problem

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I learned all about quaternions yesterday and successfully implemented them in OpenGL for my camera. I can spin about just wonderfully now with no gimbal lock. I'm making a ship-flying type game, and so obviously I want to be able to move the ship forward, in the local z direction. My code keeps track of the rotation quaternion, and with rotations I just have a small pitch or roll angle quaternion to multiply with and get the new rotation. To keep track of my world position, I have xpos, ypos, zpos. My problem seems to be getting the correct view vector out of the quaternion. My initial vector, for movement, is (0, 0, 1) because I want to move in Z. To rotate it with the quaternion, I looked up how to turn the quat into a rotation matrix. Multiplying that by (0,0,1) gives the third column of the matrix. These (I think) should be the new view vector, which I should add to my world position. Here's my code:
if (KeyDown(VK_SPACE))
	{
		xpos += 2*rotation.x*rotation.z - 2*rotation.w*rotation.y;
		ypos += 2*rotation.y*rotation.z + 2*rotation.w*rotation.x;
		zpos += rotation.w*rotation.w - rotation.x*rotation.x - rotation.y*rotation.y + rotation.z*rotation.z;
	}

...
glTranslatef(0.0f,0.0f,-6.0f);	// Move Into The Screen

// draw the ship here
	
glRotatef(114.6*acos(rotation.w),rotation.x,rotation.y,rotation.z);

glTranslatef(xpos,ypos,zpos);


From the start, when I move into Z, it works. When I pitch myself up and move into Y, it works. When I roll to one side and then pitch myself into the X, it moves in Y instead. I tried a random guess fix and swapped the + and - signs in the xpos and ypos formulae, to get the bottom row of the matrix instead. This actually fixed the x problem, so now movement on all three axes moved correctly by themselves. The big problem is still that this movement doesn't work. It seems to at first, but after a bit of flying around, I start sliding sideways, backwards, down, some combination of them, etc. Is my math wrong, or something else in my code?

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glRotate isn't going to be useful in this case. In fact, glRotate is rarely useful at all. With quaternions and OpenGL, you'll usually be constructing a matrix from the quaternion. In this case, since you are just doing a camera, it's even simpler because gluLookAt will multiply the appropriate matrix for you, without having to do it by hand.

All you need to do is write a method that rotates a vector by a quaternion. Use that to rotate {0, 0, 1} and {0, 1, 0} to get your appropriate forward and up vectors, and just plug those directly into gluLookAt(position, position + forward, up);

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Ok, that took a few times reading through, but I think I get it. Instead of rotate, do the use the entire quaternion->rotation matrix and multiply in my own method. Then use it on those two vectors, one for my forward flight direction and the other for my ship's roll / up direction. Use that for the camera instead of translating.

Will that fix my flight problem? I'm assuming I then take my transformed forward vector and add some speed multiple of it to my position vector.

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Quote:
Original post by Tesserex
Ok, that took a few times reading through, but I think I get it. Instead of rotate, do the use the entire quaternion->rotation matrix and multiply in my own method. Then use it on those two vectors, one for my forward flight direction and the other for my ship's roll / up direction. Use that for the camera instead of translating.

Will that fix my flight problem? I'm assuming I then take my transformed forward vector and add some speed multiple of it to my position vector.
Looking at the code you posted earlier:

glTranslatef(0.0f,0.0f,-6.0f);	// Move Into The Screen

// draw the ship here

glRotatef(114.6*acos(rotation.w),rotation.x,rotation.y,rotation.z);

glTranslatef(xpos,ypos,zpos);

I'm unclear as to whether this is intended to be a camera or object transform, but in either case the sequence of transforms appears to be incorrect.

Can you clarify the purpose of the transform? You mentioned that this was for a camera, but the comment 'draw the ship here' seems to indicate otherwise.

Anyway, as mentioned, when working with a rotation in quaternion form it's typical to convert it to a matrix before submitting it to OpenGL. Furthermore, the direction vectors can be extracted directly from this matrix; there's no need to perform additional vector rotations to derive these vectors.

As for 114.6, that is one 'magic number' :) I see what you're doing (converting to degrees and multiplying by two), but it would be far better to write a simple utility function to perform the conversion (even then using a named constant rather than 57.x), and then include the factor of 2 directly in the expression.

However, although it looks like this method should work, again, it would probably be better to use a matrix. Your method relies on the specifics of how a quaternion is used to represent a rotation; although it's important to understand these specifics, code that works with quaternions should treat them more like a 'black box'. In short, you should avoid mucking around with the quaternion elements directly in most cases.

If you can clarify what the purpose of the transform you posted is, we can probably point out more specifically where the code might be in error.

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Ok, for the purpose. Think Star Fox. Third person view. The ship is in the center of the screen, at it's location. The camera is a fixed distance directly behind it. So the first set of commands moves the camera back from the ship and draws the ship. Then we spin the world around the ship to orient it, then move the ship to it's location in space. Because the ship was already drawn, it's tied to the camera and they move together.

For now, though, I've removed that part and am going to get first person flying working first. It has the same problems though. I implemented some matrix stuff this time, and now my problems are backwards. The moving seems to work, but the rotating around is busted. Here's my new code:


Vector Transform(Vector v)
{
Vector nv;
nv.x = v.x*(w*w + x*x - y*y - z*z) + v.y*(2*x*y - 2*w*z) + v.z*(2*x*z + 2*w*y);
nv.y = v.x*(2*x*y + 2*w*z) + v.y*(w*w - x*x + y*y - z*z) + v.z*(2*y*z - 2*w*x);
nv.z = v.x*(2*x*z - 2*w*y) + v.y*(2*y*z + 2*w*x) + v.z*(w*w - x*x - y*y + z*z);
return nv;
}


You can probably tell, this just transforms any vector by the rotation matrix derived from the quaternion. This function is a member of my quaternion class.

Here's the keypress stuff...

if (KeyDown(VK_LEFT))
{
rotation.Multiply(rollmq);
up = rotation.Transform(yvect);
}
if (KeyDown(VK_RIGHT))
{
rotation.Multiply(rollq);
up = rotation.Transform(yvect);
}
if (KeyDown(VK_UP))
{
rotation.Multiply(pitchq);
forward = rotation.Transform(zvect);
}
if (KeyDown(VK_DOWN))
{
rotation.Multiply(pitchmq);
forward = rotation.Transform(zvect);
}
if (KeyDown(VK_SPACE))
{
position.x += forward.x;
position.y += forward.y;
position.z += forward.z;
}


"rollmq" and "pitchmq" are the tiny angle quaternions for the negative turns. I figured that the up and forward vectors need only be updated if the roll and pitch change, respectively. If I update both each time, it still doesn't work, but it's behaves differently, so this might be a clue.


gluLookAt(position.x,position.y,position.z,position.x+forward.x,position.y+forward.y,position.z+forward.z,up.x,up.y,up.z);



This is now my only camera modifying line. It seems ok, giving position, position+forward, and up.

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This isn't a complete answer to your question (I didn't look at your code carefully enough to comment in detail), but here are a few notes:

1. The problems of a) orienting and moving the ship, b) constructing an object matrix for the ship and rendering it, and c) constructing a view matrix for the camera should all be considered separately. I mention this because the code you posted seems to include elements of the solutions to all three problems, but itself is not the correct solution for any of them. Thinking about and solving the problems separately should help clear up some of this confusion.

2. Remember that when transforms are applied via OpenGL function calls, the order in which the transforms are applied is the opposite of the order in which the corresponding OpenGL function calls appear in the code.

3. The 'model transform' for an object typically consists of the transform sequence scale->rotate->translate (any of these is of course optional, and scale is often simply identity).

4. The 'view transform' that corresponds to a 'model transform' is, generally the speaking, the inverse of that transform. There are various ways the inverse can be computed. In your case it appears you're trying to do it manually by applying the inverse of the individual transforms in the opposite order. Leaving aside scale, this should translate to (translate^-1)->(rotation^1), where translate^-1 is the original translation negated, and rotation^-1 is the original rotation inverted (transpose for a matrix, conjugate for a quaternion, negation of angle or axis for an axis-angle pair).

5. Third-person cameras are a different problem. It looks like you're already taking this approach, but it would probably be best to get basic object motion and rendering and first-person camera mode working before trying to implement a proper 3rd-person camera.

I hope these notes will help you to identify some of the problems in your code. Feel free to post back if you have further questions.

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Despite not having a clue what your post was saying, it allowed me somehow to fix the problem entirely.

Using the new vector approach with gluLookAt solved my translation problems but the gimbal lock came back. That was quite annoying. My fix?

Reverse the order in which I multiplied my quaternions to add rotations.


if (KeyDown(VK_LEFT))
{
Quaternion temp = rollmq;
temp.Multiply(rotation);
rotation = temp;
//rotation.Multiply(rollmq);
}




And if anyone would like to know, I intend this to eventually become a space fighter game where you aren't limited to fighting in one plane (the 2d space kind of plane, not the vehicle). Also, I plan to control it with wiimotes :-D

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Unfortunately, you seem to be getting way ahead of yourself. You need to have a grasp on linear algebra(at least the parts that pertain to 3D graphics) and the OpenGL API. I'd suggest getting yourself a book, there are plenty of good ones out there on the subject. I personally liked "Mathematics for 3D Game Programming and Computer Graphics" and "3D Math Primer for Graphics and Game Development." Yes, it is true that you can learn plenty about all the fancy pants stuff out there just by using google, however, it *appears* as though you lack the basic understanding of what's really going on when you make these calls. It is incredibly important that you do understand that in order to use it properly.

That being said, here is the important parts of my camera class, which is far from perfect but may shine some light on it.


#import <OpenGL/gl.h>
#import <OpenGL/glu.h>

#import "OCCamera.h"

@implementation OCCamera

- (id)initWithLocation:(vector_t)loc width:(int)w height:(int)h
{
[super init];

position = loc;

screenWidth = w;
screenHeight = h;
screenRatio = (float)(screenWidth/screenHeight);
near = 1.0f;
far = 768.0;
fov = 45.0f;

rotation = quaternion_identity();
//Completely unneccesary, but a good reminder.
forward = quaternion_rotate_vector(rotation, vector3(0,0,1));
up = quaternion_rotate_vector(rotation, vector3(0,1,0));
right = quaternion_rotate_vector(rotation, vector3(1,0,0));
yaw = pitch = 0.0f;

interpolationSpeed = 1.0f;

return self;
}

- (void)animate:(float)dt
{
if(allowInterpolation)
{
elapsedTime += dt * interpolationSpeed;
if(elapsedTime > 1.0f)
{
elapsedTime = 1.0f;
allowInterpolation = false;
}
position = vector_add(initPosition, vector_scale(vector_subtract(destPosition, initPosition), elapsedTime));
rotation = Quaternion_SLERP(initRotation, destRotation, elapsedTime);

forward = quaternion_rotate_vector(rotation, vector3(0,0,1));
up = quaternion_rotate_vector(rotation, vector3(0,1,0));
right = quaternion_rotate_vector(rotation, vector3(1,0,0));

yaw = atan2(forward.x, forward.z);
pitch = acos(vector_dot_product(vector3(0, 1, 0), forward)) - OSML_PI / 2.0f;
}

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(fov, screenRatio, near, far);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

gluLookAt(position.x, position.y, position.z,
position.x + forward.x, position.y + forward.y, position.z + forward.z,
up.x, up.y, up.z);
}

- (void)rotateYaw:(double)delta
{
if(allowInterpolation)
return;

yaw += delta;
quaternion_t qPitch = quaternion_from_angle_around_axis(pitch, vector3(1,0,0));
quaternion_t qYaw = quaternion_from_angle_around_axis(yaw, vector3(0,1,0));

rotation = quaternion_product(qYaw, qPitch);

forward = quaternion_rotate_vector(rotation, vector3(0,0,1));
up = quaternion_rotate_vector(rotation, vector3(0,1,0));
right = quaternion_rotate_vector(rotation, vector3(1,0,0));
}
- (void)rotatePitch:(double)delta
{
if(allowInterpolation)
return;

pitch += delta;
quaternion_t qPitch = quaternion_from_angle_around_axis(pitch, vector3(1,0,0));
quaternion_t qYaw = quaternion_from_angle_around_axis(yaw, vector3(0,1,0));

rotation = quaternion_product(qYaw, qPitch);

forward = quaternion_rotate_vector(rotation, vector3(0,0,1));
up = quaternion_rotate_vector(rotation, vector3(0,1,0));
right = quaternion_rotate_vector(rotation, vector3(1,0,0));
}
- (void)setPitch:(double)p
{
if(allowInterpolation)
return;
pitch = p;
[self rotatePitch:0];
}
- (void)setYaw:(double)p
{
if(allowInterpolation)
return;
yaw = p;
[self rotateYaw:0];
}
- (vector_t)targetPoint:(vector_t)point distance:(float)f
{
return vector3(point.x - forward.x * f, point.y - forward.y * f, point.z - forward.z * f);
}
- (void)targetOnPoint:(vector_t)point distance:(float)f
{
if(allowInterpolation)
return;

position.y = point.y - forward.y * f;
position.z = point.z - forward.z * f;
position.x = point.x - forward.x * f;
}
- (void)orbitYaw:(double)amt aroundPoint:(vector_t)center
{
if(allowInterpolation)
return;

vector_t newPos;
quaternion_t newRot;
float radius = sqrtf(pow(position.x - center.x, 2) + pow(position.z - center.z, 2));

yawOrbit += amt;
newPos.x = center.x + cos(yawOrbit + OSML_HALF_PI) * radius;
newPos.y = position.y;
newPos.z = center.z - sin(yawOrbit + OSML_HALF_PI) * radius;

yaw += amt;
quaternion_t qPitch = quaternion_from_angle_around_axis(pitch, vector3(1,0,0));
quaternion_t qYaw = quaternion_from_angle_around_axis(yaw, vector3(0,1,0));
newRot = quaternion_product(qYaw, qPitch);

position = newPos;
[self rotateTo:newRot];
}
- (void)rotateTo:(quaternion_t)q
{
if(allowInterpolation)
return;

rotation = q;
forward = quaternion_rotate_vector(rotation, vector3(0,0,1));
up = quaternion_rotate_vector(rotation, vector3(0,1,0));
right = quaternion_rotate_vector(rotation, vector3(1,0,0));
}
- (bool)interpolateTo:(vector_t)pos withRotation:(quaternion_t)rot withSpeed:(float)speed cancelPrevious:(bool)cancel
{
if(allowInterpolation && !cancel)
return false;

interpolationSpeed = speed;
allowInterpolation = true;

elapsedTime = 0.0f;
initPosition = position;
destPosition = pos;
initRotation = rotation;
destRotation = rot;
return true;
}
- (void)moveForward:(double)amt
{
if(allowInterpolation)
return;

position.x += forward.x * amt;
position.y += forward.y * amt;
position.z += forward.z * amt;
}
- (void)moveRight:(double)amt
{
if(allowInterpolation)
return;

position.x -= right.x * amt;
position.y -= right.y * amt;
position.z -= right.z * amt;
}
- (void)moveUp:(double)amt
{
if(allowInterpolation)
return;

position.x += up.x * amt;
position.y += up.y * amt;
position.z += up.z * amt;
}
- (void)moveTo:(vector_t)pos
{
if(allowInterpolation)
return;

position = pos;
}
@end

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Well, first of all, it's fixed, thank you everyone for your insight.

Second, I'll not take offense to your comments, but your assumptions were wrong, Longjumper. I do have a grasp of linear algebra. I've been through a college course on it. Just last semester, in fact, with a primer on it (especially how it pertains to transformations) in my previous calculus 3 class. The only thing that was new to me here was the quaternion itself. I'm also in a class called Numerical Methods right now. You can probably guess I'm a CS major.

Thanks again to everyone.

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No offense was intended, of course. And as always, I will admit when I am wrong, and in this case I may be. However, I took a class in linear algebra and numerical methods some years back, and it is only recently that I have truly grasped it intuitively. Perhaps I was just projecting myself onto you, though. ;)

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Ok I have a different question, but it's still about the camera so I decided not to make a new thread. It's about which of two methods to use for third person chase cam. It seems to me there are two equally valid possibilities:

A.
1) Translate a set Z distance from the origin
2) Draw the player object at the origin (after rotating properly)
3) gluLookAt the world based on player position

B.
1) gluLookAt
2) Draw the player at the correct position it stores

This second method I know requires calculating the camera forward vector to have a consistent length maintained in chase, and also finding the proper camera position in the direction behind the player. Does the first method require this as well, or are the gluLookAt parameters simpler? I had what seemed to be a working 3rd person camera but then I guessed (and still am not sure) that when I rotated pitch, the player object was technically moving in space while the camera spun.

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You will want to use the forward and up vectors of the player, scale them appropriately, and use that as the position of the camera in gluLookAt. The center of the camera(reference point) will be the player's position, and you will have to do some vector manipulation to determine the up vector for gluLookAt.

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      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
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      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!
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