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OpenGL Why is emulating fixed functionality using shaders so slow?

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Hi, i am trying to implement Microsofts PixelMotionBlur using OpenGL. In order to using only one render pass i need to draw to two frame buffer attachments: 1. the original image, 2. the vertex velocity information. For the original image i normaly want to use the opengl fixed functions, but to do it in one pass, the vertex shader has to calculate everything the fixed function would do: lighting, texture appliance, etc. I only implemented the fixed function lighting but it seems already at this point, that the shader supported rendering is much slower than the usal opengl pipeline. But why? If it stays this way, i will render in two passes, because of a better performance. But i though it would be vise versa. Could my someone explain why the same calculations made in a shader instead of the fixed function pipeline is so much slower? Greetz, Achilleos.

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I'm not convinced the slowdown you're observing is actually a result of the shader, but rather some other aspect of your rendering method.

It could depend on your hardware as well. Also remember that shaders are not about speed, they are about control. Allegedly many cards don't even have fixed function paths anymore, they simply execute the programmable path with a specific program. Custom shaders won't make your code faster, unless perhaps if you write uselessly trivial shaders, which does not appear to be your problem here anyways.

In any case, can you describe your overall rendering techniques in more detail?

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i am using a nvidia 8800 gtx, therefore i thought shader should reach the same speed than fixed function.

Below are to images, the first is the is created using only a post production shader which uses 2 fbo attachted images to generate the final image (the velocity buffers are not implemented so far, so you see only an offset of the rendering).

The second image uses a shader pair in addition for the object rendering (the post production shader renders to a window filling quad after that). The source is as follows:


#version 110

void ApplyLighting(in vec3 ecPosition3,
in vec3 normal,
in bool LocalViewer,
in int NumEnabledLights,
in bool SeperateSpecular,
out vec4 color);

void main(void)
{
// calculate position
gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;

// calculate eye coordinate position
vec4 ecPosition = gl_ModelViewMatrix * gl_Vertex;
vec3 ecPosition3 = vec3(ecPosition) / ecPosition.w;

// calculate normal
vec3 normal = gl_NormalMatrix * gl_Normal;

// apply the lighting (fixed function)
vec4 color;
ApplyLighting(ecPosition3, normal, true, 2, false, color);

// write back the color information
gl_FrontColor = color;
}




// removed images from server

If you pay attention to the fps displayed at the bottom line, you can figure out the performance loss. The fps are restricted to a maximum of 60 (vsync). And i have to defend myself: There is a cube mapped ball lying in the middle. The cube map is dynamic rendered and currently refreshed every frame (6 additional renderings of the scene).
So the application is not slow, but i dont want a performance decrease at this early state of the advanced motion blur i am up to implement.
(there is a complex physics engine and a robot control system running in the background...)

ps: why on earth is the gamedev.net website so slow?

[Edited by - Achilleos on June 6, 2007 1:07:13 PM]

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So the original runs at 60FPS (capped by vsync) and the "slower" one at 48FPS? 60FPS means you take 0.0166 seconds per frame. 48FPS means you take 0.0208 seconds per frame. That's a difference of 0.0042 seconds. Four milliseconds. That's not nearly as pathological a slowdown as your original post led me to believe.

In any case, try using ftransform() and spitting out a simple flat color from the PS. Try not binding any shader parameters you're binding -- are you doing that every frame, are you reloading the shader every frame? Et cetera. There's nothing necessarily broken with the shader you posted as far as I can tell, except the bools make me suspicious; they might be the first place I'd look, as conditionals are not necessarily the nicest things to have in shaders.

The idea is to determine where this four ms frame speed decrease is coming from. It will hard because its so small. By making the shader trivial, you might be able to tell if its the shader code itself (particularly those conditionals) or the CPU-side setup for the shader.

It's tricky to profile GPUs.

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Or maybe your performance Problems are elsewhere.
I would like to know what you are doing to get a gf8 down to 50 FPS with such a tiny scene? I'm rendering a huge terrain atm that has lots of polys does soft lod (no sudden changes in lod levels visible) and uses a fragment shader that mixes 3 textures out of a 4th, still at 1000 FPS.

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Quote:

I would like to know what you are doing to get a gf8 down to 50 FPS with such a tiny scene?

He's not doing anything, he's just waiting for the vsync:

Quote:

The fps are restricted to a maximum of 60 (vsync).


It's not a performance issue.

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A few comments:

1] Disable vsync when doing any profiling

2] the camera position in the scenes don't look identical, which makes me think that you moved the camera manually to get there. For more accurate comparisons you should have completely deterministic scenes, relying on no user input to setup the scene.

3] Are you averaging your framerate since the beginning of the app or keeping a running average?

4] The 2nd screenshot appears to have point sampling enabled judging by the highly pixelated screenshot whilst no such artefacts are visible in the 1st screenshot. What is the reason for this difference?

5] Try break down your scene into multiple parts. Profile each part independently - this will let you isolate the differences more easily.

6] your vshader should probably be using glPosition = ftransform()

7] ApplyLighting is not using any texture lookups is it?

8] Are your shaders performing dynamic branching in the shader? are you looping over your lights?

9] What does your pixel shader look like?

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I found the problem. I am currently implementing a collection of motion blur algorithms. The horrorable fps came from an unreseted value. For the accumulation buffer motion blur method i increased my renderPasses variable to 12 and never decreased it after choosing another method from the menu.

Now i am running rendering a frame in less than a ms.

The scene is that slow because there are multiple render contexts which render to different targets. The images are passed through some imaging routines for segmentation and classification. From this information simulated actuators are altered and the physics engine makes another tick.

The fps of the window i am testing in, is set invalid on change or on physics update.

Here's another image.

some scene

The application is really huge. And customizable to the tips. Thats makes it hard for me to optimize things. I working for the project since january this year. And i found a graphics engine writte in 1998 with little tweaks. There were mistakes in the source like creating a display list per triangle...

Greetz.

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Quote:
Original post by FReY
A few comments:

1] Disable vsync when doing any profiling

I currently improved the frames per second counter. So profiling should be fine now.

Quote:
Original post by FReY
2] the camera position in the scenes don't look identical, which makes me think that you moved the camera manually to get there. For more accurate comparisons you should have completely deterministic scenes, relying on no user input to setup the scene.

I used a presentation camera mode which forces the display to circle the camera aroung the selected object and redraw the scene. But your are right a simple mechanism to constantly force updates should be implemented.

Quote:
Original post by FReY
3] Are you averaging your framerate since the beginning of the app or keeping a running average?

changed the fps counter, as mentioned at point 1.

Quote:
Original post by FReY
4] The 2nd screenshot appears to have point sampling enabled judging by the highly pixelated screenshot whilst no such artefacts are visible in the 1st screenshot. What is the reason for this difference?

I have antialiasing disabled. Driver forced multisample works with fixed funciton but not with post production shader which use a frame buffer object as input. I haven't read anywhere that its possible... Beside that, i am agreeing your sharp eye. But i don't know where mistake is. I am rendering into a fbo and use the image as input into a fragment shader when rendering a fullscreen quad. Mipmapping settings don't seem to affect the quality. The fbo is the same pixel size (width and height) as the window, so nearest filtering should be okay, but i use the linear filter method. otherwise it looks worse...

Quote:
Original post by FReY
7] ApplyLighting is not using any texture lookups is it?

8] Are your shaders performing dynamic branching in the shader? are you looping over your lights?

Yes, its simply a function which runs through the lights and adds the effects (point, directional and spot light) as discribed in OpenGL Shading Language Second Edition by Randi Rost.

Quote:
Original post by FReY
9] What does your pixel shader look like?

:) It simple does nothing. Writes the the the color to gl_FragData[0] and vec4(0.0, 0.0, 1.0, 1.0) tp gl_FragData[1]. I am currently implementing... and wondered about the performance decreasing this early implementation step.
But now everything seems great!

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      Invoking Draw Command
      The final step is to set states that are not part of the PSO, such as render targets, vertex and index buffers. Diligent Engine uses Direct3D11-syle API that is translated to other native API calls under the hood:
      ITextureView *pRTVs[] = {m_pRTV}; m_pContext->SetRenderTargets(_countof( pRTVs ), pRTVs, m_pDSV); // Clear render target and depth buffer const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); m_pContext->ClearDepthStencil(nullptr, CLEAR_DEPTH_FLAG, 1.f); // Set vertex and index buffers IBuffer *buffer[] = {m_pVertexBuffer}; Uint32 offsets[] = {0}; Uint32 strides[] = {sizeof(MyVertex)}; m_pContext->SetVertexBuffers(0, 1, buffer, strides, offsets, SET_VERTEX_BUFFERS_FLAG_RESET); m_pContext->SetIndexBuffer(m_pIndexBuffer, 0); Different native APIs use various set of function to execute draw commands depending on command details (if the command is indexed, instanced or both, what offsets in the source buffers are used etc.). For instance, there are 5 draw commands in Direct3D11 and more than 9 commands in OpenGL with something like glDrawElementsInstancedBaseVertexBaseInstance not uncommon. Diligent Engine hides all details with single IDeviceContext::Draw() method that takes takes DrawAttribs structure as an argument. The structure members define all attributes required to perform the command (primitive topology, number of vertices or indices, if draw call is indexed or not, if draw call is instanced or not, if draw call is indirect or not, etc.). For example:
      DrawAttribs attrs; attrs.IsIndexed = true; attrs.IndexType = VT_UINT16; attrs.NumIndices = 36; attrs.Topology = PRIMITIVE_TOPOLOGY_TRIANGLE_LIST; pContext->Draw(attrs); For compute commands, there is IDeviceContext::DispatchCompute() method that takes DispatchComputeAttribs structure that defines compute grid dimension.
      Source Code
      Full engine source code is available on GitHub and is free to use. The repository contains tutorials, sample applications, asteroids performance benchmark and an example Unity project that uses Diligent Engine in native plugin.
      Atmospheric scattering sample demonstrates how Diligent Engine can be used to implement various rendering tasks: loading textures from files, using complex shaders, rendering to multiple render targets, using compute shaders and unordered access views, etc.

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

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

      Future Work
      The engine is under active development. It currently supports Windows desktop, Universal Windows, Linux, Android, MacOS, and iOS platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and Metal backend is in the plan.
    • By LifeArtist
      Good Evening,
      I want to make a 2D game which involves displaying some debug information. Especially for collision, enemy sights and so on ...
      First of I was thinking about all those shapes which I need will need for debugging purposes: circles, rectangles, lines, polygons.
      I am really stucked right now because of the fundamental question:
      Where do I store my vertices positions for each line (object)? Currently I am not using a model matrix because I am using orthographic projection and set the final position within the VBO. That means that if I add a new line I would have to expand the "points" array and re-upload (recall glBufferData) it every time. The other method would be to use a model matrix and a fixed vbo for a line but it would be also messy to exactly create a line from (0,0) to (100,20) calculating the rotation and scale to make it fit.
      If I proceed with option 1 "updating the array each frame" I was thinking of having 4 draw calls every frame for the lines vao, polygons vao and so on. 
      In addition to that I am planning to use some sort of ECS based architecture. So the other question would be:
      Should I treat those debug objects as entities/components?
      For me it would make sense to treat them as entities but that's creates a new issue with the previous array approach because it would have for example a transform and render component. A special render component for debug objects (no texture etc) ... For me the transform component is also just a matrix but how would I then define a line?
      Treating them as components would'nt be a good idea in my eyes because then I would always need an entity. Well entity is just an id !? So maybe its a component?
      Regards,
      LifeArtist
    • By QQemka
      Hello. I am coding a small thingy in my spare time. All i want to achieve is to load a heightmap (as the lowest possible walking terrain), some static meshes (elements of the environment) and a dynamic character (meaning i can move, collide with heightmap/static meshes and hold a varying item in a hand ). Got a bunch of questions, or rather problems i can't find solution to myself. Nearly all are deal with graphics/gpu, not the coding part. My c++ is on high enough level.
      Let's go:
      Heightmap - i obviously want it to be textured, size is hardcoded to 256x256 squares. I can't have one huge texture stretched over entire terrain cause every pixel would be enormous. Thats why i decided to use 2 specified textures. First will be a tileset consisting of 16 square tiles (u v range from 0 to 0.25 for first tile and so on) and second a 256x256 buffer with 0-15 value representing index of the tile from tileset for every heigtmap square. Problem is, how do i blend the edges nicely and make some computationally cheap changes so its not obvious there are only 16 tiles? Is it possible to generate such terrain with some existing program?
      Collisions - i want to use bounding sphere and aabb. But should i store them for a model or entity instance? Meaning i have 20 same trees spawned using the same tree model, but every entity got its own transformation (position, scale etc). Storing collision component per instance grats faster access + is precalculated and transformed (takes additional memory, but who cares?), so i stick with this, right? What should i do if object is dynamically rotated? The aabb is no longer aligned and calculating per vertex min/max everytime object rotates/scales is pretty expensive, right?
      Drawing aabb - problem similar to above (storing aabb data per instance or model). This time in my opinion per model is enough since every instance also does not have own vertex buffer but uses the shared one (so 20 trees share reference to one tree model). So rendering aabb is about taking the model's aabb, transforming with instance matrix and voila. What about aabb vertex buffer (this is more of a cosmetic question, just curious, bumped onto it in time of writing this). Is it better to make it as 8 points and index buffer (12 lines), or only 2 vertices with min/max x/y/z and having the shaders dynamically generate 6 other vertices and draw the box? Or maybe there should be just ONE 1x1x1 cube box template moved/scaled per entity?
      What if one model got a diffuse texture and a normal map, and other has only diffuse? Should i pass some bool flag to shader with that info, or just assume that my game supports only diffuse maps without fancy stuff?
      There were several more but i forgot/solved them at time of writing
      Thanks in advance
    • By RenanRR
      Hi All,
      I'm reading the tutorials from learnOpengl site (nice site) and I'm having a question on the camera (https://learnopengl.com/Getting-started/Camera).
      I always saw the camera being manipulated with the lookat, but in tutorial I saw the camera being changed through the MVP arrays, which do not seem to be camera, but rather the scene that changes:
      Vertex Shader:
      #version 330 core layout (location = 0) in vec3 aPos; layout (location = 1) in vec2 aTexCoord; out vec2 TexCoord; uniform mat4 model; uniform mat4 view; uniform mat4 projection; void main() { gl_Position = projection * view * model * vec4(aPos, 1.0f); TexCoord = vec2(aTexCoord.x, aTexCoord.y); } then, the matrix manipulated:
      ..... glm::mat4 projection = glm::perspective(glm::radians(fov), (float)SCR_WIDTH / (float)SCR_HEIGHT, 0.1f, 100.0f); ourShader.setMat4("projection", projection); .... glm::mat4 view = glm::lookAt(cameraPos, cameraPos + cameraFront, cameraUp); ourShader.setMat4("view", view); .... model = glm::rotate(model, glm::radians(angle), glm::vec3(1.0f, 0.3f, 0.5f)); ourShader.setMat4("model", model);  
      So, some doubts:
      - Why use it like that?
      - Is it okay to manipulate the camera that way?
      -in this way, are not the vertex's positions that changes instead of the camera?
      - I need to pass MVP to all shaders of object in my scenes ?
       
      What it seems, is that the camera stands still and the scenery that changes...
      it's right?
       
       
      Thank you
       
    • By dpadam450
      Sampling a floating point texture where the alpha channel holds 4-bytes of packed data into the float. I don't know how to cast the raw memory to treat it as an integer so I can perform bit-shifting operations.

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