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OpenGL Graphics Hardware simultaneous or asynchronous?

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I guess this is a question about how the operating system of a (WinXP) computer interacts with the graphics accelerator card. I don't completely understand the issues regarding wether the GPU and graphics card are actually operating "concurrently" with the rest of the computer, or wether the GPU must complete all its processing before returning control of the code back to the executable/Operating System. What difference this makes to a programmer will become clear after I show an example. Let me be a little more explicit about what I am trying to ask here. We have two possibilities: [Possibility A] The GPU sits idle and waits for commands from an executable in memory. Upon receiving a command from the executable, it then stops the processing of any further code, performs the action, and then returns control of the computer back to the executable. Possibility A means that the CPU in a computer runs asynchronously with the GPU. The "acceleration" gained from a graphics accelerator here means that the GPU is merely compressing the time that it takes to perform 3D graphics, rather than actually loading the work off the main CPU while the CPU does "other things". [Possibility B] The GPU is never idle, and is constantly processing graphics operations regardless of what the rest of the computer is doing, and regardless of the speed at which the rest of the computer is doing it. In this scenario, the GPU runs *simultaneously*, or "concurrently", with the CPU and the operating system, and the executable itself. An executable that is issuing openGL functions is actually "dispatching" commands to the GPU; but after dispatching such commands, it does not sit there and wait for a result, but goes on to the rest of the code in the main executable. I have a few large books on programming 3D games using graphics hardware, and none of them address this issue with any clarity. I don't know whether I am programming a machine that is engaging in Possibility A, or one that is engaging in Possibility B!! If you think that this issue doesn't matter or whatever, consider the following code from a callback function for a WinXP executable: // Data initialized elsewhere before OnIdle() Vector4f* avInput = <array of vectors> Vector4f* avOutput = <array of transformed vectors> float * afPIn = (float *)avInput; float * afPOut = (float *)avOutput; // BOOL CGameApp::OnIdle(LONG lCount) { // Let the base class perform its overhead tasks. CWinApp::OnIdle(lCount); if( firstidlecall ) { firstidlecall = false; // Never do this again // Perform initialization. glMatrixMode(GL_MODELVIEW); } else { // Otherwise get the results of commands // issued the last time. glGetFloatv(GL_MODEL_VIEW_MATRIX, afPOut ); glPopMatrix(); } AppDefined_UpdateMatrix(avInput); glPushMatrix(); glMultMatrix3f(afPIn); return TRUE; // call ::OnIdle() again. } If Possibility A is going on, then placing the glMultMatrix3f() right before the return call will not gain any speed over just performing all the gl operations in a row and waiting for the result. However, if the GPU is running "concurrently" with the CPU then the glMultMatrix3f() may be performed while the rest of the system is performing other tasks, such as dispatching messages and executing other active threads elsewhere in memory. This issue is very important if you are considering writing code that is going to use the GPU as a "second processor" in the computer meant to perform computations other than 3D graphics. If they do not run cuncurrently, then you are really not loading anything off onto the graphics card, but merely comparing the relative speed of the GPU against the same operations performed in software.

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Possibility A is actually synchronous, not asynchronous.

At the hardware level, the CPU and GPU are highly asynchronous: they can operate independently of one another. In fact, the CPU runs independently of nearly everything: it sends queries through the various buses (including the PCI Express or AGP bus used by the GPU) and to the various computing units, and then starts doing other things while waiting for the results. If there are no other things to be done, it stalls (becomes idle until it gets the results to do something else).

At the software level, things are a little bit more complex. The CPU interacts with the peripherials by sending data along buses using protocols described by drivers. If the protocol provided by the driver involves waiting for the GPU computation to end, then this is what the CPU will do. If the protocol provided by the driver involves queuing commands and using another thread to send the batches to the GPU when it's done with the previous one, then the CPU will be able to do other things. It all depends on the driver.

OpenGL itself is asynchronous by design. It was designed to be able to send rendering commands across a network back in a time when networks were slow, and is still used today to send very costly and complex rendering commands without losing interactivity. So, I would assume that, unless the driver is very nasty about it (for instance, on embedded systems with limited thread support), OpenGL should be able to make asynchronous calls.

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Original post by ToohrVyk
Possibility A is actually synchronous, not asynchronous.

[snip]

OpenGL itself is asynchronous by design. It was designed to be able to send rendering commands across a network back in a time when networks were slow, and is still used today to send very costly and complex rendering commands without losing interactivity. So, I would assume that, unless the driver is very nasty about it (for instance, on embedded systems with limited thread support), OpenGL should be able to make asynchronous calls.


So you seem to be saying that some of the commands are asynchronous and some of them are not, depending on the driver used. If I am writing a WinXP single thread application that hopes to move some of the computational burden to the GPU, what steps should I take to ensure that I'm not simply idling to wait for results?

In animated games, I have noticed that it is up to the application itself to tell the graphics card when to swap the rendered "backbuffer" onto the screen. I assume that the application simply idles while that takes place, and regains control after the graphics card has finished doing this. It seems to me that it HAS to work that way if the application code itself is telling the graphics card when to swap in the "backbuffer". The alternative would be the graphics card itself updating the frames by itself. But this simply does not go on, am I wrong?


It seems this problem cannot be solved by simply using multiple threads, since multitasking is really an illusion created by the Operating System that juggles pieces of threads so quickly that the human eye is tricked into thinking they are really happening at the "same time". So even if I were rendering every even row using software, and having some other thread render every odd row of an image using the GPU, that they really are not happening at the same time, since the control of either thread is performed to the exclusion of the other. (Maybe I'm wrong?)

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The key to it comes down to two things;
1) the command buffer
2) reading back results

When you submit instructions to the graphics card they are queued up for execution by the driver and the GPU works it way through this buffer. It's kept topped up while there are things to draw, however certain operations act as a break in this stream of data because they rely on the results from operations before it.

A good example is glReadPixels; for the readback to happen all the commands executed before it must completed, the readback can then take place and once that has completed the GPU can continue to execute drawing commands.

However, while it is waiting for the read to take place the CPU has stalled and is waiting on the GPU to finish work.

The other way a stall can happen is if the command buffer gets full and the driver needs to wait to be able to flush some of the work to the GPU's buffers before continuing. There isn't anything you can really do about this one.

When it comes to the backbuffer swapping I believe current drivers (NV and AMD Vista anyways) just insert the swap into the command stream and return right away. But dont' quote me on that [smile]

In the end, avoiding readbacks is the best way to avoid stalls, as well arranging your loop so that while the rendering is going on you are updating your logic, this should overlap things nicely.

(granted, PBOs allow non-stalling async transfer of data, which can remove some of the 'sync points' which occur; these are basically any time data needs to be sent or pulled back from the card).

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Quote:
Original post by phantom
The key to it comes down to two things;
1) the command buffer
2) reading back results

[snip]

A good example is glReadPixels; for the readback to happen all the commands executed before it must completed, the readback can then take place and once that has completed the GPU can continue to execute drawing commands.

However, while it is waiting for the read to take place the CPU has stalled and is waiting on the GPU to finish work.



Ok I think I really have an idea of how this works now. The main executable sends commands to the graphics card in a command buffer. The graphics card pulls the commands off the command buffer at whatever pace it operates. The the main CPU will idle if it needs some data from the GPU, (glReadPixels()) and this idle time will depend on how backed up the command buffer has become. I think it would be interesting to write the code for a driver for a modern GPU.

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Phantom's answer is basically correct for most modern GPUs.

To be more specific, the GPU has a command processor that DMAs commands from a buffer in system memory in kernel space. The driver is responsible for allocating this buffer and configuring the GPU to use it.

However, not all GPUs will do this... on some systems (perhaps embedded systems with low memory), the GPU will operate in a mode where all commands are sent directly to the card via register writes.

About the frame buffer swapping, the CPU is free to run on its own once the command has been sent to the card (either via DMA or direct register writes). In fact, by the time the CPU reaches the next frame, it is possible that the GPU has not yet swapped the previous frame or is only partially done swapping the frame. This is actually what causes tearing to occur in your image and it also explains why enabling VSYNC slows things down, since the CPU has to wait until it knows the GPU is done swapping the frame before drawing the next frame.

Thats what I know to the best of my knowledge.. please correct me if i'm wrong about anything :D

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Quote:
Original post by Clapfoot
About the frame buffer swapping, the CPU is free to run on its own once the command has been sent to the card (either via DMA or direct register writes). In fact, by the time the CPU reaches the next frame, it is possible that the GPU has not yet swapped the previous frame or is only partially done swapping the frame. This is actually what causes tearing to occur in your image and it also explains why enabling VSYNC slows things down, since the CPU has to wait until it knows the GPU is done swapping the frame before drawing the next frame.


Swapping buffers is generally a very fast operation.

Tearing happens when the buffers are swapped while the frame is actively being scanned (because actually DISPLAYING the image on the monitor isn't instantaneous). Enabling VSYNC tells the driver to only swap buffers during the vertical blanking period; therefore, with VSYNC on, you can't swap buffers more often than the monitor refresh rate.

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      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 reenigne
      For those that don't know me. I am the individual who's two videos are listed here under setup for https://wiki.libsdl.org/Tutorials
      I also run grhmedia.com where I host the projects and code for the tutorials I have online.
      Recently, I received a notice from youtube they will be implementing their new policy in protecting video content as of which I won't be monetized till I meat there required number of viewers and views each month.

      Frankly, I'm pretty sick of youtube. I put up a video and someone else learns from it and puts up another video and because of the way youtube does their placement they end up with more views.
      Even guys that clearly post false information such as one individual who said GLEW 2.0 was broken because he didn't know how to compile it. He in short didn't know how to modify the script he used because he didn't understand make files and how the requirements of the compiler and library changes needed some different flags.

      At the end of the month when they implement this I will take down the content and host on my own server purely and it will be a paid system and or patreon. 

      I get my videos may be a bit dry, I generally figure people are there to learn how to do something and I rather not waste their time. 
      I used to also help people for free even those coming from the other videos. That won't be the case any more. I used to just take anyone emails and work with them my email is posted on the site.

      I don't expect to get the required number of subscribers in that time or increased views. Even if I did well it wouldn't take care of each reoccurring month.
      I figure this is simpler and I don't plan on putting some sort of exorbitant fee for a monthly subscription or the like.
      I was thinking on the lines of a few dollars 1,2, and 3 and the larger subscription gets you assistance with the content in the tutorials if needed that month.
      Maybe another fee if it is related but not directly in the content. 
      The fees would serve to cut down on the number of people who ask for help and maybe encourage some of the people to actually pay attention to what is said rather than do their own thing. That actually turns out to be 90% of the issues. I spent 6 hours helping one individual last week I must have asked him 20 times did you do exactly like I said in the video even pointed directly to the section. When he finally sent me a copy of the what he entered I knew then and there he had not. I circled it and I pointed out that wasn't what I said to do in the video. I didn't tell him what was wrong and how I knew that way he would go back and actually follow what it said to do. He then reported it worked. Yea, no kidding following directions works. But hey isn't alone and well its part of the learning process.

      So the point of this isn't to be a gripe session. I'm just looking for a bit of feed back. Do you think the fees are unreasonable?
      Should I keep the youtube channel and do just the fees with patreon or do you think locking the content to my site and require a subscription is an idea.

      I'm just looking at the fact it is unrealistic to think youtube/google will actually get stuff right or that youtube viewers will actually bother to start looking for more accurate videos. 
    • By Balma Alparisi
      i got error 1282 in my code.
      sf::ContextSettings settings; settings.majorVersion = 4; settings.minorVersion = 5; settings.attributeFlags = settings.Core; sf::Window window; window.create(sf::VideoMode(1600, 900), "Texture Unit Rectangle", sf::Style::Close, settings); window.setActive(true); window.setVerticalSyncEnabled(true); glewInit(); GLuint shaderProgram = createShaderProgram("FX/Rectangle.vss", "FX/Rectangle.fss"); float vertex[] = { -0.5f,0.5f,0.0f, 0.0f,0.0f, -0.5f,-0.5f,0.0f, 0.0f,1.0f, 0.5f,0.5f,0.0f, 1.0f,0.0f, 0.5,-0.5f,0.0f, 1.0f,1.0f, }; GLuint indices[] = { 0,1,2, 1,2,3, }; GLuint vao; glGenVertexArrays(1, &vao); glBindVertexArray(vao); GLuint vbo; glGenBuffers(1, &vbo); glBindBuffer(GL_ARRAY_BUFFER, vbo); glBufferData(GL_ARRAY_BUFFER, sizeof(vertex), vertex, GL_STATIC_DRAW); GLuint ebo; glGenBuffers(1, &ebo); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebo); glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices,GL_STATIC_DRAW); glVertexAttribPointer(0, 3, GL_FLOAT, false, sizeof(float) * 5, (void*)0); glEnableVertexAttribArray(0); glVertexAttribPointer(1, 2, GL_FLOAT, false, sizeof(float) * 5, (void*)(sizeof(float) * 3)); glEnableVertexAttribArray(1); GLuint texture[2]; glGenTextures(2, texture); glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_2D, texture[0]); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); sf::Image* imageOne = new sf::Image; bool isImageOneLoaded = imageOne->loadFromFile("Texture/container.jpg"); if (isImageOneLoaded) { glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, imageOne->getSize().x, imageOne->getSize().y, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageOne->getPixelsPtr()); glGenerateMipmap(GL_TEXTURE_2D); } delete imageOne; glActiveTexture(GL_TEXTURE1); glBindTexture(GL_TEXTURE_2D, texture[1]); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); sf::Image* imageTwo = new sf::Image; bool isImageTwoLoaded = imageTwo->loadFromFile("Texture/awesomeface.png"); if (isImageTwoLoaded) { glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, imageTwo->getSize().x, imageTwo->getSize().y, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageTwo->getPixelsPtr()); glGenerateMipmap(GL_TEXTURE_2D); } delete imageTwo; glUniform1i(glGetUniformLocation(shaderProgram, "inTextureOne"), 0); glUniform1i(glGetUniformLocation(shaderProgram, "inTextureTwo"), 1); GLenum error = glGetError(); std::cout << error << std::endl; sf::Event event; bool isRunning = true; while (isRunning) { while (window.pollEvent(event)) { if (event.type == event.Closed) { isRunning = false; } } glClear(GL_COLOR_BUFFER_BIT); if (isImageOneLoaded && isImageTwoLoaded) { glActiveTexture(GL_TEXTURE0); glBindTexture(GL_TEXTURE_2D, texture[0]); glActiveTexture(GL_TEXTURE1); glBindTexture(GL_TEXTURE_2D, texture[1]); glUseProgram(shaderProgram); } glBindVertexArray(vao); glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, nullptr); glBindVertexArray(0); window.display(); } glDeleteVertexArrays(1, &vao); glDeleteBuffers(1, &vbo); glDeleteBuffers(1, &ebo); glDeleteProgram(shaderProgram); glDeleteTextures(2,texture); return 0; } and this is the vertex shader
      #version 450 core layout(location=0) in vec3 inPos; layout(location=1) in vec2 inTexCoord; out vec2 TexCoord; void main() { gl_Position=vec4(inPos,1.0); TexCoord=inTexCoord; } and the fragment shader
      #version 450 core in vec2 TexCoord; uniform sampler2D inTextureOne; uniform sampler2D inTextureTwo; out vec4 FragmentColor; void main() { FragmentColor=mix(texture(inTextureOne,TexCoord),texture(inTextureTwo,TexCoord),0.2); } I was expecting awesomeface.png on top of container.jpg

    • By khawk
      We've just released all of the source code for the NeHe OpenGL lessons on our Github page at https://github.com/gamedev-net/nehe-opengl. code - 43 total platforms, configurations, and languages are included.
      Now operated by GameDev.net, NeHe is located at http://nehe.gamedev.net where it has been a valuable resource for developers wanting to learn OpenGL and graphics programming.

      View full story
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