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OpenGL Need help with OpenGL rendering thread, C++ and win32 API

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I've had some success starting out with OpenGL using the basic code in NeHe's tutorials. ( ) This all works fine when rendering frame by frame, but I've now got to the stage where I would like to have two threads running, one to render, and the other to perform some real time analysis of data from a MIDI keyboard. My first attempt uses the _beginthread() function to start off a thread, the thread function contains an infinite loop with a call to the DrawGLScene() method which was working fine in the single thread case. All this does is put a blank window on the screen, and put my processor use to 100%. I didn't think it would be that simple heh. Is my rendering thread blocking other threads in the OS from actually updating the screen? How would I go about solving this issue? (I can tell the thread is actually working - if I put a OutputDebugStr() in the loop, I see it over and over in the output window in VisStudio) I am a beginner with C++, although I am very familiar with Java and OO in general. I am a beginner with the windows API also. I'm also trying to avoid using the MFC. Once this problem is solved there will be no more windows code to write, the rest will be analysis and openGL.

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Your problem probably comes from not having the opengl context active in the right thread.

I'm assuming you create your window and context at start up and then spawn a different thread to draw to it? This aint gonna work. Instead you need to release it from the orignal context (i forget how I admit, but its an important step, look at some window destruction code to get an idea of how) and then set it as current in the thread you want todo the drawing.

And having your threads yeild when they are done processing is a good idea [smile]

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Thanks - I'm rather clueless, I'm used to the soft fuzzy world of JAVA where you create a thread and it goes....

Is it possible to set up the rendering context when the thread gets created instead of doing it when creating the window? There's only ever going to be one rendering thread.

How do I set a context as current in a thread?

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Don't try to move contexts between threads, you'll run into all sorts of problems, at least when your app is running on windows. You should do all your opengl stuff in a single thread, however it sounds like that is what you want anyway.

Creating a rendering context is a seperate operation from creating a window, however whatever library you are using to set up your context may do both at the same time. Using the wgl commands in windows you would do something like:

hwnd = CreateWindow(...)
hdc = GetDC(hwnd)
format = ChoosePixelFormat(hdc, ...)
SetPixelFormat(hdc, format, ...)
hglrc = wglCreateContext(hdc)
wglMakeCurrent(hdc, hglrc)

That's how you do it in C/C++ using windows platform sdk to create the window and wgl to set up the context. I haven't done ogl with java before but I'm sure all that is happening under the hood somewhere, at least on a windows implementation. The problem is that java may do all that for you in one step so you can't jump in the middle with your seperate thread. You'll need to dig into the docs a bit and find out.

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Argh! Well I've made some progress - I can get some graphics back on the screen, and they render like they did before I tried adding threads, but I can't seem to find out how to get a win32 thread to yield as was suggested. Whenever I try and move the window/resize or do anything other than watch the graphics, the whole thing hangs.
Heres a rough outline of what is where in my code, it's a total mess cos I don't know what I 'm doing:

GLvoid ResizeGLScene(GLsizei width, GLsizei height) { NeHe's window resizing code }

GLvoid KillGLWindow() { NeHe's window killing code }

BOOL CreateGLWindow(params) { NeHe's window creatin code }

void ThreadFunc(void* m) {
CreateGLWindow("MidiLights",640,480,16,false); //Window created
(including Render context and device context) inside thread function as you suggested.
for(;;) { rend->DrawGLScene(); SwapBuffers(hDC); } //

int WINAPI WinMain(params) {
rend = new CRenderer;
MSG msg;
BOOL done = FALSE;

_beginthread(ThreadFunc,0,null); //Thread created here

while(!done) { message loop }


LRESULT CALLBACK WndProc(params) { message handlers }

The CRenderer class just holds a few methods full of OpenGL calls, including the main rendering routine, DrawGLScene(). This is so I can write several different rendering classes, that hopefully share all the windows boilerplate and threading code. (I'll only ever be using one at a time - I'm not trying to get two threads rendering at once, just one rendering with another thread doing some other processing).

Hopefully the above outline will at first make you scream in horror, but then make it obvious where i've messed it up...

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shove a Sleep(0); after the call to swap buffers and in the while loop at the bottom.

I dont know if there is another solution for win32 threads, I use the boost::thread system for my threading which comes with a boost::yeild() function [grin]

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Original post by 0beron

while(!done) { message loop }

Does this message loop use GetMessage() or PeekMessage()? I agree with Phantom that it sounds like one thread is starving the other. You'll want to use GetMessage() here to get the message loop to yeild control. You'll also want to have the rendering thread yield (with sleep(0)) so that the message loop can handle events. What exactly do you mean by hang? Does the window stop responding to inputs and give you a "not responding" error? Or does it stop rendering into the window? Or both?

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Guest Anonymous Poster
I'll make it REALLY easy on you.

That's basically everything I use for my GL test apps. It's far from perfect, and generally a mess. Most of the stuff isn't even used anymore, but it'll prove that you *CAN* split the rendering from the processing and window events. Just don't try to replicate it using SDL, because it wont work.

You'll notice there's very few comments in there, and the ones that are there are generally me complaining about something else. Ignore them. The doLookAt(...) function doesn't work quite right (gimbal lock). Fix it if you need to, or use doLookFromXY(...) function.

I don't think there's any major problems with it. If any of you spot any problems with it, I'd be more than happy to know about them. Just don't ask me how to use it. If you want to take the time to rewrite a few lines in Basic_GL_App.h, you can reintroduce unicode to it (just change the class name and window names to wchar_t* types, and make sure UNICODE is defined).

Tear out everything useful, stick that in a class, and use it as your template. Just make sure you find ways to block the rendering and updating threads if they're both fiddling with objects, or you could find your apps crashing. ;)

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@solias, yes it uses PeekMessage(), will change that. When I say it hangs, I mean both - the window goes white, including the menu bar, and it stops responding altogether.

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Multi-threading is always tricky to debug. Having both threads yield is a good first step. Are you using windows xp by any chance?

From MSDN on GetMessage():

Windows XP: If a top-level window stops responding to messages for more than several seconds, the system considers the window to be hung and replaces it with a ghost window that has the same z-order, location, size, and visual attributes. This allows the user to move it, resize it, or even close the application. However, these are the only actions available because the application is actually hung. When in the debugger mode, the system does not generate a ghost window.

If both the rendering thread and the message thread are hanging you might be getting into a deadlock situation somehow. Look for any dependencies between the threads.

You might also want to make sure that the message loop isn't doing anything to affect the client area of the window. Make sure the window is created with a NULL background brush and you don't do anything in response to WM_PAINT.

Personally I'm not really sure that having the window message handeling code in a seperate thread from the rendering code is all that useful. The main advantage is that you can prevent the window from hanging if your rendering loop takes too long, however if you hit the message queue every frame or so your game would be at unplayable frame rates long before you got in trouble with windows for missing messages. The main place I would see this being an issue is when doing resource loading. When I do a level load I'm careful to hit the message loop fairly often (this is a good time to update the progress bar too).

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well, to me it makes sense, as it completely removes your windowing system from the scope of the game, with only a couple of functions needed to perform actions such as pausing/resuming and quiting, which means your game and its logic is free to carry on as it wants, free from any platform dependancies, like ye olde dos games [grin]
and from a cross platform stand point, once you've got the little... quirks.. removed your code is nicely parceled away and safe from any windowing changes [grin]

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Yes - I run XP

Ok - after a bit more fiddling, it is definitely a starvation problem. Putting one OutputDebugStr() in the top of the message loop, and another in the render routine shows that the render routine is the only one getting run.

I'm only explicitly creating one thread at the moment - the one to do rendering. Does the message loop need a thread to run in or does the OS provide one?

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When you run a program it techinically IS a thread, so when you create the 2nd thread to render you'll have the first one (main one) which handles the message pump and the 2nd one handles the drawing.

You need to make the render routine yeild so that the message pump gets a chance to run, as I said, sleep(0) or sleep(1) should do it, if it doesnt you'll have to see how to make it yeild via the system you are using.

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Here's an interesting sample from Microsoft. Of course, it doesn't help that much on Linux, OSX, etc. :-) I've got multiple rendering threads in my new engine, up and running on Windows and Linux+X11, and using SDL to create the windows no less, but I tell you, if I never have to code against the X11 API again it'll be too soon. Anyway, it can be done pretty easily on Windows, as that sample demonstrates. The only problem is all the normal synchronization issues that come with multithreaded programs.

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Aha - I've got somewhere. I switched back to using Peek Message, and put a timer on the render routine to cap the frame rate. Now the threads interleave with one another - I can see that from the Output window in Vis studio.
Windows no longer complains that the application is not responding, but you can't move the window or use its menus. If the window is covered by another and then uncovered, the window frame never redraws itself. Is there another windows message that I need to handle or something?

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not as far as I know, my own window framework only handles 3 msgs (the create and the two that go with destory) and while its only single threaded it works perfectly.

I've recently rigged it for multithread however, so I'm going to test that probably later today and see what issues I run into.

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OK, I've just knocked up a quick multithread OpenGL Program via my window framework anmd using boost::thread for my threading work, and it works perfectly [grin], I'm not doing any special handling of messages, infact I'm only dealing with a create message and the message which are needed to destory it, everything is passing to the default handler.

Its two loops, one infinite which does the drawing (with a yeild at the end) and another which works the message pump via Peekmessage, also with a yeild in it.

For the reference of heeen, this is on an ATI 9800xt using the Cat4.12 drivers and this code was totally untested in a multithread enviroment before now [grin]

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would you be so kind and post a binary or, even better, the source code so we can test it on our systems? so far all of my recent attempts failed. maybe theres some other tidbit like service packs or whatever that we're missing.

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The above link contains two exes, SimpleTest is a single thread version, just to check things work fine on your system [wink]
Multithread Test is the same program but rejiggled for multithreaded windowing.

I do plan on releasing the source etc soon, I'm just considering the interface to my windowing framework before making the first beta release, I'll post here when I do it, hopefully before new year [grin]

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ok, i just thought, what the heck is it with that yield function... so i read it temporarily suspends the current thread in favor for some other thread, and guess what? it just worked! simply yield the render thread somewhere in its main loop and it works!

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Cracked it! - I took the advice of Solias and in a pm from heen and recombined the rendering with the main window message loop, leaving the thread out on it's own. I intend to use it to do analysis on MIDI data, which then drives a visualisation. The data's asynchronous, and I want to call a 'beat' method at particular times, and that's what the thread will do.
I've got a program now that does exactly what it did before I added threads, but now it flashes another bunch of polygons on the screen every second thanks to a counter in the thread function. Thanks everyone for your help!

I've now got the problem of giving the thread it's own message loop so I can notify it of incoming MIDI messages. I've put up another post on this, but since it's not really openGL related I put it here:

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      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.
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