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OpenGL Irregular Framerates

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Hello out there, I am experiencing quite a disturbing effect in my opengl application. Every now and then (usually at around every say 20 frames), my frame time doubles for one frame and the next frame finishes at around 1 ms. As all my animation is time based, the motion appears quite jerky, though the app is running at a moderate frame rate (~ 30 fps @ ATI 9600) In plain numbers, this is an excerpt from what I log : ... Sun Oct 08 17:28:46 2006 DEBUG Update Time 0.236902 Sun Oct 08 17:28:46 2006 DEBUG Render Time 40.363510 Sun Oct 08 17:28:46 2006 DEBUG Update Time 0.301156 Sun Oct 08 17:28:46 2006 DEBUG Render Time 45.085339 Sun Oct 08 17:28:46 2006 DEBUG Update Time 0.304229 Sun Oct 08 17:28:46 2006 DEBUG Render Time 48.914038 Sun Oct 08 17:28:46 2006 DEBUG Update Time 0.300597 Sun Oct 08 17:28:46 2006 DEBUG Render Time 89.839630 // <-- DOUBLED ! Sun Oct 08 17:28:46 2006 DEBUG Update Time 0.343619 Sun Oct 08 17:28:46 2006 DEBUG Render Time 1.100978 // BLAZING FAST ALL OF A SUDDEN ! Sun Oct 08 17:28:46 2006 DEBUG Update Time 0.240813 Sun Oct 08 17:28:46 2006 DEBUG Render Time 43.929606 ... Could this be in conjunction with the vertical sync ? I am using the wglSwapIntervalEXT extension though... Has anybody an idea where to start looking or what I could do about it? I might well interpolate between the update times, but that seems more like a fix then a real solution. Thanks in advance for any suggestions, Christian [Edited by - Novative on October 8, 2006 4:12:41 PM]

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The obvious suggestion would be ask your profiler where all the time is going.

Alternatively, it could be that the time is spent outside the game. Yes, that happens, if other processes need the CPU. (an extra 40 ms (if it is milliseconds you're measuring) sounds a bit excessive for that though)

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Thanks for the suggestions :) Unfortunately it does not seem to be the code as such that gives rise to this problem, instead what really stuns me, is that the following frame completes in about 1 ms, though it runs through all the same code that averagely needs 40 ms to complete otherwise. How can this be possible ?

The effect is repeatable (with varying time between the peaks) so I am tempted to rule out the influence of other processes...and am left rather puzzled about this rather annoying behaviour.

Thanks anyway :)

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Unfortunately not, this is why this problem troubles me so much. I can tell by the jerky movements that the problem still persists when I have the logging turned off.

Maybe I try a different machine any time soon, just to rule out some weird influences.

Is there any logical explanation why my code can finish rendering in 1 ms ? It seems as if two frames get batched up (thus taking roughly twice the time) and the opengl calls of the succeeding frame are ignored/delayed (thus the heavy time saving). But this is beyond my knowledge of opengl (windows?) synchronization.

P.S: The code is not multi-threaded, its a plain update/renderloop in double buffer mode under WinXP.

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In addition to posting the code, maybe upload a stripped down version of the app. (simple enough to duplicate the issue) for others to try.

If the problem is happening across multiple computers, focus on your code. If other computers are not affected, then the issue would seem to be with another process running in the background sucking up your resources (spyware?).

Also be sure to check for any Sleep(rand()%100); calls that might be mysteriously placed throughout your code [grin].

- Dan

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I had this problem once too, and i found that i had memory leaks. The framerate would go very low for a millisecond every few seconds. It was something to do with how the OS (mac osX in my case) was handling the page files; run your program, look in task manager and note down the amount of memory its using (virtual and physical) and come back an hour later and look.
My application was leaking one struct of about 24 bytes per frame, and managed to use up all 768mb of my physical memory within four hours of execution. look out for memory leaks when using malloc()!

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[edit]seeing as your probelm is to do with the time rendering is taking, my suggestions are probably wrong. However, how are you timing how long it takes to render?[/edit]

I had a simmilar problem when I was using the performance timer incorrectly.
My guess is you're losing precision in your timer code, which could have the effect of returning times that are shorter than, or longer than the actual elapsed time for one frame.
eg. once I corrected my timer code and compared it with the old version I found results like this (nb. values are from memory and are illustrative):
actual | returned
33ms | 16ms
31ms | 30ms
31ms | 30ms
31ms | 80ms
32ms | 16ms
36ms | 60ms

This caused the game to animate very jerky, appearing smooth for a few frames, then jumping, then stalling, the going smooth again...

A "quick fix" is to average the timer for the last few frames, but thats a real hack... Perhaps you could post your timer code?

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Thanks for all the valuable input :)

I checked my timer implementation (thanks @PhilMorton for the hint :)) against simple GetTickCount() difference(am using QueryPerformanceCounter otherwise) around my render-code.

The timings differ within their natural resolution, but the overall picture is the same (with the second value being the GetTickCount difference) :

Mon Oct 09 07:34:21 2006 DEBUG Render Time 34.136461 / 31
Mon Oct 09 07:34:21 2006 DEBUG Render Time 74.885775 / 78
Mon Oct 09 07:34:21 2006 DEBUG Render Time 0.895365 / 0 // !!!
Mon Oct 09 07:34:21 2006 DEBUG Render Time 34.011306 / 47
Mon Oct 09 07:34:21 2006 DEBUG Render Time 75.542283 / 63
Mon Oct 09 07:34:21 2006 DEBUG Render Time 0.910730 / 0 // !!!!

I also checked for memory leaks (thanks @ speciesUnknown) , but at least there I seem to have done my homework ;)

Tried it on my laptop just right now, its the same. I put together a little binary package that should contain all that is needed to run my little sample application. Please note however that the source is not thoroughly tested and that I do not assume that it will run on any hardware (unfortunately for now FBO and VBO support is hardcoded ...)

I would be really grateful if someone might take a look at it.

All you need to do is start the program, let it run for a few seconds (use arrow keys for basic orbiting around if you feel that the vast sample scene is a little too boring to look at;)) and then check the log file. I added a little marker, that shows up at the end of the line whenever the actual render time is 1.5 higher than the previous. This falsely marks a few spots where rendering time is higher due to changed viewing distance, but most of the time it hits the spots I am looking for.

Thanks for the support and in advance for everyone that risks having a look. here is the link to the binary :

Sample App [1.2 MB]

Please report, if any files are missing.

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Works fine for me, render time is about 1ms, update time 0.005.
on Athlon64 XP 3500+, Geforce 6600GT, 768Mb RAM.

ch.

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Thanks a lot for trying :) You're right, the scene does not impose much load onto either GPU nor CPU, but I didnt want to waste network bandwidth with a huge scene to download.

I had a look at the logfile and though not as clear as my local values (i should be thankful for my slow computer I guess ;)) the trend is still there.

I finegrained my time measurements more and arrived at the conclusion that its all about the swapping time. It varies from ~1 ms to ~10 ms and thus turning my time based animation into sth. rather useless.

Does anyone have some recipe for keeping the swap times in a moderate range ? Or can give a detailed explanation of what is going wrong ? I assume its something about the vertical sync, but the effect should then be constant, shouldnt it ?


I wonder then how to setup a proper time based animation scheme that can handle this type of varying update times (as a result of my varying frame times).
But maybe I start a new post for this.


Thanks to everyone, I really appreciate your help !


Excerpts from recent loggings :

normal run :

Mon Oct 09 13:16:17 2006 DEBUG PreFrameTime 0.051403
Mon Oct 09 13:16:17 2006 DEBUG InFrameTime 0.830552
Mon Oct 09 13:16:17 2006 DEBUG Swap time: 0.421841
Mon Oct 09 13:16:17 2006 DEBUG Render Time 2.564851 / 0

a) 90% of all cases are caused by a buffer swap time which peeks every now and then and takes up 5-20 times longer than usually, suggesting that I run into some kind of vertical sync-miss. (somebody please correct me, if this is a wrong conclusion)

Mon Oct 09 13:16:17 2006 DEBUG PreFrameTime 0.049727
Mon Oct 09 13:16:17 2006 DEBUG InFrameTime 0.821333
Mon Oct 09 13:16:17 2006 DEBUG Swap time: 10.032839
Mon Oct 09 13:16:17 2006 DEBUG Render Time 12.226694 / 16

b) the rest is caused by some misterious and yet to be resolved time loss, suggesting that I am maybe scheduled away by the OS.

NOTE : Swap is the last thing I do before I return from the render function and measure the render time.

Mon Oct 09 13:16:17 2006 DEBUG PreFrameTime 0.050286
Mon Oct 09 13:16:17 2006 DEBUG InFrameTime 0.825524
Mon Oct 09 13:16:17 2006 DEBUG Swap time: 0.460952
Mon Oct 09 13:16:17 2006 DEBUG Render Time 8.515887 / 15

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disable vsync + it should be much smoother
i expect this is cause youre rendering close to the buffer refresh

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So are you using the vsync to time your app? In so that you expect that each frame is N milliseconds long and advance by that each frame? Or are you taking the time-delta between last and current frame and use that for animation?

The latter approach should work with any framerates, no matter how variable they are.

And there's nothing wrong in interpolating the times. You could store last 4 or more states (just positions) and interpolate between them, based on how much time has gone between them, rather than discreetly updating each frame. This is actually a way you can improve physics modelling anyway, so in a way it is a "correct" solution.

ch.

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No, I am not using the vsync to time my app. I simply keep it going and take the delta time between the last update and the current to advance my scene.

Unfortunately the varying frame rates cause the motion to appear jerky every once in a while, because the objects would "jump" a little, when the last frame suddenly took twice as long as the one before (as the delta time would be much higher and thus the way objects move). This kills any smooth movements.

This is no problem if the framerates are high enough (as the delta times are rather small and even doubling them makes not much difference positionwise), but when I am around 30fps it gets noticable dependend on the speed of the object.

Maybe I should really go and interpolate update times to smooth out the sudden peaks. The result of interpolating is an incorrect state though (as objects at time t are not where they are supposed to be, but rather at some time t' =! t), which might annihilate, when t varies around t'). But this is just a theoretical thought.

Have you ever applied this in practice ?

Thanks for the hint :)

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Quote:
Original post by Novative
Have you ever applied this in practice ?


No, I only read about it, and even those were using it for physics modeling. You might wanna look up midpoint-method or runge-kutta if you're out for specific algorithms, even though the midpoint is the brute force method that you'd think first too.

ch.

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      // 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 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
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