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OpenGL Getting depth values

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I'm attempting to read out depth values for every screen coordinate in a 640*480 window at each frame. I know this has received attention here before, but the application is slightly unusual, and I would appreciate any advice on the best approach. I need to extract _only_ the matrix of orthogonal depth data that results from a particular viewpoint on the scene. Choices regarding lighting, or textures, or even whether the data is displayed to the screen at all, are _not_ requirements of the application. Strangely - I suppose - I'm using OpenGL despite not requiring any kind of visualisation of the results it gives. Currently I do display each frame to the screen because it allows me to read depth components across the entire window with glReadPixels(). I have heard this described as bad practice(?) and I am aware that there are also various performance issues related to the type of buffer you read into, its alignment and system hardware etc. Despite quite a bit of playing around with glReadPixels(), I am not able to achieve an acceptable level of performance. I'm aware that pixel buffer objects might give a performance improvement, but I'm not sure whether either of these approaches will offer the best solution? Reading old posts on the forum has made me aware of feedback mode. This seems a potentially better type of approach, as I don't require that the results be displayed to screen. I have no previous experience of this technique, but was considering using glfeedbackbuffer() with GL_3D as the feedback buffer type to try and recover the depth data for the entire window. Is this a valid use of the feedback mode, and is it likely to offer a performance increase over the, display to screen then read depths with glReadPixels() approach, described above? Thanks in advance for any help/comments. [For anyone interested in where the application requirements come from, it is an implementation of a particle filter http://en.wikipedia.org/wiki/Particle_filter I use OpenGL to draw an articulated 3D object, consisting of about 20 component parts, each with between 1 and 3 degrees of freedom. This object is viewed from a fixed point and is used for comparison with video image evidence (by this I mean a frame of real-world video from a video camera). I must probe around 1000 object configurations for depth data to compare with every individual frame of video evidence. With video evidence running at 30-60Hz, there are tens of thousands of configurations per second to be probed for depth data. Although the application need not run in real time, it must be manageable. At the moment, my glReadPixels() approach gives ~12fps which equates to over an hour to process 1 second of video evidence. As there is no need to visualise any of the output, only to grab the xyz data, I am hopeful that a performance gain is possible, but perhaps not].

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i dont think feedback is what u want ( i can see it being a lot slower )
i just ran a very old benchmark of mine on my nvidia gf7600gs
im getting ~150million pixels sec with glReadPixels( GL_DEPTH );
ie 640x480 > 400fps

have u looked into PBO (theres info + a demo on the nvidia developer site)

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zeds,

That's an interesting result. When I remove my glReadPixels() call I get ~70fps, when I add it in framerate drops to ~12fps.

-If you take out your call to glReadPixels() what kind of performance increase do you get on your benchmark framerate i.e. is it anything like my jump of about 5x, above?

-Could you tell me how you're calling glReadPixels()? How many depth values does your benchmark code read per call. My code is below, I'm trying to take all ~300,000 depth values in the window at once.

Here's how I make my call:
float *fmem = malloc(640*480*sizeof(float));
glReadPixels(0, 0, 640, 480, GL_DEPTH_COMPONENT, GL_FLOAT, fmem);

I think the PBO idea is a good one, but I want to make sure of some things before I move on from glReadPixels(). My fps results above are based on a 1000 frame long test, where the 1000 glReadPixels() calls add 70 seconds in total, versus a run where they aren't called. That looks like under 5 million pixels per second coming back to the app.

-Could I be suffering from the lack of a decent graphics card here? Or perhaps I'm making my call to glReadPixels() incorrectly?

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Still don't know what graphics card I have in this machine. But using GPUBench I get the following results for glReadPixels() (http://graphics.stanford.edu/projects/gpubench/test_readback.html has details). They don't read GL_DEPTH_COMPONENT but I was still interested to see them (window size for the test is 512*512 by default):

Fixed Hostmem GL_RGBA Mpix/sec: 46.54 MB/sec: 177.53
Fixed Hostmem GL_ABGR_EXT Mpix/sec: 1.48 MB/sec: 5.66
Fixed Hostmem GL_BGRA Mpix/sec: 46.23 MB/sec: 176.36
Float Hostmem GL_RGBA Mpix/sec: 12.55 MB/sec: 191.48
Float Hostmem GL_ABGR_EXT Mpix/sec: 0.47 MB/sec: 7.11
Float Hostmem GL_BGRA Mpix/sec: 12.47 MB/sec: 190.22

I've looked at the GPUBench source code, and made some very slight changes to my glReadPixels() calls to bring my code in line with theirs. My performance is pretty much unchanged, however.

I think I will attempt to give feedback mode a try before I move on. I'll post if I conclude anything other than what zeds predicted above.

Regarding PBOs, I'm concerned that all they will give me is the potential for a non-blocking call to read the depth info. As I don't have much work I can give the app to do in the meantime (before I actually try to use the depth data), I don't think I have much chance of a performance increase. Quote from Dominik Göddeke's tutorial below might be interesting to anyone else considering this approach

"Conventional transfers require a pipeline stall on the GPU to ensure that the data being read back is synchronous with the state of computations. PBO-accelerated transfers are NOT able to change this behaviour, they are only asynchronous on the CPU side. This behaviour cannot be changed at all due to the way the GPU pipeline works. This means in particular that PBO transfers from the GPU will not deliver any speedup with the application covered in this tutorial, they might even be slower than conventional ones. They are however asynchronous on the CPU: If an application can schedule enough work between initiating the transfer and actually using the data, true asynchronous transfers are possible and performance might be improved in case the data format allows this. ... To benefit from PBO acceleration, a lot of independent work needs to be scheduled between initiating the transfer and requesting the data".

Full tutorial available at http://www.mathematik.uni-dortmund.de/~goeddeke/gpgpu/tutorial3.html

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Quote:

Here's how I make my call:
float *fmem = malloc(640*480*sizeof(float));
glReadPixels(0, 0, 640, 480, GL_DEPTH_COMPONENT, GL_FLOAT, fmem);


i hope youre not doing that each frame ie declaring the memory.

my results are from an old benchmarking app i wrote many years ago (from memory even my gf2mx at the time did >10million pixs)

1000x readpixels of 640x480 GL_DEPTH_COMPONENT with GL_FLOAT should noway near take 70secs.

heres the output from my testing (as u can see depth values should be pretty close to color values)
thus if u have
Fixed Hostmem GL_RGBA Mpix/sec: 46.54 MB/sec: 177.53
u should be seeing something similar WRT depth (which youre not)
try removing everything except for the readpixels + see if thats truly the bottleneck


glReadPixels: DEPTH_COMPONENT -- UNSIGNED_BYTE 170.111 Mpixels/sec
glReadPixels: DEPTH_COMPONENT -- UNSIGNED_SHORT 170.111 Mpixels/sec
glReadPixels: DEPTH_COMPONENT -- FLOAT 145.572 Mpixels/sec
glReadPixels: DEPTH_COMPONENT -- UNSIGNED_INT 140.837 Mpixels/sec
glReadPixels: DEPTH_STENCIL_NV -- UNSIGNED_INT_24_8_NV 150.722 Mpixels/sec
---
glReadPixels: LUMINANCE -- UNSIGNED_BYTE 144.398 Mpixels/sec
glReadPixels: LUMINANCE -- UNSIGNED_SHORT 23.865 Mpixels/sec
glReadPixels: LUMINANCE -- UNSIGNED_INT 16.529 Mpixels/sec
glReadPixels: LUMINANCE -- FLOAT 25.871 Mpixels/sec
glReadPixels: ALPHA -- UNSIGNED_BYTE 186.673 Mpixels/sec
glReadPixels: ALPHA -- UNSIGNED_SHORT 184.746 Mpixels/sec
glReadPixels: ALPHA -- UNSIGNED_INT 184.746 Mpixels/sec
glReadPixels: ALPHA -- FLOAT 175.333 Mpixels/sec
glReadPixels: RED -- UNSIGNED_BYTE 171.744 Mpixels/sec
glReadPixels: RED -- UNSIGNED_SHORT 144.398 Mpixels/sec
glReadPixels: RED -- UNSIGNED_INT 119.305 Mpixels/sec
glReadPixels: RED -- FLOAT 150.722 Mpixels/sec
glReadPixels: RGB -- UNSIGNED_BYTE 141.954 Mpixels/sec
glReadPixels: BGR -- UNSIGNED_BYTE 163.580 Mpixels/sec
glReadPixels: RGBA -- UNSIGNED_BYTE 149.380 Mpixels/sec
glReadPixels: BGRA -- UNSIGNED_BYTE 165.191 Mpixels/sec
glReadPixels: RGB -- FLOAT 45.222 Mpixels/sec
glReadPixels: BGR -- FLOAT 46.668 Mpixels/sec
glReadPixels: RGB -- UNSIGNED_SHORT_5_6_5 154.718 Mpixels/sec
glReadPixels: RGB -- UNSIGNED_SHORT_5_6_5_REV 148.061 Mpixels/sec
glReadPixels: RGBA -- FLOAT 38.000 Mpixels/sec
glReadPixels: BGRA -- FLOAT 37.680 Mpixels/sec
glReadPixels: RGBA -- UNSIGNED_INT_8_8_8_8 166.834 Mpixels/sec
glReadPixels: BGRA -- UNSIGNED_INT_8_8_8_8 142.029 Mpixels/sec
glReadPixels: RGBA -- UNSIGNED_INT_8_8_8_8_REV 149.380 Mpixels/sec
glReadPixels: BGRA -- UNSIGNED_INT_8_8_8_8_REV 166.730 Mpixels/sec

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I'm not doing the malloc each frame. Sorry, that is misleading.
The good benchmark is what is leaving me so confused. I know you're right that it should be much faster. If I remove just the one glReadPixels() line _only_, then the 1000 frame run does indeed complete 70 seconds faster (about 15sec in total). There's something wrong here.

I found out yesterday that the card in this machine is an ATI EAX300SE 128Mb PCIe.

The only explanation I can come up with at the moment is an ATI driver problem for Linux. (Now I've said that it's bound to be me making a stupid coding mistake).

1) My benchmarks were indeed good, but they were run under windows.
2) I do all my OpenGL work in Debian Linux.
3) I have seen people mention ATI Linux driver problems on other forums, specifically mentioning glReadPixels() e.g. http://www.gpgpu.org/forums/viewtopic.php?t=3353&view=previous&sid=3f7fb23c04d396ca28cd5493ff624753

Don't know what the best next step is. I have an NVidia G-Force 6 Series 6600GT PCIe sitting on my desk but switching them over could be a problem as I don't own this machine. I've yet to look at whether any more recent ATI drivers are available.

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Found another PC running Debian Linux, very similar spec _but_ with an NVidia graphics card. I ran exactly the same code on both my PC (ATI card) and the alterative machine (NVidia card), results are below.

1000 frame test, duration:

ATI:
Window size 640*512: 3.32sec (readback on), 8sec (readback off)
Window size 214*512: 1.23sec (readback on)

NVidia:
Window size 640*512: 19sec (readback on), 4sec (readback off)
Window size 214*512: 10sec (readback on)

[The readback off cases aren't entirely fair as I also dropped a big array loop every frame, that I shouldn't have done. To give an idea, ATI would be 12sec with readback off and the array loop left in. So you could scale up the 4sec Nvidia result a little.]

But regardless of that, and the fact that I don't know what model the Nvidia card is - it appears faster in general rendering than the ATI... I'm sure that there is some problem with the ATI card's readback under Linux. See the jump up to 3 mins 32secs. An overhead of ~200 seconds. [I was wrong to quote an overhead of 70sec on readback for 1000*glReadPixels(0,0,640,480,...) in earlier posts. It was for 1000*glReadPixels(0,0,214,512,...).

Perhaps this could be helpful info if someone is struggling with slow glReadPixels() under Linux in the future.

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Is it possible to upload the video evidence to the GPU and do the comparison there instead? That would possibly yield an increase in speed.

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Jerax,
Yes, I think that's a nice idea. Looking at gpgpu.org the kind of techniques I'd need to employ for general purpose GPU computations look relatively tough (to me, at least) but I think you're right that it's the way to go for performance increases. I'll be testing the approach further using the readback technique for now, but if it's successful then I'll look again at this option.

Re. glReadPixels(), I've replaced my machine's ATI EAX300SE 128Mb PCIe with the NVidia G-Force 6 Series 6600GT 128Mb PCIe. The final result for my benchmark under Linux is now:

1000 frame test, duration:

NVidia 6600GT 128Mb PCIe:
Window size 640*512: 16sec (readback on)

This is manageable for my application.

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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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