• Advertisement
Sign in to follow this  

OpenGL Valve OpenGL Tips and Tricks

This topic is 1524 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

Advertisement

Is the "Vertex Attribute Object" they mention as being slow in this video the same thing as Vertex Arrays (eg glGenVertexArrays)

and if so, what kinds of things would make them slower than rebinding the buffers/calling glVertexAttribPointer every draw?

Share this post


Link to post
Share on other sites

Is the "Vertex Attribute Object" they mention as being slow in this video the same thing as Vertex Arrays (eg glGenVertexArrays)

and if so, what kinds of things would make them slower than rebinding the buffers/calling glVertexAttribPointer every draw?

 

Yes, that's VAOs.

 

Really, all Valve have given us is their conclusions; we don't have their code, we don't have their test cases, we don't have their profiling data, and we don't know to what extent this was based on hardware vendor advice (it's interesting to note here that id Software don't use VAOs either - and in their case they have released their code so we can cross-check and confirm).

 

If you think about it, changing a VAO involves swapping out one huge chunk of state and swapping in another similarly huge chunk - the enabled arrays, their pointers and the buffers used for them.  It's easy enough to concieve of scenarios where not using VAOs may be more efficient - maybe you just wanted to change one pointer but keep everything else the same, or maybe you wanted to change the buffers (and remember that they're not using GL4.3 so no vertex attrib binding) but keep everything else, or maybe your GPU vendor just has a bad implementation of VAOs in their driver?  Again, without the missing information from Valve it's hard to draw conclusions - we don't really know what kind of vertex formats they're using, how often they're changing them, and whether their usage patterns are consistent and sensible, or borderline insane.

 

Without all of that the only conclusion we can validly draw is that Valve have found one case where using VAOs is slower (and they're not giving us the full information we need to test and/or support their findings), but that doesn't necessarily hold true for all cases.  Profile your own code, form your own conclusions that are appropriate for your own program, and use whichever approach gives you the best performance for your own use cases.

Share this post


Link to post
Share on other sites


it's interesting to note here that id Software don't use VAOs either

And neither does tri-Ace, and that’s because…

 


the only conclusion we can validly draw is that Valve have found one case where using VAOs is slower

…I’m responsible for performance inside of tri-Ace and I have yet to find a case at all in which a VAO is faster than manual switching, when manual switching is done properly.

 

It’s not like they found a few cases in which the performance was better for lack of VAO’s, but rather that their searches, id Software’s searches, and my own searches yielded no results in the pursuit of a better-case scenario for VAO’s.  VAO’s simply do not offer better performance than you can get on your own via your own redundant state-tracking, and as Valve mentioned that is likely never to change, since it requires a scope outside of the driver’s range.

 

I’m covering this in my upcoming book.

 

 

L. Spiro

Share this post


Link to post
Share on other sites
In theory something like a VAO SHOULD be faster because the driver can cache and validate upfront the various buffers bound and convert it to a sane format.

In reality not all your streams/buffers are going to be static AND bind-to-edit allowances mean they probably don't cache to such/any degree. (bind vao, bind new buffer, opps edited vao...).

The 4.4 extension 'GL_ARB_multi_bind' will probably end up being the fastest way of doing it as you can use one API call to set multiple streams at once which, assuming the api is sane, should let you set 'static' stream data and then bind in 'instance' data as needed afterwards.

As for everything else from Valve on this; given they are pushing a Linux/OpenGL based OS I'd take what they say with varying degrees of salt.

Share this post


Link to post
Share on other sites

Just to be clear on this, I've also benchmarked VAOs as being slower, even in the case where you create and bind a single VAO during startup then write the rest of your code as if VAOs didn't exist.  I stopped short of saying "VAOs are always slower" because I can guarantee that somebody, somewhere, right now undoubtedly has a case where they are actually faster.

 

I haven't benchmarked VAOs combined with GL4.3 vertex attrib binding but I suspect that this may be a faster path than pre-GL4.3 usage because it can involve just swapping out buffer specifications, leaving the rest of the vertex format intact.  Valve of course aren't using that because they must target pre-GL4.3 hardware.

 

Despite all of this I still use VAOs because I find them convenient for state management, the performance impact is not too high, and there are bigger bottlenecks in GL anyway (updating dynamic buffer objects, for example, although I'm hoping that GL4.4 buffer storage will resolve much of that).

Share this post


Link to post
Share on other sites

I'm surprised VAO should be slower because not only is it fewer API calls, and not only can the driver cache and validate updront the various buffers, but it can also cache the validation. This is admittedly less expensive than in the case of a FBO (which is why it's faster to switch 2 FBOs than to add/remove attachments to a single one), but still it necessarily means touching fewer objects spread out in memory, and thus fewer cache misses.

 

That said, it surprises me they're discouraging MapBuffer, too. In my experience, MapBufferRange is just about the same as CopyBufferSubData, with the difference that you can offload the copy to another thread. And if the GPU sync really bites you as they suggest, there's still MAP_UNSYNCHRONIZED_BIT which you can use as described by Hrabcak and Masserann in Cozzi/Riccio's book. That not only avoids synchronization and lets you offload the copy to another thread, but it also avoids having the driver perform memory allocation and reclamation work.

Surely the Valve guys would know about that technique?

Share this post


Link to post
Share on other sites

I'm surprised VAO should be slower because not only is it fewer API calls, and not only can the driver cache and validate updront the various buffers, but it can also cache the validation. This is admittedly less expensive than in the case of a FBO (which is why it's faster to switch 2 FBOs than to add/remove attachments to a single one), but still it necessarily means touching fewer objects spread out in memory, and thus fewer cache misses.

 

Depends on Valve's usage, to be honest.  E.g. a common enough scenario is to use the same vertex format and layout but to change the buffers; without GL_ARB_vertex_attrib_binding it's not possible to do this without respecifying the entire VAO, so there's not only no caching going on in this scenario, but also the extra overhead of VAO respecification and revalidation (at which point in time you may as well not be using VAOs at all).

 

I highly doubt that Valve are using GL_ARB_vertex_attrib_binding as many AMD cards, and all Intel cards, don't support it, and Valve's products must run on that hardware.

 

I'd also draw your attention to their earlier observation (in the same presentation) about GL being chatty but efficient, and not to judge a piece of code by number of calls.  It's easy enough to concieve of a single API call that does a lot more work than multiple calls, so it really depends on the amount of work that each API call has to do.  If - as I suspect - most vendors implement VAOs primarily as a user-mode software wrapper, with lazy state changes calling into kernel mode to flush changed VAO states to the hardware when a draw call is made, the API overhead of single call versus multiple calls should really be very minimal.

 

That said, it surprises me they're discouraging MapBuffer, too. In my experience, MapBufferRange is just about the same as CopyBufferSubData, with the difference that you can offload the copy to another thread. And if the GPU sync really bites you as they suggest, there's still MAP_UNSYNCHRONIZED_BIT which you can use as described by Hrabcak and Masserann in Cozzi/Riccio's book. That not only avoids synchronization and lets you offload the copy to another thread, but it also avoids having the driver perform memory allocation and reclamation work.

Surely the Valve guys would know about that technique?

 

Valve definitely know about this technique because it's the way D3D buffer updates work, so they've been using it in D3D for over 10 years now; it's very straightforward to port D3D discard/no-overwrite code to MapBufferRange (the API calls used match up very well) so they must have another reason for not using MapBufferRange.  Again, I'd suggest that this reason is because GL_ARB_map_buffer_range may not be available on all of their target hardware.  Raw MapBuffer (i.e. without "Range" ) has several problems so BufferSubData is definitely to be preferred over that in cases where MapBufferRange isn't available.

Share this post


Link to post
Share on other sites

I don't respecify VAO attributes in compatibility mode at all

I only do it once, and after that I just upload new data once it changes.. set and forget

 

Am I doing it wrong?

GL_ARB_vertex_attrib_binding is for opengl 4.2, which I can understand not many cards will support

 

But 3.x features are wildly, and widely supported.. I'm going to be liberally using 3.x features

Are VAOs still subpar? I don't see how

I guess since I'm using compatibility mode that I wouldn't know if I broke the 3.x rules easily

But the VAOs seriously never needs attrib updates after that first time

Share this post


Link to post
Share on other sites

I don't respecify VAO attributes in compatibility mode at all

I only do it once, and after that I just upload new data once it changes.. set and forget

 

Am I doing it wrong?

 

That sounds fine; I'd expect it to run as well as possible provided you don't change the buffer size during your "upload new data" step.  If you do change the buffer size, the driver would need to specify a new buffer behind the scenes, swap it out with the old, and wait until pending draw calls on the old have completed before freeing it, which could lead to VAO respecification (it would be driver internal behaviour and is not specified by OpenGL however).

 

GL_ARB_vertex_attrib_binding is for opengl 4.2, which I can understand not many cards will support

 

In theory the basic functionality behind vertex attrib binding should be able to work on any GL2.0+ hardware as it's a fairly clean duplicate of the way D3D9+ specifies vertex formats.  In practice the inclusion of instancing entry points, the L and I versions of glVertexAttribFormat, and the requirement for a non-zero VAO to be bound limit it's ability to work on downlevel hardware.  Vendors could however implement it as an extension for 3.0+ hardware if they wished, and there seems no good reason for why they have not done so.

Share this post


Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
  • Advertisement
  • Popular Tags

  • Advertisement
  • Popular Now

  • Similar Content

    • 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
    • By TheChubu
      The Khronos™ Group, an open consortium of leading hardware and software companies, announces from the SIGGRAPH 2017 Conference the immediate public availability of the OpenGL® 4.6 specification. OpenGL 4.6 integrates the functionality of numerous ARB and EXT extensions created by Khronos members AMD, Intel, and NVIDIA into core, including the capability to ingest SPIR-V™ shaders.
      SPIR-V is a Khronos-defined standard intermediate language for parallel compute and graphics, which enables content creators to simplify their shader authoring and management pipelines while providing significant source shading language flexibility. OpenGL 4.6 adds support for ingesting SPIR-V shaders to the core specification, guaranteeing that SPIR-V shaders will be widely supported by OpenGL implementations.
      OpenGL 4.6 adds the functionality of these ARB extensions to OpenGL’s core specification:
      GL_ARB_gl_spirv and GL_ARB_spirv_extensions to standardize SPIR-V support for OpenGL GL_ARB_indirect_parameters and GL_ARB_shader_draw_parameters for reducing the CPU overhead associated with rendering batches of geometry GL_ARB_pipeline_statistics_query and GL_ARB_transform_feedback_overflow_querystandardize OpenGL support for features available in Direct3D GL_ARB_texture_filter_anisotropic (based on GL_EXT_texture_filter_anisotropic) brings previously IP encumbered functionality into OpenGL to improve the visual quality of textured scenes GL_ARB_polygon_offset_clamp (based on GL_EXT_polygon_offset_clamp) suppresses a common visual artifact known as a “light leak” associated with rendering shadows GL_ARB_shader_atomic_counter_ops and GL_ARB_shader_group_vote add shader intrinsics supported by all desktop vendors to improve functionality and performance GL_KHR_no_error reduces driver overhead by allowing the application to indicate that it expects error-free operation so errors need not be generated In addition to the above features being added to OpenGL 4.6, the following are being released as extensions:
      GL_KHR_parallel_shader_compile allows applications to launch multiple shader compile threads to improve shader compile throughput WGL_ARB_create_context_no_error and GXL_ARB_create_context_no_error allow no error contexts to be created with WGL or GLX that support the GL_KHR_no_error extension “I’m proud to announce OpenGL 4.6 as the most feature-rich version of OpenGL yet. We've brought together the most popular, widely-supported extensions into a new core specification to give OpenGL developers and end users an improved baseline feature set. This includes resolving previous intellectual property roadblocks to bringing anisotropic texture filtering and polygon offset clamping into the core specification to enable widespread implementation and usage,” said Piers Daniell, chair of the OpenGL Working Group at Khronos. “The OpenGL working group will continue to respond to market needs and work with GPU vendors to ensure OpenGL remains a viable and evolving graphics API for all its customers and users across many vital industries.“
      The OpenGL 4.6 specification can be found at https://khronos.org/registry/OpenGL/index_gl.php. The GLSL to SPIR-V compiler glslang has been updated with GLSL 4.60 support, and can be found at https://github.com/KhronosGroup/glslang.
      Sophisticated graphics applications will also benefit from a set of newly released extensions for both OpenGL and OpenGL ES to enable interoperability with Vulkan and Direct3D. These extensions are named:
      GL_EXT_memory_object GL_EXT_memory_object_fd GL_EXT_memory_object_win32 GL_EXT_semaphore GL_EXT_semaphore_fd GL_EXT_semaphore_win32 GL_EXT_win32_keyed_mutex They can be found at: https://khronos.org/registry/OpenGL/index_gl.php
      Industry Support for OpenGL 4.6
      “With OpenGL 4.6 our customers have an improved set of core features available on our full range of OpenGL 4.x capable GPUs. These features provide improved rendering quality, performance and functionality. As the graphics industry’s most popular API, we fully support OpenGL and will continue to work closely with the Khronos Group on the development of new OpenGL specifications and extensions for our customers. NVIDIA has released beta OpenGL 4.6 drivers today at https://developer.nvidia.com/opengl-driver so developers can use these new features right away,” said Bob Pette, vice president, Professional Graphics at NVIDIA.
      "OpenGL 4.6 will be the first OpenGL release where conformant open source implementations based on the Mesa project will be deliverable in a reasonable timeframe after release. The open sourcing of the OpenGL conformance test suite and ongoing work between Khronos and X.org will also allow for non-vendor led open source implementations to achieve conformance in the near future," said David Airlie, senior principal engineer at Red Hat, and developer on Mesa/X.org projects.

      View full story
    • By _OskaR
      Hi,
      I have an OpenGL application but without possibility to wite own shaders.
      I need to perform small VS modification - is possible to do it in an alternative way? Do we have apps or driver modifictions which will catch the shader sent to GPU and override it?
  • Advertisement