• Advertisement
Sign in to follow this  

OpenGL Hybrid vsync

This topic is 2880 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

From http://www.opengl.org/discussion_boards/ubbthreads.php?ubb=showflat&Number=196709&page=3
Quote:
Just for the record - for my personal, immediate purposes, the following setup seems to work fine :)
if( dt>24) {
  if(vsync) {
    wglSwapIntervalEXT(0);
    vsync=false;
  }
}
else if( dt<12){
  if(!vsync) {
    wglSwapIntervalEXT(1);
    vsync=true;
  }
}
SwapBuffers();
dt being the time between the start of the last render, and the start of the current - ie. one frame lag.
Is this still helpful if triple-buffering is enabled?

Share this post


Link to post
Share on other sites
Advertisement
Quote:
Original post by Prune
Is this still helpful if triple-buffering is enabled?


Define "helpful". In your application when you try it do you have better visual results? If so, yes, if not, no.

-me

Share this post


Link to post
Share on other sites
What is that code supposed to?(Fixing frame rate?) And why do you think it works? Depending on the driver settings wglSwapIntervalEXT may or may not affect v-sync.
Also as end used I would not happy to see an application messing up with v-sync like that since I often want it to be enable to overcome screen tearing.

Share this post


Link to post
Share on other sites
I don't think that snippet is any helpful at all, and has never been. At least, not without a lot of "ifs" and "whens".

The problem with VSYNC is that it will only allow refresh/N frames per second (so, in the case of a 60Hz refresh, it will allow for 60, 30, 20, 15, 12, 8, ... fps). On the positive side, it avoids tearing artefacts, and for many games 1/2 the refresh rate is still quite acceptable.

Now, it may happen that your rendering is a bit too slow, maybe running at just 59 fps, which would be acceptable, but due to VSYNC it will drop to 30 fps which is not acceptable (maybe in a fast-paced ego-shooter).

This code snippet tries to deal with the problem by collecting some more or less reliable data (last frame time), compare it to some hardcoded value and turn VSYNC on/off on the fly.

Here's is the first flaw already. Not only is 24 ms a really weird frame time, this corresponds to something like 42 fps -- few if any devices will have a refresh rate like this -- but also you don't really know the real refresh rate, so hardcoding it is a kind of stupid idea. Typical monitors can have refresh rates going from 50 to 120 Hz with maybe a dozen steps in between. So... 24 may be good for one particular monitor, but that's about it.
Also, different games have different fps needs, so regardless of the actual hardware, this number may not be the one you want. For a top-down strategy game, 15 fps may be perfectly acceptable. For a fast ego-shooter, 60 fps may not be enough.
Plus, measuring the frame time in a reliable and precise way is not quite trivial, either. And, whatever you measure is (obviously) after the fact. It may be better to avoid the problem in the first place.

I think the right solution here is to let the user choose. Usually I hate it when people just play the "let the user choose" card, but in this case, I think it is really justified.
Not only is this much less work and trouble, but it also is a lot more reliable. And, it accounts for the fact that different people are... well... different. Some people maybe don't mind a bit of tearing but really hate low frame rates. And then, tearing might annoy the hell out of others. You don't know.
Lastly, you don't even know if you can change VSYNC at all. Most drivers allow the user to turn VSYNC on/off globally, so whatever you do might not be good for anything after all.

Share this post


Link to post
Share on other sites
Quote:
Just for the record - for my personal, immediate purposes, the following setup seems to work fine :)

if( dt>24) {
if(vsync) {
wglSwapIntervalEXT(0);
vsync=false;
}
}
else if( dt<12){
if(!vsync) {
wglSwapIntervalEXT(1);
vsync=true;
}
}
SwapBuffers();


dt being the time between the start of the last render, and the start of the current - ie. one frame lag.
That is very interesting. I had the same question with no answers a few months ago, and I came out with almost the same idea, but for some reason, I could never get it to work. V-sync never got switched on, or never got switched off, no matter what values I used it the else-if conditions (I also had the two different values to introduce some kind of hysteresis), just didn't work.
So I decided to go the user selection way, and the user can switch to lower resolution, decide how many particles are drawn etc.

Share this post


Link to post
Share on other sites
Quote:
Original post by samoth
This code snippet tries to deal with the problem by collecting some more or less reliable data (last frame time), compare it to some hardcoded value and turn VSYNC on/off on the fly.

Of course, it is trivial to extend to some statistic measure relying on a few past frames, rather than a single frame.

Quote:
Here's is the first flaw already. Not only is 24 ms a really weird frame time, this corresponds to something like 42 fps -- few if any devices will have a refresh rate like this -- but also you don't really know the real refresh rate, so hardcoding it is a kind of stupid idea.

I am myself very curious about this choice. As I wrote in the first post, I copied this snipped from elsewhere. The following comment is one of the replies on that forum:
Quote:
Mikkel Gjoel: you are right, you snippet works very well !
This is really nice to the eye, even with black+white vertical stripes in variable high-speed horizontal scroll, my worst case as far as display refresh is concerned.
I don't get why your first threshold value works so well, but I could not get better results with a smaller value. However 85 Hz monitor would mean around 11-12 milliseconds, not 24 right ?

There was no explanation in reply though; I myself posted in the thread but got no answer of any sort.
I cannot begin to imagine the reason for the choice, whether latencies for wglSwapIntervalEXT's effect come into play, or what... I'd love to hear some suggestions. Perhaps the choice is simply empirical, but given that a different random user also found this the smallest value that made a difference, it's unlikely that the writer of the snipped had some unusual refresh rate.

Quote:
And, whatever you measure is (obviously) after the fact.

That's hardly specific to this issue. Consider a physics simulation where you're determining the delta-t. There is at least a one-frame lag, some smoothness of the dt variation over most frames is always assumed since you cannot predict discontinuous events that would cause a sudden change (since they usually result from user action or arise from system complexity). This is a general aspect of any feedback-based controller system, where the variable is not necessarily just time but could be temperature or whatever.

Quote:
I think the right solution here is to let the user choose. Usually I hate it when people just play the "let the user choose" card, but in this case, I think it is really justified.
Not only is this much less work and trouble, but it also is a lot more reliable. And, it accounts for the fact that different people are... well... different. Some people maybe don't mind a bit of tearing but really hate low frame rates. And then, tearing might annoy the hell out of others. You don't know.
Lastly, you don't even know if you can change VSYNC at all. Most drivers allow the user to turn VSYNC on/off globally, so whatever you do might not be good for anything after all.

I'm lucky to be writing for a hardware and software platform that I specify (touchscreen kiosks), which removes these concerns. But even someone without that privilege can provide the user with such an option in addition to having vsync always on or always off, maybe with even tunable threshold dt. (By the way, the NVIDIA driver lets one to force vsync on and off, but also has a setting allowing the application to select it.) So I would really like to get a bit deeper into this issue in order to get best results.

This algorithm is, as you explained, geared to prevents halving of framerate for cases where some portion of frames might exceed one refresh time. The problem of course is that those frames would appear with tearing, so it's a specific compromise between lag to next frame display and visual artifacts.
This got me into thinking about the other solution, triple buffering, which presents a different compromise--adding a lag time of an extra frame to act as a safety margin for the cases where some frames take longer than a vertical retrace. But wouldn't it make sense, in cases where the draw takes less than half of a vertical refresh, to dynamically switch to double-buffering and thus improve latency? This would be somewhat analogous to the dynamically using vsync in the snippet above, and could be combined. But how can one turn triple buffering on/off from the application, given there seems to be no WGL/GLX extension as there is for vsync? Does NVIDIA have any way to ask the driver for it programmatically? Worst case, I'd imagine one could manually handle two back buffers in the application and turn triple-buffering off in the driver...

Then there is the question of how NVIDIA's driver setting of "Maximum pre-rendered frames" affects the above considerations...

Share this post


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

  • Advertisement
  • Advertisement
  • Popular Now

  • Advertisement
  • Similar Content

    • 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?
    • By xhcao
      Does sync be needed to read texture content after access texture image in compute shader?
      My simple code is as below,
      glUseProgram(program.get());
      glBindImageTexture(0, texture[0], 0, GL_FALSE, 3, GL_READ_ONLY, GL_R32UI);
      glBindImageTexture(1, texture[1], 0, GL_FALSE, 4, GL_WRITE_ONLY, GL_R32UI);
      glDispatchCompute(1, 1, 1);
      // Does sync be needed here?
      glUseProgram(0);
      glBindFramebuffer(GL_READ_FRAMEBUFFER, framebuffer);
      glFramebufferTexture2D(GL_READ_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
                                     GL_TEXTURE_CUBE_MAP_POSITIVE_X + face, texture[1], 0);
      glReadPixels(0, 0, kWidth, kHeight, GL_RED_INTEGER, GL_UNSIGNED_INT, outputValues);
       
      Compute shader is very simple, imageLoad content from texture[0], and imageStore content to texture[1]. Does need to sync after dispatchCompute?
  • Advertisement