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Why do I get sRGB framebuffer only when I set non-zero alpha size to SDL2?

I am trying to display a typical OpenGL color triangle in the correct gamut by following this guide and consulting the SDL2 documentation on how to enable SRGB support in the default framebuffer.

This is the code I wrote that draws a triangle:

#include <SDL.h> // Header file generated with glLoadGen #include "gl_core_3_3.h" #include <cstdlib> void sdl_loop(SDL_Window* window); static const char* const vertexSource = R"( #version 330 in vec2 position; in vec3 color; out vec3 vs_color; void main() { gl_Position = vec4(position, 0.0, 1.0); vs_color = color; } )"; static const char* const fragmentSource = R"( #version 330 in vec3 vs_color; out vec4 fragColor; void main() { fragColor = vec4(vs_color, 1.0); } )"; static const float vertices[] = { // XYRGB -0.9f, -0.9f, 1.0f, 0.0f, 0.0f, 0.9f, -0.9f, 0.0f, 1.0f, 0.0f, 0.0f, 0.9f, 0.0f, 0.0f, 1.0f, }; static const unsigned short indices[] = { 0, 1, 2 }; void sdl_loop(SDL_Window* window) { glClearColor(0.0f, 0.1f, 0.2f, 1.0f); glEnable(GL_FRAMEBUFFER_SRGB); auto program = glCreateProgram(); auto vertexShader = glCreateShader(GL_VERTEX_SHADER); auto fragmentShader = glCreateShader(GL_FRAGMENT_SHADER); glShaderSource(vertexShader, 1, &vertexSource, nullptr); glShaderSource(fragmentShader, 1, &fragmentSource, nullptr); glCompileShader(vertexShader); glCompileShader(fragmentShader); glAttachShader(program, vertexShader); glAttachShader(program, fragmentShader); glBindFragDataLocation(program, 0, "fragColor"); glLinkProgram(program); glDetachShader(program, vertexShader); glDetachShader(program, fragmentShader); glDeleteShader(vertexShader); glDeleteShader(fragmentShader); auto positionAttr = glGetAttribLocation(program, "position"); auto colorAttr = glGetAttribLocation(program, "color"); unsigned vao; glGenVertexArrays(1, &vao); glBindVertexArray(vao); unsigned vertexBuffer; glGenBuffers(1, &vertexBuffer); glBindBuffer(GL_ARRAY_BUFFER, vertexBuffer); glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW); unsigned indexBuffer; glGenBuffers(1, &indexBuffer); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indexBuffer); glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW); size_t stride = 5 * sizeof(float); size_t offset = 0; glEnableVertexAttribArray(positionAttr); glVertexAttribPointer(positionAttr, 2, GL_FLOAT, false, stride, (void*) offset); offset += 2 * sizeof(float); glEnableVertexAttribArray(colorAttr); glVertexAttribPointer(colorAttr, 3, GL_FLOAT, false, stride, (void*) offset); offset += 3 * sizeof(float); glUseProgram(program); SDL_Event evt; for (;;) { while (SDL_PollEvent(&evt)) { if (evt.type == SDL_QUIT) { goto out; } } glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT); glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_SHORT, nullptr); SDL_GL_SwapWindow(window); } out: glDeleteProgram(program); glDeleteBuffers(1, &vertexBuffer); glDeleteBuffers(1, &indexBuffer); glDeleteVertexArrays(1, &vao); } int main() { if (SDL_Init(SDL_INIT_EVERYTHING) < 0) { SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Couldn't initialize SDL: %s", SDL_GetError()); SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't initialize SDL", SDL_GetError(), nullptr); return EXIT_FAILURE; } SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3); SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 3); SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1); SDL_GL_SetAttribute(SDL_GL_MULTISAMPLEBUFFERS, 1); SDL_GL_SetAttribute(SDL_GL_MULTISAMPLESAMPLES, 8); SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8); SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8); SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8); // Commenting this line out doesn't give you an sRGB framebuffer on Intel GPUs SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 8); SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24); SDL_GL_SetAttribute(SDL_GL_STENCIL_SIZE, 8); SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE); SDL_GL_SetAttribute(SDL_GL_CONTEXT_FLAGS, SDL_GL_CONTEXT_DEBUG_FLAG); SDL_GL_SetAttribute(SDL_GL_FRAMEBUFFER_SRGB_CAPABLE, 1); auto* window = SDL_CreateWindow("Hello OpenGL 3!", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 800, 600, SDL_WINDOW_SHOWN | SDL_WINDOW_OPENGL); if (window == nullptr) { SDL_LogError( SDL_LOG_CATEGORY_APPLICATION, "Couldn't initialize SDL window: %s", SDL_GetError()); SDL_ShowSimpleMessageBox( SDL_MESSAGEBOX_ERROR, "Couldn't create SDL window", SDL_GetError(), nullptr); return EXIT_FAILURE; } auto context = SDL_GL_CreateContext(window); SDL_GL_MakeCurrent(window, context); SDL_GL_SetSwapInterval(1); ogl_LoadFunctions(); sdl_loop(window); SDL_GL_DeleteContext(context); SDL_DestroyWindow(window); SDL_Quit(); return EXIT_SUCCESS; }

I work on Ubuntu 16.04 (with HWE drivers), and the machine I use has both an Intel HD 4000 GPU and a special nvidia graphics processor with Optimus. Theoretically, I should get an sRGB framebuffer. However, I only get the sRGB framebuffer when running under the nvidia GPU or using ApiTrace. If I work under Intel GPU, I get only sRGB framebuffer if I specify non-zero alpha size with SDL2. If I specify a zero size for alpha (or not at all), I get a linear framebuffer instead.

Here's what my example looks like when working under the nvidia GPU (and Intel GPU with non-zero alpha-bit size): nvidia GPUs and Intel GPUs with non-zero alpha bit size

Here's what it looks like when working under Intel GPUs with a zero alpha-bit size (sRGB is incorrect): Intel GPU with zero alpha bit size

And this is what I get with ApiTrace: Apitrace

I also ran glxinfo -t , and from what I can say, I should get sRGB support even with zero alpha bits set ( EDIT : it is not. The output of glxinfo should display s es in the sRGB column, but for some reason the reason for this does not happen):

 40 GLX Visuals Vis Vis Visual Trans buff lev render DB ste rgbas aux dep ste accum buffer MS MS ID Depth Type parent size el type reo sz sz sz sz flt rgb buf th ncl rgba num bufs caveats -------------------------------------------------------------------------------------------------------------------- 0x 21 24 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 0 0 None 0x 22 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 0 0 None 0x b7 24 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 0 0 0 0 0 0 0 0 None 0x b8 24 TrueColor 0 32 0 rgba 0 0 8 8 8 8 . . 0 0 0 0 0 0 0 0 0 None 0x b9 24 TrueColor 0 32 0 rgba 0 0 8 8 8 8 . . 0 24 8 0 0 0 0 0 0 None 0x ba 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 0 0 0 0 0 0 0 0 None 0x bb 24 TrueColor 0 24 0 rgba 0 0 8 8 8 0 . . 0 0 0 0 0 0 0 0 0 None 0x bc 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 0 0 None 0x bd 24 TrueColor 0 24 0 rgba 0 0 8 8 8 0 . . 0 24 8 0 0 0 0 0 0 None 0x be 24 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 16 16 16 16 0 0 Slow 0x bf 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 0 0 None 0x c0 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 16 16 16 0 0 0 Slow 0x c1 24 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 0 0 0 0 0 0 4 1 None 0x c2 24 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 0 0 0 0 0 0 8 1 None 0x c3 24 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 4 1 None 0x c4 24 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 8 1 None 0x c5 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 0 0 0 0 0 0 4 1 None 0x c6 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 0 0 0 0 0 0 8 1 None 0x c7 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 4 1 None 0x c8 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 8 1 None 0x c9 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 0 0 0 0 0 0 0 0 None 0x ca 24 DirectColor 0 32 0 rgba 0 0 8 8 8 8 . . 0 0 0 0 0 0 0 0 0 None 0x cb 24 DirectColor 0 32 0 rgba 0 0 8 8 8 8 . . 0 24 8 0 0 0 0 0 0 None 0x cc 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 0 0 0 0 0 0 0 0 None 0x cd 24 DirectColor 0 24 0 rgba 0 0 8 8 8 0 . . 0 0 0 0 0 0 0 0 0 None 0x ce 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 0 0 None 0x cf 24 DirectColor 0 24 0 rgba 0 0 8 8 8 0 . . 0 24 8 0 0 0 0 0 0 None 0x d0 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 0 0 None 0x d1 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 16 16 16 16 0 0 Slow 0x d2 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 0 0 None 0x d3 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 16 16 16 0 0 0 Slow 0x d4 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 0 0 0 0 0 0 4 1 None 0x d5 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 0 0 0 0 0 0 8 1 None 0x d6 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 4 1 None 0x d7 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 8 1 None 0x d8 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 0 0 0 0 0 0 4 1 None 0x d9 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 0 0 0 0 0 0 8 1 None 0x da 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 4 1 None 0x db 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 8 1 None 0x 76 32 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 0 0 None 64 GLXFBConfigs: Vis Vis Visual Trans buff lev render DB ste rgbas aux dep ste accum buffer MS MS ID Depth Type parent size el type reo sz sz sz sz flt rgb buf th ncl rgba num bufs caveats -------------------------------------------------------------------------------------------------------------------- 0x 77 0 TrueColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 0 0 0 0 0 0 0 0 None 0x 78 0 TrueColor 0 16 0 rgba 0 0 5 6 5 0 . . 0 0 0 0 0 0 0 0 0 None 0x 79 0 TrueColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 16 0 0 0 0 0 0 0 None 0x 7a 0 TrueColor 0 16 0 rgba 0 0 5 6 5 0 . . 0 16 0 0 0 0 0 0 0 None 0x 7b 0 TrueColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 24 8 0 0 0 0 0 0 None 0x 7c 0 TrueColor 0 16 0 rgba 0 0 5 6 5 0 . . 0 24 8 0 0 0 0 0 0 None 0x 7d 24 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 0 0 0 0 0 0 0 0 None 0x 7e 24 TrueColor 0 32 0 rgba 0 0 8 8 8 8 . . 0 0 0 0 0 0 0 0 0 None 0x 7f 24 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 0 0 None 0x 80 24 TrueColor 0 32 0 rgba 0 0 8 8 8 8 . . 0 24 8 0 0 0 0 0 0 None 0x 81 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 0 0 0 0 0 0 0 0 None 0x 82 24 TrueColor 0 24 0 rgba 0 0 8 8 8 0 . . 0 0 0 0 0 0 0 0 0 None 0x 83 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 0 0 None 0x 84 24 TrueColor 0 24 0 rgba 0 0 8 8 8 0 . . 0 24 8 0 0 0 0 0 0 None 0x 85 0 TrueColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 16 0 0 0 0 0 0 0 None 0x 86 0 TrueColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 16 0 16 16 16 0 0 0 Slow 0x 87 32 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 0 0 None 0x 88 24 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 16 16 16 16 0 0 Slow 0x 89 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 0 0 None 0x 8a 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 16 16 16 0 0 0 Slow 0x 8b 0 TrueColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 0 0 0 0 0 0 4 1 None 0x 8c 0 TrueColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 0 0 0 0 0 0 8 1 None 0x 8d 0 TrueColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 16 0 0 0 0 0 4 1 None 0x 8e 0 TrueColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 16 0 0 0 0 0 8 1 None 0x 8f 24 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 0 0 0 0 0 0 4 1 None 0x 90 24 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 0 0 0 0 0 0 8 1 None 0x 91 24 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 4 1 None 0x 92 24 TrueColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 8 1 None 0x 93 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 0 0 0 0 0 0 4 1 None 0x 94 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 0 0 0 0 0 0 8 1 None 0x 95 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 4 1 None 0x 96 24 TrueColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 8 1 None 0x 97 0 DirectColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 0 0 0 0 0 0 0 0 None 0x 98 0 DirectColor 0 16 0 rgba 0 0 5 6 5 0 . . 0 0 0 0 0 0 0 0 0 None 0x 99 0 DirectColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 16 0 0 0 0 0 0 0 None 0x 9a 0 DirectColor 0 16 0 rgba 0 0 5 6 5 0 . . 0 16 0 0 0 0 0 0 0 None 0x 9b 0 DirectColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 24 8 0 0 0 0 0 0 None 0x 9c 0 DirectColor 0 16 0 rgba 0 0 5 6 5 0 . . 0 24 8 0 0 0 0 0 0 None 0x 9d 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 0 0 0 0 0 0 0 0 None 0x 9e 24 DirectColor 0 32 0 rgba 0 0 8 8 8 8 . . 0 0 0 0 0 0 0 0 0 None 0x 9f 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 0 0 None 0x a0 24 DirectColor 0 32 0 rgba 0 0 8 8 8 8 . . 0 24 8 0 0 0 0 0 0 None 0x a1 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 0 0 0 0 0 0 0 0 None 0x a2 24 DirectColor 0 24 0 rgba 0 0 8 8 8 0 . . 0 0 0 0 0 0 0 0 0 None 0x a3 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 0 0 None 0x a4 24 DirectColor 0 24 0 rgba 0 0 8 8 8 0 . . 0 24 8 0 0 0 0 0 0 None 0x a5 0 DirectColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 16 0 0 0 0 0 0 0 None 0x a6 0 DirectColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 16 0 16 16 16 0 0 0 Slow 0x a7 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 0 0 None 0x a8 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 16 16 16 16 0 0 Slow 0x a9 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 0 0 None 0x aa 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 16 16 16 0 0 0 Slow 0x ab 0 DirectColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 0 0 0 0 0 0 4 1 None 0x ac 0 DirectColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 0 0 0 0 0 0 8 1 None 0x ad 0 DirectColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 16 0 0 0 0 0 4 1 None 0x ae 0 DirectColor 0 16 0 rgba 1 0 5 6 5 0 . . 0 16 0 0 0 0 0 8 1 None 0x af 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 0 0 0 0 0 0 4 1 None 0x b0 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 0 0 0 0 0 0 8 1 None 0x b1 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 4 1 None 0x b2 24 DirectColor 0 32 0 rgba 1 0 8 8 8 8 . . 0 24 8 0 0 0 0 8 1 None 0x b3 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 0 0 0 0 0 0 4 1 None 0x b4 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 0 0 0 0 0 0 8 1 None 0x b5 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 4 1 None 0x b6 24 DirectColor 0 24 0 rgba 1 0 8 8 8 0 . . 0 24 8 0 0 0 0 8 1 None

Why am I not getting sRGB framebuffer in this case? Is it possible that I got into an Intel driver error?

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1 answer

Apparently, the culprit is this error in the Intel Mesa driver (thanks to the user genpfault user SO to find it in freedesktop bugzilla).

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Source: https://habr.com/ru/post/1269102/

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