SDL2/SDL3 WGL: Context creation fails when trying to create float framebuffer

[mariob92]

I want to create an OpenGL context with SDL_GL_FLOATBUFFERS on Windows. However, the window creation fails with the error message No matching GL pixel format available. My GPU supports the extension WGL_ARB_pixel_format_float, so it should be possible to create such a context.

I looked into the SDL code and it seems that the attributes array is not filled correctly.

Instead of

    if (_this->gl_config.floatbuffers) {
        *iAttr++ = WGL_TYPE_RGBA_FLOAT_ARB;
    }

it should be like

    if (_this->gl_config.floatbuffers) {
        *iAttr++ = WGL_PIXEL_TYPE_ARB;
        *iAttr++ = WGL_TYPE_RGBA_FLOAT_ARB;
    }

because WGL_TYPE_RGBA_FLOAT_ARB is an value for the parameter WGL_PIXEL_TYPE_ARB (every attribute is a {type, value} pair). This also causes that the following attributes will be messed up if the user requests a float buffer.

This is my test code:

#include <SDL2/SDL.h>
#include <iostream>

int main(int argc, char *argv[])
{
    SDL_Init(SDL_INIT_VIDEO);

    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 3);
    SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 16);
    SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 16);
    SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 16);
    SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 16);
    SDL_GL_SetAttribute(SDL_GL_FLOATBUFFERS, SDL_TRUE);

    SDL_Window *window = SDL_CreateWindow("Float Framebuffer", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 1024, 576, SDL_WINDOW_OPENGL);
    std::cout << SDL_GetError() << '\n';
    SDL_GLContext context = SDL_GL_CreateContext(window);
    SDL_GL_MakeCurrent(window, context);

    int r, g, b, a, fb;
    SDL_GL_GetAttribute(SDL_GL_RED_SIZE, &r);
    SDL_GL_GetAttribute(SDL_GL_GREEN_SIZE, &g);
    SDL_GL_GetAttribute(SDL_GL_BLUE_SIZE, &b);
    SDL_GL_GetAttribute(SDL_GL_ALPHA_SIZE, &a);
    SDL_GL_GetAttribute(SDL_GL_FLOATBUFFERS, &fb);
    std::cout << "r: " << r << " g: " << g << " b: " << b << " a: " << a << " float fb: " << fb << '\n';

    SDL_GL_MakeCurrent(window, nullptr);
    SDL_GL_DeleteContext(context);
    SDL_DestroyWindow(window);
    SDL_Quit();

    return 0;
}

[mariob92]

X11 is also affected:

SDL/src/video/x11/SDL_x11opengl.c

Line 574 in f3ac2d9

attribs[i++] = GLX_RGBA_FLOAT_TYPE_ARB;

Cocoa may also:

SDL/src/video/cocoa/SDL_cocoaopengl.m

Line 346 in f3ac2d9

attr[i++] = NSOpenGLPFAColorFloat;

(don't know if this is correct)