This piece of SDL2 code draws some white pixels on-screen using OpenGL, then grabs the pixels field of the window's SDL_Surface and loops through it, printing out the values of the contents. Even though I just drew a white triangle, the loop shows that there's nothing but zeros in that buffer (the code just prints 0 to standard out over and over).

How can I actually get at the modified pixel buffer, in something like RGB or ARGB or RGBA format?

#include <SDL2/SDL.h>
#include <SDL2/SDL_opengl.h>
#include <GL/glu.h>

int main()
{
    SDL_Init(SDL_INIT_VIDEO | SDL_INIT_EVENTS);

    const int WINDOW_WIDTH  = 100;
    const int WINDOW_HEIGHT = 100;
    SDL_Window *window = SDL_CreateWindow("OpenGL Test", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, WINDOW_WIDTH, WINDOW_HEIGHT, SDL_WINDOW_OPENGL);
    
    SDL_GLContext gl_context = SDL_GL_CreateContext(window);
    SDL_GL_SetSwapInterval(1);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    GLenum error = glGetError();
    if( error != GL_NO_ERROR )
    {
        printf( "Error initializing OpenGL! %s\n", gluErrorString(error));
    }

    glClearColor(0, 0, 0, 1);
    
    int quit = 0;
    SDL_Event event;
    while (!quit)
    {
        while (SDL_PollEvent(&event))
        {
            switch (event.type)
            {
                case SDL_QUIT:
                quit = 1;
                break;
            }
        }

        glBegin(GL_TRIANGLES);
        glColor3f(255, 255, 255);
        glVertex2f(0, 0);
        glVertex2f(0, 1);
        glVertex2f(1, 0);
        glEnd();

        SDL_GL_SwapWindow(window);

        SDL_Surface *surface = SDL_GetWindowSurface(window);
        int pixel_depth = SDL_BYTESPERPIXEL(surface->format->format);
        char *pixels = (char*) surface->pixels;
        int max_value = 0;
        for (int i = 0; i < WINDOW_WIDTH * WINDOW_HEIGHT * pixel_depth; i  )
        {
            if (pixels[i] > max_value)
            {
                max_value = pixels[i];
            }
        }
        SDL_FreeSurface(surface);

        SDL_Log("%d", max_value);
    }

    SDL_Quit();

    return 0;
}

CodePudding user response：

SDL_GetWindowSurface() doesn't work with OpenGL:

You may not combine this with 3D or the rendering API on this window.

Use glReadPixels() instead.

CodePudding user response：

Use PBO to read data from from the pixel buffer.

glReadBuffer(GL_COLOR_ATTACHMENT0);
writeIndex = (writeIndex   1) % 2;
readIndex = (readIndex   1) % 2;
glBindBuffer(GL_PIXEL_PACK_BUFFER, pbo[writeIndex]);
                    // copy from framebuffer to PBO asynchronously. it will be ready in the NEXT frame
glReadPixels(0, 0, SCR_WIDTH, SCR_HEIGHT, GL_RGBA, GL_UNSIGNED_BYTE, nullptr);
                    // now read other PBO which should be already in CPU memory
glBindBuffer(GL_PIXEL_PACK_BUFFER, pbo[readIndex]);
unsigned char* Data = (unsigned char *)glMapBuffer(GL_PIXEL_PACK_BUFFER, GL_READ_ONLY);                    
glUnmapBuffer(GL_PIXEL_PACK_BUFFER);
glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);