This opengl code snippet writes the current color buffer to a raw grayscale image. Photoshop recognizes this format as Photoshop Raw.

int screenshot(char *filename) {
    FILE* imgfile;
    unsigned char* pixels;
    unsigned char* flipped;
    int viewport[4];
    int width, height, i;
 
    glGetIntegerv(GL_VIEWPORT, viewport);
    width = viewport[2];
    height = viewport[3];
 
    pixels =  malloc(width*height);
    glReadPixels(0, 0, width, height, 
                 GL_LUMINANCE, GL_UNSIGNED_BYTE, pixels);
 
    flipped =  malloc(width*height);
    for (i=0; i < height; i++)
        memcpy(flipped + width*i, 
               pixels + width*(height-i-1), width);
    free(pixels);
 
    imgfile = fopen(filename, "wb");
    fwrite(flipped, width*height, 1, imgfile);
 
    free(flipped);
    fclose(imgfile);
 
    return 1;
}

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