#define PROCESS_IMAGE( type , n ) \
    int process_image_##type(void *img, int width, int height ) \
    {                                                          \
        int x, y;                                              \
                                                               \
        for (y = 0; y < height; y++) {                         \
            for (x = 0; x < width; x++) {                      \
                if      (n == 0) foo(img, x, y);               \
                else if (n == 1) bar(img, x, y);               \
                else             baz(img, x, y);               \
            }                                                  \
        }                                                      \
        return 0 ; \
    }

    PROCESS_IMAGE( foo , 0 )
    PROCESS_IMAGE( bar , 1 )
    PROCESS_IMAGE( baz , 2 )