save graphics output as Windows Bitmap(*.bmp)

Uint32 rmask, gmask, bmask, amask;

    /* SDL interprets each pixel as a 32-bit number, so our masks must depend
       on the endianness (byte order) of the machine */
#if SDL_BYTEORDER == SDL_BIG_ENDIAN
    rmask = 0xff000000;
    gmask = 0x00ff0000;
    bmask = 0x0000ff00;
    amask = 0x000000ff;
#else
    rmask = 0x000000ff;
    gmask = 0x0000ff00;
    bmask = 0x00ff0000;
    amask = 0xff000000;
#endif

w=400;
h=500;
unsigned char * pixels = new unsigned char[w*h*4]; // 4 bytes for RGBA
    glReadPixels(0,0,w, h, GL_RGBA, GL_UNSIGNED_BYTE, pixels);

     SDL_Surface * surf = SDL_CreateRGBSurfaceFrom(pixels, width, height, 8*4, width*4, rmask, gmask, bmask, amask);                                                                                      
    SDL_SaveBMP(surf, filename);

    SDL_FreeSurface(surf);
    delete [] pixels;