Fade between two 16bpp images w/ dithering

fade-swag-meme_dither.c

#include <stdio.h>
#include <math.h>
#include <SDL.h>
#include <SDL_image.h>

int main(int argc, char *argv[])
{
  SDL_Surface *vid = SDL_SetVideoMode(320, 240, 16, 0);
  SDL_Surface *image  = IMG_Load("pepe.png");
  SDL_Surface *image2 = IMG_Load("spurdo.png");
  image  = SDL_ConvertSurface(image, vid->format, 0);
  image2 = SDL_ConvertSurface(image2, vid->format, 0);

  if (!vid || !image)
  {
    printf("Image loading/Video setting failed, faggot. %s\n", IMG_GetError());
    return -1;
  }

  while (1)
  {
    SDL_Event ev;
    while (SDL_PollEvent(&ev))
    {
      switch(ev.type)
      {
      case SDL_QUIT:
        return 0;
      default: break;
      }
    }

    SDL_FillRect(vid, NULL, 0);

    Uint32 mul_a = (Uint32)((sinf(SDL_GetTicks() / 1000.f)+1.f) * 8388607.f);
    Uint32 mul_b = mul_a ^ 0xFFFFFF;
    Uint32 r, g, b, err_r=0, err_g=0, err_b=0;

    Uint16 c;
    Uint16 c2;
    Uint8 *dest = vid->pixels;
    Uint8 *src  = image->pixels;
    Uint8 *src2 = image2->pixels;
    int i, j;

    for (i=0; i<image->h; i++)
    {
      for (j=0; j<image->w; j++)
      {
        c  = *(Uint16*)src;
        c2 = *(Uint16*)src2;

        r =
          (((c  >> 11) & 0b11111 ) * mul_a) +
          (((c2 >> 11) & 0b11111 ) * mul_b) +
          err_r;

        g =
          (((c  >>  5) & 0b111111) * mul_a) +
          (((c2 >>  5) & 0b111111) * mul_b) +
          err_g;

        b =
          (((c       ) & 0b11111 ) * mul_a) +
          (((c2      ) & 0b11111 ) * mul_b) +
          err_b;

        err_r = r & 0xFFFFFF;
        err_g = g & 0xFFFFFF;
        err_b = b & 0xFFFFFF;
        r &= 0x1F000000;
        g &= 0x3F000000;
        b &= 0x1F000000;

        *(Uint16*)dest = (r >> 13) | (g >> 19) | (b >> 24);
        //skip color (2 bytes = 16bit)
        dest += 2;
        src  += 2;
        src2 += 2;
      }

      //skip line (line width - width * color depth)
      dest += vid->pitch - (image->w * 2);
      src  += image->pitch - (image->w * 2);
      src2 += image->pitch - (image->w * 2);
    }

    SDL_Flip(vid);
  }


  return 0;
}