util.c

#define COLOR_SUBTRACT(c,r) (((((r)&0x3)>=((c)&0x3))?0:(((c)&0x3)-((r)&0x3)))|((((r)&0xC)>=((c)&0xC))?0:(((c)&0xC)-((r)&0xC)))|((((r)&0x30)>=((c)&0x30))?0:(((c)&0x30)-((r)&0x30))))

void _SMS_loadBGPaletteafterColorSubtraction (const void *palette, const unsigned char subtraction_color) {
  unsigned char i;
  SMS_setNextBGColoratIndex(0);
  // precalculate the first four colors so that we tie up the CPU long enough to miss the bottom border
  unsigned char new_colors[4];
  for (i=0;i<4;i++)
    new_colors[i] = COLOR_SUBTRACT(((unsigned char *)(palette))[i],subtraction_color);
  for (i=0;i<4;i++)
    SMS_setColor(new_colors[i]);
    
  for (i=4;i<16;i++)
    SMS_setColor(COLOR_SUBTRACT(((unsigned char *)(palette))[i],subtraction_color));
}

void _SMS_loadSpritePaletteafterColorSubtraction (const void *palette, const unsigned char subtraction_color) {
  unsigned char i;
  SMS_setNextSpriteColoratIndex(0);
  // precalculate the first four colors so that we tie up the CPU long enough to miss the bottom border
  unsigned char new_colors[4];
  for (i=0;i<4;i++)
    new_colors[i] = COLOR_SUBTRACT(((unsigned char *)(palette))[i],subtraction_color);
  for (i=0;i<4;i++)
    SMS_setColor(new_colors[i]);

  for (i=4;i<16;i++)
    SMS_setColor(COLOR_SUBTRACT(((unsigned char *)(palette))[i],subtraction_color));
}

void fade_in(const void *bg_palette, const void *sprite_palette, unsigned int sprite_palette_bank)
{
  SMS_displayOff();
  for (int i = sizeof(color_reduction) - 1; i >= 0; i--)
  {
    SMS_waitForVBlank();

    if (bg_palette != NULL)
      _SMS_loadBGPaletteafterColorSubtraction(bg_palette, color_reduction[i]);

    SMS_waitForVBlank();
    if (sprite_palette != NULL)
    {
      SMS_saveROMBank();
      SMS_mapROMBank(sprite_palette_bank);
      _SMS_loadSpritePaletteafterColorSubtraction(sprite_palette, color_reduction[i]);
      SMS_restoreROMBank();
    }
    if (i == sizeof(color_reduction) - 1)
      SMS_displayOn();
  }
}

void fade_out(const void *bg_palette, const void *sprite_palette, unsigned int sprite_palette_bank)
{
  for (int i = 0; i < sizeof(color_reduction); i++)
  {
    SMS_waitForVBlank();

    if (bg_palette != NULL)
      _SMS_loadBGPaletteafterColorSubtraction(bg_palette, color_reduction[i]);

    SMS_waitForVBlank();
    if (sprite_palette != NULL)
    {
      SMS_saveROMBank();
      SMS_mapROMBank(sprite_palette_bank);
      _SMS_loadSpritePaletteafterColorSubtraction(sprite_palette, color_reduction[i]);
      SMS_restoreROMBank();
    }
  }
  SMS_displayOff();
}