Filter = enum
  none = 0, sub = 1, up = 2, average = 3, paeth = 4

proc unapply(filter: Filter; bpp: int; scanline: var string; last_scanline: string) {.
    raises: [ValueError], tags: [].}

proc apply(filter: Filter; bpp: int; scanline, last_scanline: string; res: var string) {.
    raises: [ValueError], tags: [].}

proc choose_filter(img: PngImage; scanline, last_scanline: string): Filter {.
    raises: [ValueError], tags: [].}