static Image2B binaryImageOutline(IImage2B image) {
  int w = image.w(), h = image.h();
  ret image == null ?: binaryImageFromFunction(w+2, h+2,
    (x, y) ->
      !(
        // white if pixel was black before
        getBoolPixelDefaultTrue(image, x, y) ? false
        // otherwise, black if surrounded by at least one black pixel
      : (
        getBoolPixelDefaultTrue(image, x-1, y-1)
      || getBoolPixelDefaultTrue(image, x, y-1)
      || getBoolPixelDefaultTrue(image, x+1, y-1)
      || getBoolPixelDefaultTrue(image, x-1, y)
      || getBoolPixelDefaultTrue(image, x+1, y)
      || getBoolPixelDefaultTrue(image, x-1, y+1)
      || getBoolPixelDefaultTrue(image, x, y+1)
      || getBoolPixelDefaultTrue(image, x+1, y+1))));
}