Using a Stack which holds an Image and a layer on top of that.

The layer on top is smaller than the Image. It is supposed to gray out only a the part of the underlying image.

The idea is to move this greyscale filtering layer around. Which is why it has to sit in its own layer; the grayscale filter cannot be the direct parent of the Image.

Stack(
  children: [
    Image.asset('bla', fit: BoxFit.contain),
    Positioned( /* WILL MOVE AROUND LATER */
      top: 50,
      left: 50,
      right: 50,
      bottom: 50,
      child: Colors.red.withAlpha(100),  /* GRAYSCALE FILTER HERE */
    )
  ]
)