I see there are various ways to use one image as a mask for another.
But I'd like to construct a mask of a rectangular shape with an arbitrary position and size within the pix. I suppose I could handle arbitrary position by loading an image file with the rectangle, and [pix_roll]-ing it (twice, once for each dimension), but that wouldn't help with size.
It seems like there should be some pix object to fill a pix region with a color or pattern. But, I can't find it.