(graymap-hmirror! graymap)
This procedure mirrors the graymap destructively and horizontally thus exchanging right and left like a RL-mirror.
The graymap is modified.
The mirrored graymap is returned.
> (define image (graymap-readpng "example.png"))
image
> (graymap-writepng (graymap-hmirror! image) "example-hmirrored.png")
ok
>