(bitmap-half bitmap)
A new bitmap with half the width and half the height is created, every second row and every second column is not copied to the resulting bitmap.
The resulting bitmap is returned.
> (define bm (bitmap-create 100 100))
bm
> (bitmap-half bm)
#<bitmap orow:-1 ocol:-1 xres:0 yres:0 h:50 w:50>
>