capture(region) gives (partly) black images

Mac 10.6 also
capture(selectRegion()) gives a black picture in temp file.

my tests gave:
- capture(x,y,w,h) always works with any w and h if both x and y are 0

the buggy capture is only with x and/or y > 0.

if both x and y are greater than w and h the captured image is totally black.

if x and/or y are < w/h respectively, you will have the upper left part of the image in the lower right part of the captured image.

if you add the x and y value to w and h respectively, you will get a capture with
- width = x+w and height = y+h
- the wanted image in the lower right corner with the relative position x,y
- and the rest being black.

example:
capture wanted: capture(200, 100, 200, 100)
you say capture(200, 100, 400, 200)

If you now could say capture on the captured image, capture(200, 100, 200, 100) would give you the wanted image.

Since I related a question on partly black captures in multi monitor environments, I tested with the following monitor configuration with Mac 10.6:
primary: 1920x1200, secondary on the left at (-1280,0) with 1280x960

captures on primary:
there is an additional x-offset (to the above mentioned ones with single monitor) with the width of the secondary monitor.

captures on secondary:
same effect as for a single monitor situation, with the single (0,0) being in this case (-1280,0).

so the above example would read:
capture wanted: capture(-1080, 100, 200, 100)
you say capture(-1080, 100, 400, 200)
and at least you can see your wanted image in the lower right part of the captured image.