Why doesn't "xwd -root" work in Ubuntu Unity2d?

Question

linux

Noticed strange behavior xwd -rootin Ubuntu 12.04 (Unity 2d).
Launched:
xwd -root -out test.screen.root.xwd
Result opened with gimp:

Launched:

xwd -root | xwdtopnm | pnmtopng > Screenshot.root.png

Received:
xwdtopnm: writing PPM file
libpng warning: Invalid sBIT depth specified
Opened with Image Viewer:

Why is that? Where to look for the reason for this behavior?

Answer 1

Here's an explanation from the unity2d developers:
bugs.launchpad.net/unity-2d/+bug/1081674/comments/1

Answer 2

Sorry for offtopic, but rather you are on the ubuntu forum ...