I tested this in Oneiric (Ghostscript 9.04) and there it works. Closing
...
** Changed in: ghostscript (Ubuntu)
Status: New => Fix Released
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https://bugs.launchpad.net/bugs/537401
** Attachment added: "Actual result"
http://launchpadlibrarian.net/40794868/gs.png
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Ghostscript doesn't render my masked objects
https://bugs.launchpad.net/bugs/537401
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** Attachment added: "Expected result"
http://launchpadlibrarian.net/40794862/poppler.png
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Ghostscript doesn't render my masked objects
https://bugs.launchpad.net/bugs/537401
You received this bug notification because you are a member of Ubuntu
Bugs, which is subscribed to Ubuntu.
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** Attachment added: "A pdf with /Mask /S /Alpha"
http://launchpadlibrarian.net/40794840/test_pdf_out.pdf
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Ghostscript doesn't render my masked objects
https://bugs.launchpad.net/bugs/537401
You received this bug notification because you are a member of Ubuntu
Bugs, which is subscribed to U