Can the background used to create a picture bump be set
to transparant. This would potentially make the
creation of documents in which renders are used
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While it would be desirable to have full alpha channel support in the framebuffers so that we could readily keep track of transparency, this simply isn't worth the effort right now given other priorities. You can render to a framebuffer as it is and set the background color to some unique value (default is 0/0/1), making it pretty simple to post-process the image with your favorite image editor to remove the background from the image. Several of the BRL-CAD tools rely on the use of a special user-specified background-color for image processing in the manner you suggest even.
The mod to libfb, however, to support a proper additional alpha channel would be more work than I think we're currently willing/capable of investing in this. Thank you for the submission, though, and we'll keep note of the feature request in our TODO file.