Maybe it’s too late for you, but this question comes out from time to time; for me it came tonight as I needed that extra channel to do some 3D-2D compositing and the various zblur/showzbuf sequence plugins didn’t work as expected…
Under linux you can use the ‘sgitopnm’ command, part of the ‘netpbm’ utilities, to extract the channels from an irix+zbuffer image (a.k.a. sgi image format).
sgitopnm -channel NUMBER image.sgi >image.NUMBER
where NUMBER is:
3 alpha (useful, I didn’t know it was there!)
4 I haven’t figured out yet
5 sort of stratified zbuffer
6 zbuffer (very useful for compositing)
7 always gray (???)
I hope it will help any other linux user out there.