funny you should mention a chroma key because keying has been on my mind. Just started using zmatte for post work.. RTalpha subtraction and convolution as you mentioned will be a wonderful tool as well. what i would love to see is the ability for GEM to read different file types like .flt, .iv, .pfb etc.. and have be able to tweak those inside a gemwin...
zmatte and the other keying tools in a package like shake are great tools for professional post - the keying in GEM isn't quite that caliber however. the thing about real-time software is that the processing has to happen in order and within a strict schedule, which is not the same as post workflows. i'll look into those formats but don't hold your breath. ;)
and of course a way to dump out to .mov or .mpg or avi is a must at some point
you can actually save image sequences on OSX using pix_write now. it uses Quicktime's GraphicExporter so by saving the sequence to a folder you can open the first image using 'Open Image Sequence' and it will assemble a QT .mov. Right now the only option is photo-jpeg set to high quality, but i can add any of the oterh supported QT image formats like .tiff .sgi .png and so on. let me know what you need.
oh and the pix_record object is on the list, the thing holding it up is cross-platform requirements and me sitting down and pounding out the object (that QT book is around here somewhere....)
cgc
Pat