Re: unpremultiply in combustion?

maybe to get the steam out of the discussion before it gets really hot.
of course, the transparency of your fg object was taken into account by your 3d program to produce the rendered image. it would have done so, too, when there's something behind your transparent fg object. it's simply the way objects at different distances to each other get "packed" together to render the final image, that's the rgb info in it. you might relate it to an "over node"/operation in a compositing program. if i have a fg layer and a bg layer and i want to combine them together, it's done in the following manner: (fg*alpha)+(bg*(1-alpha)) in plain words, you take the rgb date of your fg layer and multiply them with the fg alpha values. you take the bg rgb values and multiply them by the inverse fg alpha values. then, you add both. that's just one small step of how a 3d renderer comes up with a picture, which i'm far from being expert in. since you usually have a lot of layers to comp in your composite it was handy to incorporate the first step of this "over operation" in the image file. the image was "pre"-multiplied, that is for further compositiong. in your case, the cg fg object with it's transparency/alpha information was composited/layed over a solid bg. when you choose to save that file "pre-multiplied" with a separate alpha file your 3d app gave you the above result as rgb info in a tga file and a separated file which contains the alpha. but that's not what is generally referred to as "pre-multiplied". the term itself refers to a way how transparency information can be stored in an image file. the alternative is "non-premultiplied" or "straight". in any case, the terms refer to alpha information in the image file.
now, you have a final rgb image and the alpha info which was used to derive that result, as well as, you know the bg color. it is fair to say, that you can reverse engineer that over operation which was done by your 3d app. unfortunately, c* doesn't have a divide node or operation (in the rgb arithmetics, for instance). but as i mentioned in my first reply, there might be options.

in case i wrote something which was not correct,please correct me.

cheers,
rayk