CompuServe Messages

There are a couple of separate issues going on here. I. Accumulated alpha. The alpha channel IS accumulated throughout the compositing chain. That's an important feature that we spent quite a bit of time working on to get right. Your comment about the alpha switches in the IXP's being useless is inaccurate, though, because if you do a single-layer renderings with an additional light-effect layer, the alpha switches work perfectly. If someone wanted to write an IXP that saved the accumulated alpha channel from all previous layers, wiped the alpha channel for the subsequent layer, and then another IXP that restored the accumulated alpha after the light-effect layer was done, it would allow n-layer compositing with light effects and alpha-switch control. But the point is that it works perfectly in a 2-layer situation. II. Alpha output from IXP's. As for the alpha output from IXPs such as FLARE, HILITE, etc., Alpha is not modified by these processes because IT SHOULD NOT BE. The reason for this is that lighting effects which are additive in nature do NOT mask out what is behind them — they add to it. If you take a multiple-exposure photograph of red, green and blue light sources with different overlapping cross-star filters, the overlapping star points from the different color layers ADD UP — Exposing red, then green, then blue does not result in a blue image (as you seem to expect) — it adds up to white. Tom has some slides that he took as experiments while doing the IXP's that I could dig up if I need to to provide someone actual documented photographic proof of this, but you can try it yourself. In the computer, if we generated alpha, compositing the resulting image over another would mask out the other image rather than adding the flare color to it. There ya go, – G