> That little ditty is also very likely why PNG has zero uptake in visual effects and CGI.
The main reason why CG studios can't use PNG is because both renderers and compositors always output pre-mult images. Yes you can choose to un-pre-mult them after rendering, but the native result of a blending operation is always a pre-mult color, regardless of the input sources. Unpremultiplied blending still results in premultiplied colors.
Since everyone knows that un-premultiplying (dividing) is to be avoided at all costs, it means you can't render or comp something and then save the file in PNG.
> Associated (aka premultiplied) alpha is the _sole_ means to embody both occlusion and emission.
I would suggest avoiding thinking of image colors as emissive. That's a material property, and using RGBA to encode material properties is only something you'd do if you were stuck in a weird fixed-function pipeline with no choice, or if you were really really low on disk space. Otherwise, emission colors go in their own separate emission channel that doesn't have an alpha value.
Feel free to reference the original Porter Duff paper regarding "luminescent pixels" or Alvy Smith's opinion on the matter as relayed by Zap Andersson in the legendary Adobe thread.
Remember that a ray tracing engine uses associated alpha as that is the sole format it can generate. Only associated alpha models emission and occlusion.
Yes you're right; I wasn't arguing with you about that. I always feel like calling it additive rather than emissive. But just because you can represent additive colors in premult images doesn't mean you should, and I'd speculate wildly that it occurs less often in production than halo problems. Someone who writes lens flare and rainbow shaders is going to scold me for saying that though...
In the context of the OP's article, and of artists who paint images with transparency in them, worrying about emissive colors isn't really an issue. Artists very rarely paint pre-mult images, they can't work with premult images, generally speaking. No doubt a few people who know what they're doing do it, but I can't personally say I've ever seen an artist painted premult image with emissive colors, nor do I recall ever seeing a software rendered layer with emissive colors either. Is this common now? I've been out of film & games for a few years now.
I'm already familiar with everything you referenced; and I can vouch that it's all very good stuff so thanks for sharing, especially the Adobe thread. I hope others here benefit. It's amusing that an entire industry knows who Chris Cox is because of this thread, right? :P
The main reason why CG studios can't use PNG is because both renderers and compositors always output pre-mult images. Yes you can choose to un-pre-mult them after rendering, but the native result of a blending operation is always a pre-mult color, regardless of the input sources. Unpremultiplied blending still results in premultiplied colors.
Since everyone knows that un-premultiplying (dividing) is to be avoided at all costs, it means you can't render or comp something and then save the file in PNG.
> Associated (aka premultiplied) alpha is the _sole_ means to embody both occlusion and emission.
I would suggest avoiding thinking of image colors as emissive. That's a material property, and using RGBA to encode material properties is only something you'd do if you were stuck in a weird fixed-function pipeline with no choice, or if you were really really low on disk space. Otherwise, emission colors go in their own separate emission channel that doesn't have an alpha value.