5 ms·

I'm not sure I understand your concern. If the software converts all your assets into a premultiplied form, the bleeding you applied won't hurt anything even if

by mark-r 10y ago

I'm not sure I understand your concern. If the software converts all your assets into a premultiplied form, the bleeding you applied won't hurt anything even if it doesn't help. Yes, it's extra work that shouldn't be necessary - but we often find ourselves living in an imperfect world. I completely agree that premultiplied alpha should be used everywhere. I'd even go a step farther and say that you should use high bit depth linear values too, but that's a topic for another day.

theoh 10y ago
Alvy Ray Smith may have the canonical paper on this, from 1995: http://alvyray.com/Memos/CG/Microsoft/4_comp.pdf http://alvyray.com/Memos/CG/Microsoft/4_comp.pdf
ioquatix 10y ago
A 16bpp linear pre-multiplied format would be awesome. sRGB is a pain to use in practice: - Slow to convert to linear colour space (requires a pow).. except - GPUs use an 8-bit lookup table to convert input sRGB to output linear value. - This doesn't work for more than 8 bits as the table gets excessively large very quickly. It's a pity PNG doesn't have flag to mark the image data as being pre-multiplied.
- foolrush 10y ago
  That little ditty is also very likely why PNG has zero uptake in visual effects and CGI. Associated (aka premultiplied) alpha is the _sole_ means to embody both occlusion and emission. Unassociated (aka straight or key) alpha cannot represent these facets. Consider a candle flame that exists as mostly emission and low to no occlusion. With associated alpha, you can use zero alpha triplets with non-zero emission RGB to represent this real-world scenario. With unassociated alpha? Impossible.
  - Dylan16807 10y ago
    > Consider a candle flame that exists as mostly emission and low to no occlusion. I don't think associated alpha helps much here. You can special case pixels that are doing pure emission, but when a pixel is doing both you need the lighting to affect the color of the occlusion but not affect the color of the emission.
    - foolrush 10y ago
      It ends up being a pure add emission. It can't be done with unassociated alpha due to the multiplication.
      - Dylan16807 10y ago
        Sure, you can do a pure add. But that means you need to dedicate specific objects to being purely emissive, to avoid blurring at the boundaries. And once you've separated the objects, you don't really need to have purely-emissive textures and non-emissive textures in the same file, with the same exact format. You might as well store emissive textures as RGB and save on memory. Using a different format can even benefit you. You're less likely to accidentally blend emissive and non-emissive pixels, and you're less likely to accidentally apply lighting calculations to emissions.
        foolrush 10y ago
        Except associated alpha models emission and occlusion via the operation. You don't need zero alpha either, as any ratio can occlude partially and emit partially as well. No hacking needed.
        Dylan16807 10y ago
        You can't do both in a single pixel, unless you turn off lighting entirely and make everything fullbright. Let's have blue-tinted pane of glass, (0, 0, .5, .5). And a red tinted pane, (.5, 0, 0, .5). Then a blue glow, (0, 0, .5, 0). And a red glow, (.5, 0, 0, 0). If you have your blue glass glow red, and your red glass glow blue, both combinations come out as (.5, 0, .5, .5). Under 99% of lighting conditions, it will look wrong. If you put it in darkness, it will look overwhelmingly wrong. Even if occlusion and emission are the same color it doesn't work. A dark blue object that glows brightly, and a bright blue object that glows dimly, both will have the same RGBA.
  - ioquatix 10y ago
    I've been throwing around the idea of making a new intermediate file format which can contain only pre-multiplied transparency. But, do you know any file format already supports this?
    - mark-r 10y ago
      TIFF can do premultiplied alpha. The problem with TIFF is that it's such a grab bag of options, it makes it difficult to know what you're dealing with.
      - foolrush 10y ago
        EXR is associated by convention. TIFF mandates it, although with an additional TIFF tag for an "opacity" channel that behaves as unassociated IIRC.
      - ioquatix 10y ago
        Thanks - I didn't know that - anything else apart from TIFF?
  - dahart 10y ago
    > 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.
    - foolrush 10y ago
      It is emissive. 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. https://forums.adobe.com/thread/369637 https://forums.adobe.com/thread/369637 Here is a sample of the candle referenced by Zap: https://i.imgur.com/bnXyAl1_d.jpg?maxwidth=640&shape=thumb&fidelity=high https://i.imgur.com/bnXyAl1_d.jpg?maxwidth=640&shape=thumb&f... 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. Further reading: http://lists.openimageio.org/pipermail/oiio-dev-openimageio.org/2011-December/004709.html http://lists.openimageio.org/pipermail/oiio-dev-openimageio.... https://groups.google.com/forum/m/#!topic/ocio-dev/ZehKhUFqhjc https://groups.google.com/forum/m/#!topic/ocio-dev/ZehKhUFqh...
      - dahart 10y ago
        > It is emissive. 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
dahart 10y ago
Just pointing out that attempting to solve the real problem should be tried first, before jumping to work-arounds. If the software converts to pre-multiplied, then there'd be no halo problem and no bleeding necessary, right? You're right that bleeding won't hurt assets if you're comping, but it will hurt assets if you're not converting to premult and then do texture filtering or mipmapping or blurring. My concern is with using bleeding is the article's suggestion to use bleeding as a first resort, rather than a last resort (as an artist). It's a hack that totally works in a lot of cases, but it's still a hack. I've watched artists in film and games use random combinations of bleeding, (un)pre-multiply, gamma, and other stuff whenever something goes wrong with matting, and often it's not the right solution. A lot of people are scared of understanding premultiplied alpha - and the technical name isn't doing anyone any favors - and instead of figuring out the right solution they try every combination of hacks until it works. General misunderstanding and superstition about premultiplied alpha is the most common reason I've seen for people using un-premultiply nodes in production.

by mark-r 10y ago

theoh 10y ago

Alvy Ray Smith may have the canonical paper on this, from 1995: http://alvyray.com/Memos/CG/Microsoft/4_comp.pdf http://alvyray.com/Memos/CG/Microsoft/4_comp.pdf

ioquatix 10y ago

A 16bpp linear pre-multiplied format would be awesome. sRGB is a pain to use in practice: - Slow to convert to linear colour space (requires a pow).. except - GPUs use an 8-bit lookup table to convert input sRGB to output linear value. - This doesn't work for more than 8 bits as the table gets excessively large very quickly. It's a pity PNG doesn't have flag to mark the image data as being pre-multiplied.

foolrush 10y ago
That little ditty is also very likely why PNG has zero uptake in visual effects and CGI. Associated (aka premultiplied) alpha is the _sole_ means to embody both occlusion and emission. Unassociated (aka straight or key) alpha cannot represent these facets. Consider a candle flame that exists as mostly emission and low to no occlusion. With associated alpha, you can use zero alpha triplets with non-zero emission RGB to represent this real-world scenario. With unassociated alpha? Impossible.
- Dylan16807 10y ago
  > Consider a candle flame that exists as mostly emission and low to no occlusion. I don't think associated alpha helps much here. You can special case pixels that are doing pure emission, but when a pixel is doing both you need the lighting to affect the color of the occlusion but not affect the color of the emission.
  - foolrush 10y ago
    It ends up being a pure add emission. It can't be done with unassociated alpha due to the multiplication.
    - Dylan16807 10y ago
      Sure, you can do a pure add. But that means you need to dedicate specific objects to being purely emissive, to avoid blurring at the boundaries. And once you've separated the objects, you don't really need to have purely-emissive textures and non-emissive textures in the same file, with the same exact format. You might as well store emissive textures as RGB and save on memory. Using a different format can even benefit you. You're less likely to accidentally blend emissive and non-emissive pixels, and you're less likely to accidentally apply lighting calculations to emissions.
      - foolrush 10y ago
        Except associated alpha models emission and occlusion via the operation. You don't need zero alpha either, as any ratio can occlude partially and emit partially as well. No hacking needed.
        Dylan16807 10y ago
        You can't do both in a single pixel, unless you turn off lighting entirely and make everything fullbright. Let's have blue-tinted pane of glass, (0, 0, .5, .5). And a red tinted pane, (.5, 0, 0, .5). Then a blue glow, (0, 0, .5, 0). And a red glow, (.5, 0, 0, 0). If you have your blue glass glow red, and your red glass glow blue, both combinations come out as (.5, 0, .5, .5). Under 99% of lighting conditions, it will look wrong. If you put it in darkness, it will look overwhelmingly wrong. Even if occlusion and emission are the same color it doesn't work. A dark blue object that glows brightly, and a bright blue object that glows dimly, both will have the same RGBA.
- ioquatix 10y ago
  I've been throwing around the idea of making a new intermediate file format which can contain only pre-multiplied transparency. But, do you know any file format already supports this?
  - mark-r 10y ago
    TIFF can do premultiplied alpha. The problem with TIFF is that it's such a grab bag of options, it makes it difficult to know what you're dealing with.
    - foolrush 10y ago
      EXR is associated by convention. TIFF mandates it, although with an additional TIFF tag for an "opacity" channel that behaves as unassociated IIRC.
    - ioquatix 10y ago
      Thanks - I didn't know that - anything else apart from TIFF?
- dahart 10y ago
  > 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.
  - foolrush 10y ago
    It is emissive. 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. https://forums.adobe.com/thread/369637 https://forums.adobe.com/thread/369637 Here is a sample of the candle referenced by Zap: https://i.imgur.com/bnXyAl1_d.jpg?maxwidth=640&shape=thumb&fidelity=high https://i.imgur.com/bnXyAl1_d.jpg?maxwidth=640&shape=thumb&f... 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. Further reading: http://lists.openimageio.org/pipermail/oiio-dev-openimageio.org/2011-December/004709.html http://lists.openimageio.org/pipermail/oiio-dev-openimageio.... https://groups.google.com/forum/m/#!topic/ocio-dev/ZehKhUFqhjc https://groups.google.com/forum/m/#!topic/ocio-dev/ZehKhUFqh...
    - dahart 10y ago
      > It is emissive. 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

dahart 10y ago

Just pointing out that attempting to solve the real problem should be tried first, before jumping to work-arounds. If the software converts to pre-multiplied, then there'd be no halo problem and no bleeding necessary, right? You're right that bleeding won't hurt assets if you're comping, but it will hurt assets if you're not converting to premult and then do texture filtering or mipmapping or blurring. My concern is with using bleeding is the article's suggestion to use bleeding as a first resort, rather than a last resort (as an artist). It's a hack that totally works in a lot of cases, but it's still a hack. I've watched artists in film and games use random combinations of bleeding, (un)pre-multiply, gamma, and other stuff whenever something goes wrong with matting, and often it's not the right solution. A lot of people are scared of understanding premultiplied alpha - and the technical name isn't doing anyone any favors - and instead of figuring out the right solution they try every combination of hacks until it works. General misunderstanding and superstition about premultiplied alpha is the most common reason I've seen for people using un-premultiply nodes in production.