> that doesn't matter unless you color-scale the image (like multiply by 2 to make it brighter) before displaying it.
It does if you stack several image layers. Let's say I have a particular color tone. Then I use that as background color. And I also stack 10 layers of the same color with alpha 0.05 on top of that. If you use premultiplied colors then this will actually result in a different color.
Due to rounding those colors often tend to be more greyish too. So if draw some vector graphics and have multiple basic shapes with semi-transparent edges (aliasing!) stacked on top of each other you can get some ugly fringes.
Yeah true, I suppose there is rounding error. Is this something that has actually happened to you, or are you saying it's a problem in theory? I'd be hard pressed to come up with a real-world use case for 10 of the same color being comped. For this to be a problem, the visible elements being comped would have to be exactly the same color, without any gradient or noise at all...
Even if it did happen, the error is bounded - with 10 layers of the same color, the maximum error in any channel is 5, and the average error is 2.5. It's pretty hard to say it would be wildly and noticeably different to most people even with 8 bit color channels, but I certainly have met some film directors and CG supervisors who were very color sensitive. It would be literally invisible in anything higher than 8 bits.
I'm curious -- why do you say the rounded colors would tend toward gray? Rounding error can happen in both directions, so I would expect rounding errors to cause a uniformly distributed error -- some colors would get slightly more saturated, some less, some of them would shift hue, and some would be unaffected.
You lost me on the edges part & aliasing. Rounding errors will not be visible as fringes, so if you're seeing fringes, you have some other problem... perhaps failure to pre-multiply! ;) Tell me more about stacking shapes and getting fringes. Is this stacking 10 of the same shape in the same place, or at the edge crossings of different shapes? What kind of fringes are you seeing?
Anyway, I've never witnessed a case in 20 years of film & game production where rounding errors caused a visible, detectable problem, but I'd love to know if there real cases where it's an issue!
> Is this something that has actually happened to you
In a toy project where I tried to automatically generate SVG shapes and rendering them to a html canvas (which uses 8bit per channel premultiplied alpha). The assembled shapes occasionally overlapped and it did lead to visible color inconsistencies.
It's also a problem when working with PNGs. When you put your PNG pixels into a premultiplied space and pull them out again you actually lost information to rounding, which negates the benefits of a lossless format.
> You lost me on the edges part & aliasing. Rounding errors will not be visible as fringes, so if you're seeing fringes, you have some other problem... perhaps failure to pre-multiply! ;)
Well, if you got a solid shape then there's no alpha. But the aliasing at the edges introduces partially transparent pixels. If you then pull out the pixel data and apply it to a different canvas you get mismatched colors.
I only spent a few hours on it, so I don't recall all the details, but my conclusion was that it is inadequate for image manipulation since it is lossy and lacks precision.
It does if you stack several image layers. Let's say I have a particular color tone. Then I use that as background color. And I also stack 10 layers of the same color with alpha 0.05 on top of that. If you use premultiplied colors then this will actually result in a different color.
Due to rounding those colors often tend to be more greyish too. So if draw some vector graphics and have multiple basic shapes with semi-transparent edges (aliasing!) stacked on top of each other you can get some ugly fringes.