Investigate the idea of premultiplied alpha in PixelType

[p0nce]

• A. If we add 6 new pixel types (lap, rgbap for 8-bit, 16-bit and 32-bit FP), then we can support pre-multiplied with quite a bit of changes. The interesting thing is that PNG with the unspecified ipHone PNG does support premultiplied

or

• B. Decode unpremultiplied always, do not keep track of status, just provide ways to do it and undo it at the discretion of the user. This is simpler, but people will not get the advantages of premultiplied or less often.

• Does premultiplied alpha encodes better, and how much better in terms of space? Check with PNG and QOIX. => YES, 2x better for some QOIX overlays!

• Implement function to premultiply, or unpremultiply. This is the easy part, already exist in Dplug ImageKnob.

Big question is (above) whether to keep that information in the type system (PixelType) or not. This is NOT a LayoutOptions, and I don't think it should be a separate status also. Premultiplied alpha is better for compositing, but not for editing. It also looses a bit of data, but it repeatable.

[p0nce]

In future world, PixelType will probably explode in complexity with opEquals being non-trivial. What would be those additions?
I can think of color spaces, specific encoding, chroma sitting, etc. Some types will also not make sense, like:

 (RGB, ubyte, premultiplied, null colorspace)    // no good to have 'premultiplied alpha' without alpha

• Must choose whether to YAGNI this or not. Pixel Type would become non-POD, because of ICC. Bleh.
  • Can something with an ICC be POD? A reference type for this would be a bit meh.

[p0nce]

The reading of the color library made by Manu is interesting and points towards the likely shape of a future "Colorspace" concept, which would either the same as PixelType or PixelType would be purely derived from it.

Now, is there a way to support the less types "legacy" PixelType?

[p0nce]

Very much related to #20

[p0nce]

Important to note that some practical overlay images are largely transparent and can benefit a lot from premultiplied encoding, but not really from "ignore color for fully transparent areas" which is a less strong thing somehow. That would make the QOIX codec not lossless in 8-bit contrarily to today.

[p0nce]

Premultiplied alpha doesn't seem to strictly need ICC profile or full colorspace struct. We can still chug along, ignoring colorspace.