Add an option to decode to a premultiplied form

[jrmuizel]

Consumers of PNG images often want the alpha premultiplied. See jrmuizel/raqote#167

[fintelia]

If you want premultiplied alpha, isn't the right approach to apply it yourself before saving the PNG?

[jrmuizel]

No, PNGs are stored using unpremultiplied alpha. Premultiplying before saving will cause other png decoders to apply the alpha twice and won't help with existing images.

[jrmuizel]

I've clarified the title to make it clearer that this would probably be an optional output transformation.

[HeroicKatora]

There are at least two case where applying the transformation at decode time, instead of later, is advantageous compared to a post-processing step after the full buffer is decoded. There is no general benefit however, so their implementation mustn't have too much of a complexity overhead.

• For images with a palette it can be performed on the palette which is possibly faster.
• For images with tRNS chunks the alpha bit is a single bit. Applying a masking SIMD-AND might be faster than the equivalent full multiplications.

[jrmuizel]

Applying at decode time can also avoid cache thrashing on large images.

[yanchith]

Sorry for the noob question: If png pre-multiplies alpha when decoding, how should this interact with gamma/sRGB? Should the pre-multiply happen in linear space? If so, would png have to gamma-decode, pre-multiply, and then gamma-encode again?