Docs should say whether Rgba<T> is premultiplied or not

[federicomenaquintero]

I couldn't find the string premul in the source code except for the algorithm in the blend filter :sweat_smile:

Are images decoded with alpha premultiplied or not? It doesn't matter for my application; it can handle either format.

[fintelia]

This library neither applies pre-multiplication nor removes it. We'll return the pixel values as they're stored in the file so it is up to you to know whether your image files come pre-multiplied or not.

The main image crate however does contain some amount of code for applying blending which I believe assumes that the image is stored non-premultiplied

[sophie-h]

But is there a convention for the DynamicImage's the decoders in image-rs give?

[fintelia]

In practice, DynamicImage's are usually not premultiplied because the majority of image files don't use premultiplied alpha.

However, there's some challenges to making that a convention everywhere:

• Many formats (including PNG*) don't have a field to indicate whether they're premultipled or not, so the decoder has no idea what it is outputting.
• Applying pre-multiplication is an irreversible operation: RGB colors in fully transparent areas are lost, and precision is reduced in partially transparent areas

^{*I think the PNG spec says images are always supposed to be non-premultiplied but people sometimes ignore that.}