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 😅

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.}

[kornelski]

Do we have any formats that actually output premultiplied? (not counting non-standard PNG extensions or hacks)

I think we can document they're always uncorrelated.

Stuffing other data into lossless formats is not our problem (the alpha could be a heightmap or rgb could be normals).

[fintelia]

This issue is slightly confusing because it is opened against the image-png repository but asking about a type that is only provided in the image repository. Answers are different depending on whether you're dealing only with PNG or with any image format that image-rs supports now or in the future.

Do we have any formats that actually output premultiplied?

Yes. The TIFF format supports both premultiplied and non-premultiplied alpha. Not sure precisely how image handles it though