Potential weight calculation issue

[hannes-vernooij]

Before I create a PR to fix this potential issue I want to double check if this is intended.
In dirtyalpha.rs the alpha is subtracted from 256, this will result in values outside of the 0..=255 range when the alpha is zero. Shouldn't this be 255 instead?

[kornelski]

It's a weight, not a value. The weight can be anything, because the values are multiplied and divided by the weights, so it cancels out.

Weight 0 is used as a special case for skipping completely transparent pixels, so other alpha values intentionally get non-0 values.

[hannes-vernooij]

It might be useful to clarify this in your code by adding a comment describing this behavior.