converting to indexed with both alpha-dithering and positional dithering distorts alpha dithering
Submitted by david gowers
Link to original bug (#349882)
Description
Converting to indexed with alpha dithering and positional dithering causes the alpha-dithering to be distorted, such that the maximum effective opacity is 25% (ie. a maximum of 1 in every 4 pixels is visible.)
How to reproduce:
- make a new image
- lower the opacity of your paint tool and paint several overlapping strokes
- indexize with alpha dithering (without any other dithering)
- copy this image for comparison
- undo
- indexize with alpha dithering and positional dithering
- compare with the previously copied version
Expected result:
- Like the alpha-maximized positionally-dithered image masked by the alpha-dithered alpha values.
This only occurrs for positional dithering, not floyd-steinberg.
Version: git master