GNOME Bugzilla – Bug 349882
converting to indexed with both alpha-dithering and positional dithering distorts alpha dithering
Last modified: 2018-05-24 11:53:20 UTC
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.
Created attachment 70393 [details] shows an unindexed image, what happens, and what should happen. It's captioned. the 'expected result' image was produced by: 1. Duplicate the original image twice. 2. On the first dupe: 2.1 Transfer the layer's alpha to selection 2.2 Go into QMask mode, and edit->copy the selection mask 2.3 Exit QMask mode, paste the selection mask onto the layer, anchor it. 2.4 Invert the layer 2.5 Indexize the image to black+white, with positional dithering. 3. On the second dupe: 3.1 Threshold the alpha at 0 3.2 Indexize the result to black+white, with positional dithering. 4. Edit->copy the first dupe 5. Enter QMask mode on the second dupe, Edit->paste. 6. Clear the selection. The effect is to mask the ditherized alpha-thresholded image by the ditherized alpha channel, which in my understanding is exactly what alpha dithering is supposed to do.
Removing from 'Bugs without a response'.
-- GitLab Migration Automatic Message -- This bug has been migrated to GNOME's GitLab instance and has been closed from further activity. You can subscribe and participate further through the new bug through this link to our GitLab instance: https://gitlab.gnome.org/GNOME/gimp/issues/203.