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Gtk3/3.22 fog-of-war is too dark with fog_style "Auto"
Part of #697998.
The basic problem in this case is that the fog algorithm in
pixmap_put_overlay_tile_draw() (which dates to gna bug #21601 in 2014) darkens colours by 65% of their value, whereas the equivalent Gtk2 code in
fog_sprite() darkens colours to 65%.
(Having fixed that, there are some remaining differences between Gtk2 and Gtk3 rendering. I suspect this is some subtlety with premultiplied alpha. But this is a good enough improvement to have for now.)
#3 Updated by Jacob Nevins 11 months ago
I tried to work out which client's behaviour is more correct. By working through alpha compositing maths, I couldn't convince myself that the Gtk3 behaviour was theoretically sound.
Having fixed that, there are some remaining differences between Gtk2 and Gtk3 rendering
Attached animation illustrates this. Things like shadows at bright spots in fog remain more intense than in Gtk2. I suspect that it will show seams in hills and other multi-layered terrain if the fog darkness is artificially cranked up.
Any further work to go in a new bug.