Artifacts in rendering of Gaussian blur filter (horizontal stripes)

[Patrick Storz]

When zooming far into a blurred region (or setting a large blur radius) artifacts appear in the rendering of the Gaussian blur filter (horizontal stripes with changing color, see screenshot).

The screenshot was made with Inkscape 0.92.0 (64-bit) on Windows 10 x64 but current trunk (r15415) is affected, too.

The issue is dependent on the rendering quality set in preferences: The lower the quality setting the earlier (i.e. for lower zoom, smaller blur radius) the artifacts become visible.
Only for the highest quality setting I could not see any artifacts (but rendering gets so slow on my machine that i can't zoom in very far, so I can't completely rule out that it's affected, too).

[Patrick Storz (ede123)]

Some helpful comments from su_v via IRC:
based on tests with archived builds: not seen with 14904, seen with 14914
https://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/changes/14914
could have been exposed with the fix for the broken quality settings in
https://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/revision/14913
(i.e. the issue could have been there before that (?))
I tested with default (new) prefs for each run.
they seem to go back to early cairo-rendering merges
I do see them with 10578 (before the merge of the renderer caching branch)
I do not see them with 10579 (the merge of the caching branch)
but that's also the commit which disabled the quality prefs (based on my earlier tests)

[Patrick Storz (ede123)]

And another comment by su_v:
Bug #1651915 is not related - the lines described there also reproduce with 14904

[su_v (suv-lp)]

Confirming based on my tests as quoted from irc.

The mentioned tests had been done with archived trunk builds on OS X 10.7.5, with default (new) preferences for each run.

[Tavmjong Bah (tavmjong-free)]

Definitely related to tiling. With trunk one can now visualize the tiles by uncommenting a line in src/desktop.cpp (canvas_debug) to see the tiles. Tile edges align with the discontinuities.