[QGIS-Developer] qt and antialiasing - line pattern fills

[Nyall Dawson]

On Fri, 17 Jul 2020 at 19:34, Andreas Neumann <a.neumann at carto.net> wrote:
>
> Hi,
>
> In our efforts to improve cartographic quality, I'd also like to discuss, if we could somehow improve the quality of the line-pattern fills, and/or the antialiasing quality of the qt renderer. A very small change in angle can change the quality of a line-pattern fill in QGIS pretty or very ugly.
>
> Also: even if all buildings in the following rendering have the same distance between lines, some look much "denser" than others. Have a look at the attached file where I labeled some line fills as ugly and pretty.
>
> Is there something that QGIS could do to improve the "prettyness" of line pattern fills, or would this have to change in the qt library?
>
> Thanks if you have any thoughts on this topic,

Sorry for the delayed reply!

This is a long-standing issue in QGIS' polygon fill support. See
https://github.com/qgis/QGIS/issues/16100 (filed 2013). Basically, the
line pattern symbol type uses a trick to speed up the rendering of
features by first making a small tiled raster image of the line
pattern, and then painting the polygons using this raster brush. It's
nice and fast, and works well for canvas/server renders, but is far
from ideal for high quality exports. It's also an issue if you try to
use data defined settings on the line sub symbol -- because the
pattern is generated using the tiled image, you won't see the expected
results if you try to make the appearance of individual lines in the
pattern vary.

For reference, up until 3.14 the point pattern fill had the same
limitation. For 3.14 point pattern fills were improved to now export
the fill as vector markers (the same raster brush approach is still
used for canvas renders, where speed is more important vs quality).

What we need to do is implement a similar fix as was done for the
point pattern fill, and render each individual line in a line pattern
fill as a proper vector line (during layout exports, or when we detect
that the line subsymbol has data defined properties set).  Hopefully
something we can do for 3.16...

Nyall



>
> Andreas
>
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