[cairo] Solving the self-intersecting stroke

[Behdad Esfahbod]

Chris Wilson wrote:
> Hi guys,
> 	I've pushed to git+ssh://annarchy.freedesktop.org/~ickle/cairo a
> self-intersecting branch which contains a method to remove overlapping
> trapezoids from cairo_traps_t. This solves the self-intersecting problem
> at a cost of re-running the general Bentley-Ottmann tessellator over the
> entire input.

Very nice job Chris!

I reran my twin-font tests against it.  It shows some new bugs, but also fixes
the rendering glitches I was seeing before:

BEFORE:
http://www.gnome.org/~behdad/cairo/twin20080925.png

AFTER:
http://www.gnome.org/~behdad/cairo/twin-ickle.png

Zoom in on the 'E' in large sizes and see how the glitches on the left side
where the middle stem joins is not there anymore.


Cheers,

behdad


>>From a totally unbiased perspective ;-), cairo's output looks a look
> better with far fewer bright pixels around curves and joints. See the
> dash-curve images for an example.
> 
> One major unresolved issue is what appears to be a loss of precision
> when computing the vertices of the trapezoids for constructing the edge
> list for the second pass of the tessellator. For a good example look at
> the single pixel discrepancy inside the round end-cap around the middle
> of the dash-caps-join image. I'm still deciding the cause of the
> differences in the antialiasing, particularly if it is indeed correctly
> removing overlap of small trapezoid around the edge.
> 
> Initial estimates for the performance impact are:
> 
>     Speedups
>     ========
>     image-rgb      paint_similar_rgb_over-256    0.16 0.55% ->   0.12
> 1.28%:
>     1.36x speedup
>     ▍
>     image-rgb    paint_similar_rgb_source-256    0.16 2.20% ->   0.12
> 0.88%:
>     1.34x speedup
>     ▍
>     image-rgba  paint_similar_rgba_source-256    0.15 1.25% ->   0.12
> 1.84%:
>     1.31x speedup
>     ▎
>     image-rgba              subimage_copy-128    0.00 0.37% ->   0.00
> 0.00%:
>     1.14x speedup
>     ▏
>     image-rgba              subimage_copy-64     0.00 0.35% ->   0.00
> 0.00%:
>     1.14x speedup
>     ▏
>     image-rgba              subimage_copy-512    0.00 0.12% ->   0.00
> 0.15%:
>     1.14x speedup
>     ▏
> ...
> 
>    Slowdowns
>     =========
>     image-rgb                   world_map-800  154.42 0.11% -> 898.37
> 0.25%:
>     5.81x slowdown
>     ████▉
>     image-rgba                  world_map-800  154.65 0.09% -> 899.50
> 0.40%:
>     5.80x slowdown
>     ████▊
>     image-rgb         zrusin_another_fill-415    5.57 1.79% ->  15.40
> 1.07%:
>     2.79x slowdown
>     █▊
>     image-rgba        zrusin_another_fill-415    5.57 1.54% ->  15.13
> 0.99%:
>     2.78x slowdown
>     █▊
>     image-rgb       stroke_solid_rgb_over-64     0.39 1.24% ->   0.77
> 0.25%:
>     2.05x slowdown
>     █
>     image-rgba      stroke_solid_rgb_over-64     0.38 1.32% ->   0.77
> 0.26%:
>     2.05x slowdown
>     █
> ... lots more
> 
> 
> So... Joonas, how's your polygon rasteriser?
> 
> 
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