[OpenJDK Rasterizer] [OpenJDK 2D-Dev] Path2d needRoom very slow for huge paths

[Andrea Aime]

On Mon, Apr 6, 2015 at 11:59 PM, Jim Graham <james.graham at oracle.com> wrote:

> We use this one for fast rectangular clipping:
>> https://github.com/geotools/geotools/blob/master/modules/
>> library/main/src/main/java/org/geotools/geometry/jts/GeometryClipper.java
>>
>> It was introduced exactly because java2d does not do that, and can lead
>> to OOM easily with large
>> shapes united with a dasharray stroke.
>>
>
> If I understand the above correctly you will simplify a polygon to just
> the pieces that fall within the clip before you send them to the
> rendering/stroking engine?  You mention dashing as the main case that you
> are concerned with, but doesn't that surgery on the polygon change the dash
> pattern?  I suppose if you are just viewing the dash pattern as a periodic
> decoration then that wouldn't matter, but technically there is only one
> correct way for the in-clip dashes to appear.
>

Yes, you are right it can create visual artifacts. But the speed
(important) and stability (very important) benefits are so big that we have
no choice. Also, it seems that users are not complaining, and pretty much
every open source mapping software I've looked into has implemented
something similar.
Performance wise the thing is, we sometimes are visualizing only a tiny
part of a large geometry in a map, so quickly throwing away the parts that
we are not concerned with results in a sizeable benefit (we normally clip
at a certain distance from the visible area to avoid part of the artifacts).


>
> This does point out that our dashing engines (and stroking engines) do not
> take the clip into account at all and so generate widened and dashed
> geometry for parts of the shape that do not intersect the clip.  We should
> look at adding that.  The more responsible way to add it for dashing would
> be to compute the length of any omitted segment and consume the dash phase
> as appropriate, but that is much cheaper than generating all of the
> segments and then having them widened just to be ignored by the rendering
> phase.
>

Right

Cheers
Andrea

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