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

[Jim Graham]

It remains to be proven, but I think if we did a decent job of 
short-circuiting the geometry rejection in the actual renderer then we'd 
be easily as fast, if not faster than this technique.  Faster because it 
wouldn't require 2 stages, but since we'd likely do the computations to 
make sure that the dashing remained consistent we'd have to try it out 
and see.

On the other hand, if you amortize the cost of this clipping over 
multiple changes of view (i.e. you clip to a large surrounding area and 
let the user scroll for a bit before you invalidate your clipping area 
and perform another clip stage), then we may not be able to approach 
that in the renderer.

Still, this is a simple addition to the renderer that could perform the 
same task more accurately, if not faster...

			...jim

On 4/8/15 9:37 AM, Andrea Aime wrote:
> On Mon, Apr 6, 2015 at 11:59 PM, Jim Graham <james.graham at oracle.com
> <mailto: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
>         <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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