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.gra...@oracle.com
<mailto:james.gra...@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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