Hi Andrea,
On 4/3/15 7:01 AM, Andrea Aime wrote:
PS: GeoServer will clip and simplify the geometry before
throwing it at java2d, exactly because we know
java2d is not so good at handling this kind of complexity
Do you have a fast and efficient polygon clipper in geoserver ?
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.
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.
The dasher only needs to accumulate and account for segment lengths that
are dropped.
The stroker just needs to remember the angles at the nearest endpoint of
the omitted segements.
Another gotcha that comes up with stroking and bounds is that miter
joins can extend a fairly large distance away from the path segments
that generate them and you only know how far it extends when you have
both segments that meet at the vertex and can compute their relative
angle. There is a "miter limit" so if the segment is beyond that then
it cannot intersect the clip regardless of the angles, but between that
limit and the line width there is some computation to be done. Still,
even if the simplest and worst case assumption that any segment would
extend by the full miter limit, for extremely large or zoomed paths and
reasonable values for miter limits that still leaves quite a lot of
segments that could be trivially eliminated...
...jim
