Intersection and Snap Rounding Documentation

[flippmoke]

It is not intuitive right away how the intersection processing and the snap rounding is done in the code. Documentation should be added and put in the /docs directory of the repository.

[MaheshNaikM]

yes , please ..then i can play around with your code .I normally do area comparion with a debugged murta clipper I have which uses double precision and i also have a arbitrary precion super slow version of the same.Only problem is it is also arbitrary in execution speeds.!.
does snap rounding provided non intersecting output contours(rings).

[flippmoke]

Note -- adding previous conversation from email with @MaheshNaikM here for historical purposes:

Basically, if you took all of the points created by a vatti prior to doing your actual clipping you could collect all the integer coordinates they pass through as "hot pixels". Then as you are doing your vatti loop and finding intersections, you simply "snap" points that are prior to the intersections (and any other point additions) to the points they pass through. This is a form snap rounding - see http://doc.cgal.org/latest/Snap_rounding_2/index.html

Additionally my intersection processing is different from Angus in that it also finds the intersection location as a floating point calculation first. This helps sort properly the order of intersections prior to rounding the point back to integer coordinates for the actual intersection processing.

[flippmoke]

@MaheshNaikM

does snap rounding provided non intersecting output contours(rings).

Snap rounding will prevent intersecting segments, but will not prevent intersection at points. Intersections at points is cleaned up in the do_simple_polygons() method following the vatti algorithm.

For example - this ring has an intersection at a point and is not valid as per the OGC specification. It should be 2 separate rings to be valid.

The next image is an example of an intersection of edges that is not on a point. This will not happen as a result of the snap rounding and intersection processing from vatti.

[MaheshNaikM]

My understanding of output is that if I put the output back in for a union
no intersections should be reported.!(this fails in the original..limited
edition...Angus Vatti.If one can have non intersecting contours then i can
use them as a generator of a non intersecting collection of contours(rings
in your notation). Clipping is only one use of vattis method. I am planning
to provide a generator for a non intersecting polygon collection akin to
random number generator.!.

On Tue, Nov 1, 2016 at 7:06 PM, Blake Thompson [email protected]
wrote:

@MaheshNaikM https:/MaheshNaikM

does snap rounding provided non intersecting output contours(rings).

Snap rounding will prevent intersecting segments, but will not prevent
intersection at points. Intersections at points is cleaned up in the
do_simple_polygons() method following the vatti algorithm.

For example - this ring has an intersection at a point and is not valid as
per the OGC specification. It should be 2 separate rings to be valid.

[image: screen shot 2016-11-01 at 8 30 59 am]
https://cloud.githubusercontent.com/assets/1794907/19891372/cf225c8a-a00d-11e6-83ed-d793c6931800.png

The next image is an example of an intersection of edges that is not on
a point. This will not happen as a result of the snap rounding and
intersection processing from vatti.

[image: screen shot 2016-11-01 at 8 34 14 am]
https://cloud.githubusercontent.com/assets/1794907/19891464/43bfa58e-a00e-11e6-8138-4d25e95daeb2.png

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[MaheshNaikM]

I have a mutated version of Murtas generic polygon clipper.3 Algorithmic bugs/worms removed.Data structures changed for speedup ....gpc 2.31/2.32/2.33 had such problems that the outputs were unreliable and in reality useless.