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polygon subtraction

I was working on some skill code to generate a logo for a sheet symbol and ended up in a spot where I need to subtract two polygons. My preference is to operate on the lists of points that define the two polygons:

points1 = list(0:0 10:0 10:10 0:10 0:0)

points2=list(2:2 8:2 8:8 2:8 2:2)

points3 = myPolyptsSub(points1, points2) ;; not sure how to implement

fig=dbCreatePolygon(cv, my_lpp, points3)

alternatively, I could live with

fig1 = dbCreatePolygon(cv, my_lpp, points1)

fig2 = dbCreatePolygon(cv, my_lpp, points2)

fig3 = dbLayerAndNot(cv, my_lpp, list(fig1), list(fig2))

dbDeleteObject(fig1)

dbDeleteObject(fig2)

;; is there a db* function to merge the geometries that come back or only the leMergeShapes() function

;; in fig3 (which should be a list)

but it would be nice to be able to work directly with the points.

The reason I'm interested in working directly with points is I have some code that takes an SVG path specification string and turns it into polygon points but some cases may involve an outside shape with some cutouts. My real case is more complicated than just a square cut out of a square. Think polygons with hundreds of points.

Thanks for any suggestions.

-Dan

mschw over 8 years ago

Dear Dan,

you could use dbLayerXor(cv, mylayerName, list(fig1), list(fig2)).

Best regards,
mschw
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skillUser over 8 years ago

Based on the other response and your need to operation on the points directly you could look at the dbPointArrayXXX functions like dbPointArrayXor(). Hopefully this will help.
Best regards, Lawrence.
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drdanmc over 8 years ago

Thanks! I had missed the dbPointArray* functions. I find it interesting that those functions need a cellview ID even though they don't actually modify the cellview. I would have thought those would strictly work on points.

but this seems to work

points3 = dbPointArrayXor(cv list(points1) list(points2))
foreach(p points3
dbCreatePolygon(cv, my_lpp, p)
)

-Dan
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Andrew Beckett over 8 years ago

Dan,

I think it's needed to (at least) convert from user units to the underlying database units (so the shapes have the right resolution). May be for other things too - but that's certainly needed.

Andrew.
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