Read a polygon with a hole from a ShapeFile #80

DGuidi · 2022-01-20T15:32:53Z

To investigate #70

DGuidi · 2022-01-20T15:57:48Z

The order of vertices or orientation for a ring indicates which side of the ring is the interior of the polygon... Vertices of rings defining holes in polygons are in a counterclockwise direction. Vertices for a single, ringed polygon are, therefore, always in clockwise order.
The order of rings in the points array is not significant.

so basically testing for rings CW and CCW orientation is the correct way of doing things.
Maybe is this code that try to clean data that is inherently dirty?

…hat are not inside any shell, due mainly to invalid inputs with shell and holes not correctly oriented - as separate shells

DGuidi · 2022-01-21T10:57:45Z

The shell_bad_ccw.shp contains a single polygon, with:

a shell CCW-oriented (like a hole from ESRI specs)
a hole CW-oriented (like a shell from ESRI specs)

The idea of this PR is to let the ShapefileReader reads this kind of data as two separate shells, so a MultiPolygon is returned.

FObermaier · 2022-01-21T11:33:10Z

The idea of this PR is to let the ShapefileReader reads this kind of data as two separate shells, so a MultiPolygonis returned.

Does one polygon contain the other? In that case the result would be invalid.

DGuidi · 2022-01-21T13:15:52Z

Does one polygon contain the other? In that case the result would be invalid.

Yep, exactly (made explicit in the unit test).

I see few different strategies:

let the develop behavior: now a single polygon is read, and it's the hole (interpreted as a shell from NTS). the actual shell (interpreted as a hole from NTS) is discarded from the output because NTS doesn't find any shell that can be considered a "shell for this hole" (sorry for the poor explaination)
accept the PR behavior: both polygons are read and returned, the invalid data is appended at the end of the multipolygon and the geometry is not valid. It's up to the consumer to fix the geometry applying the logic.
throw an exception if the data is not valid (i.e.: unassigned data is found); this means that ShapefileReader.Read() returns false, because of this code and the reason must be inspected checking the windows messages (not good, but removing the catch is a breaking change).
PUT YOUR IDEA HERE

I'm open to suggestions :)

FObermaier · 2022-01-21T13:26:29Z

@DGuidi , I think we should

Order the polygon rings by their area (as if they were Polygons without holes)
Pick the largest as 1st shell
Check the others if they are contained by any of the shells and if so, make them a hole of that shell, otherwise create a new shell (in that case we have a MultiPolygon).
Repeat 3 until there are no more rings left
Build the valid [Multi]Polygon.

I think the PR I pointed you to yesterday does it in a similar way.

DGuidi · 2022-01-21T13:30:31Z

I think the PR I pointed you to yesterday does it in a similar way

~~I will do a check, thanks~~
The code in the WIP PR looks similar to the code actually in the standard lib to me

DGuidi · 2022-01-24T08:03:46Z

I'm working on resolving an issue with this geom:
MULTIPOLYGON (((-124.134 -79.199, -124.141 -79.316, -124.164 -79.431, -124.202 -79.542, -124.254 -79.647, -124.319 -79.745, -124.396 -79.833, -124.484 -79.91, -124.582 -79.975, -124.687 -80.027, -124.798 -80.065, -124.913 -80.088, -125.03 -80.095, -125.147 -80.088, -125.262 -80.065, -125.373 -80.027, -125.478 -79.975, -125.576 -79.91, -125.664 -79.833, -125.741 -79.745, -125.806 -79.647, -125.858 -79.542, -125.896 -79.431, -125.919 -79.316, -125.926 -79.199, -125.919 -79.082, -125.896 -78.967, -125.858 -78.856, -125.806 -78.751, -125.741 -78.653, -125.664 -78.565, -125.576 -78.488, -125.478 -78.423, -125.373 -78.371, -125.262 -78.333, -125.147 -78.31, -125.03 -78.303, -124.913 -78.31, -124.798 -78.333, -124.687 -78.371, -124.582 -78.423, -124.484 -78.488, -124.396 -78.565, -124.319 -78.653, -124.254 -78.751, -124.202 -78.856, -124.164 -78.967, -124.141 -79.082, -124.134 -79.199),(-124.438 -79.199, -124.443 -79.122, -124.459 -79.046, -124.483 -78.973, -124.518 -78.903, -124.561 -78.839, -124.612 -78.781, -124.67 -78.73, -124.734 -78.687, -124.804 -78.652, -124.877 -78.628, -124.953 -78.612, -125.03 -78.607, -125.107 -78.612, -125.183 -78.628, -125.256 -78.652, -125.326 -78.687, -125.39 -78.73, -125.448 -78.781, -125.499 -78.839, -125.542 -78.903, -125.577 -78.973, -125.601 -79.046, -125.617 -79.122, -125.622 -79.199, -125.617 -79.276, -125.601 -79.352, -125.577 -79.425, -125.542 -79.495, -125.499 -79.559, -125.448 -79.617, -125.39 -79.668, -125.326 -79.711, -125.256 -79.746, -125.183 -79.77, -125.107 -79.786, -125.03 -79.791, -124.953 -79.786, -124.877 -79.77, -124.804 -79.746, -124.734 -79.711, -124.67 -79.668, -124.612 -79.617, -124.561 -79.559, -124.518 -79.495, -124.483 -79.425, -124.459 -79.352, -124.443 -79.276, -124.438 -79.199)),((-124.582 -79.199, -124.586 -79.257, -124.597 -79.315, -124.616 -79.371, -124.642 -79.423, -124.674 -79.472, -124.713 -79.516, -124.757 -79.555, -124.806 -79.587, -124.858 -79.613, -124.914 -79.632, -124.972 -79.643, -125.03 -79.647, -125.088 -79.643, -125.146 -79.632, -125.202 -79.613, -125.254 -79.587, -125.303 -79.555, -125.347 -79.516, -125.386 -79.472, -125.418 -79.423, -125.444 -79.371, -125.463 -79.315, -125.474 -79.257, -125.478 -79.199, -125.474 -79.141, -125.463 -79.083, -125.444 -79.027, -125.418 -78.975, -125.386 -78.926, -125.347 -78.882, -125.303 -78.843, -125.254 -78.811, -125.202 -78.785, -125.146 -78.766, -125.088 -78.755, -125.03 -78.751, -124.972 -78.755, -124.914 -78.766, -124.858 -78.785, -124.806 -78.811, -124.757 -78.843, -124.713 -78.882, -124.674 -78.926, -124.642 -78.975, -124.616 -79.027, -124.597 -79.083, -124.586 -79.141, -124.582 -79.199),(-124.896 -79.199, -124.897 -79.181, -124.9 -79.164, -124.906 -79.148, -124.914 -79.132, -124.923 -79.117, -124.935 -79.104, -124.948 -79.092, -124.963 -79.083, -124.979 -79.075, -124.995 -79.069, -125.012 -79.066, -125.03 -79.065, -125.048 -79.066, -125.065 -79.069, -125.081 -79.075, -125.097 -79.083, -125.112 -79.092, -125.125 -79.104, -125.137 -79.117, -125.146 -79.132, -125.154 -79.148, -125.16 -79.164, -125.163 -79.181, -125.164 -79.199, -125.163 -79.217, -125.16 -79.234, -125.154 -79.25, -125.146 -79.266, -125.137 -79.281, -125.125 -79.294, -125.112 -79.306, -125.097 -79.315, -125.081 -79.323, -125.065 -79.329, -125.048 -79.332, -125.03 -79.333, -125.012 -79.332, -124.995 -79.329, -124.979 -79.323, -124.963 -79.315, -124.948 -79.306, -124.935 -79.294, -124.923 -79.281, -124.914 -79.266, -124.906 -79.25, -124.9 -79.234, -124.897 -79.217, -124.896 -79.199)))

This multipolygon is valid (two separate polygons each with a hole) but using the suggested logic the data from the shapefile is considered a polygon. I'm working on it.

when a potential hole of a shell is found, we check also the shell holes, to see if any hole contains potential hole. If so, the potential hole can not be considered a hole for the shell, but a separate geometry.

DGuidi · 2022-01-25T09:44:41Z

@FObermaier I slightly changed the logic.
Now when a potential hole of a shell is found, we check also the shell holes, to see if any hole contains the potential hole. If so, the potential hole can not be considered a hole for the shell, but a separate geometry.
It works but I think I can miss few other edge cases.

DGuidi · 2022-02-07T07:38:38Z

@FObermaier any advice on this?

FObermaier · 2022-02-07T07:45:23Z

I actually don't quite know how to proceed. There are complaints that current ShapeFile implementation is (dreadfully) slow. That is why I pointed you to the WIP/redo everyting PR. If we do thorough topology test on the rings we read, I assume it gets even worse. Maybe we can enable the thorough test conditionally, if the user of the library really wants it?

DGuidi · 2022-02-07T07:57:24Z

I actually don't quite know how to proceed.

Actually, I have the same feeling: with the "new" polygon builder logic all the tests are green, but I fear that some behavior not covered by some test can be broken.

That is why I pointed you to the WIP/redo everyting PR

that shows the same problem, actually

If we do thorough topology test on the rings we read

the "new" polygon builder logic I think is similar to the older one, in the sense that I didn't expected to be too much slower. But actually this is just a feeling, I haven't tried no comparison tests

we can enable the thorough test conditionally

I can work on it: a flag can be helpful also to build some performance test

my2cents

TODO: flag needs to be handled differently, suggestions well accepted

DGuidi · 2022-02-07T09:38:10Z

@FObermaier added a "temporary" static property as a flag, I need to think a bit better about how to manage the entire "flag" thing... suggestions are welcome!

after a brief inspection: adding a flag property to the shapefiledatareader, means that the same flag shall be copied to the shapefilereader and then to the polygonhandler, that is created in a static factory method of the shapefile class, so we must ensure that any call to the factory method should also handle the flag... additionally, the Shapefile.CreateDataReader needs to accept another parameter, just to enable the new behavior.

viable, not so complicated at all, maybe acceptable for an "experiment", but I hope to find a better alternative

…nce checks (not good vibes...)

DGuidi · 2022-02-10T15:03:16Z

I didn't expected to be too much slower

but actually - based on some poor-man's performance tests - it's much (~3X) slower

    WRITE => elapsed: '00:00:00.4138680'
    ShapeFileDataReader
    flag DISABLED => elapsed: '00:00:00.4381579'
    flag ENABLED => elapsed: '00:00:01.3507803'
    ShapeReader
    flag DISABLED => elapsed: '00:00:00.4276830'
    flag ENABLED => elapsed: '00:00:01.2875299'

Added 2 more variants to build the polygons. * Sequential Assumes that polygons are always serialized `shell[, holes][, shell[, holes][, ...]]` and thus starts a new polygon when the current ring is not contained by the current polygon. This is slightly faster than the current default implementation. * UsePolygonizer Throws all rings at the `NetTopologySuite.Operations.Polygonize.Polygonizer` and lets it build the polygon. This is really slow.

DGuidi · 2022-02-17T08:10:56Z

just a small note: tests marked as Explicit are always executed in my machine (using VS2019). this is why I used Ignore instead: now I understood that probably it's something related to my setup, and not a general problem.

FObermaier · 2022-02-17T08:13:16Z

AFAIK Explicit will always run from within the UI (VisualStudio).

FObermaier · 2022-02-17T08:20:05Z

In GDAL/OGR shapefile driver, all rings are converted to Polygons and afterwards sent to organizePolygons:
https://github.com/OSGeo/gdal/blob/363178015ae009fa1187d8c1e20b3587db43aa58/ogr/ogrgeometryfactory.cpp#L1455-L2144

I have not bisected the algorithm altogether, but it seems to be a bit of everything...

DGuidi · 2022-02-17T08:30:02Z

AFAIK Explicit will always run from within the UI (VisualStudio).

exactly what happens to me

DGuidi · 2022-02-17T08:39:33Z

"For faster computation, a polygon is considered to be inside another one if a single point of its external ring is included into the other one"

maybe we can refactor this check to speed up a bit the computation:

bool IsHoleContainedInShell(LinearRing shell, LinearRing hole) =>
   //shell.EnvelopeInternal.Contains(hole.EnvelopeInternal) &&
   PointLocation.IsInRing(hole.GetCoordinateN(0), shell.Coordinates);

FObermaier · 2022-02-17T08:59:02Z

I'd assume that without the cheap envelope precondition test we'll see a performance degeneration.

DGuidi · 2022-02-17T09:18:57Z

I'd assume that without the cheap envelope precondition test we'll see a performance degeneration.

I just mean that IsHoleContainedInShell check should just use PointLocation.IsInRing

FObermaier · 2022-02-17T09:29:39Z

I got that, but PointLocation.IsInRing is the expensive test (compared to shell.EnvelopeInternal.Contains(hole.EnvelopeInternal)). If we don't do the envelope test, the PointLocation.IsInRing test is performed more often which will lead to performance degeneration.

DGuidi · 2022-02-17T09:58:05Z

If we don't do the envelope test, the PointLocation.IsInRing test is performed more often which will lead to performance degeneration.

yep now I see your point. curiously, from performance tests I see performance improvements

FObermaier · 2022-02-17T11:43:37Z

yep now I see your point. curiously, from performance tests I see performance improvements

If you only have polygons with holes that might be true, but for multipolygons whose shell ring envelopes don't intersect, I doubt that.

DGuidi · 2022-03-22T15:06:27Z

can be this PR cancelled, considering the discussion in #81, and in particular this comment ? Maybe part of this work can be added to #69 , if worthy

investigating #70

f67a8ba

DGuidi self-assigned this Jan 20, 2022

#70 update PolygonHandler to handle "unassigned" holes - i.e: holes t…

fbdc1bf

…hat are not inside any shell, due mainly to invalid inputs with shell and holes not correctly oriented - as separate shells

DGuidi requested review from airbreather and FObermaier January 21, 2022 09:25

DGuidi marked this pull request as ready for review January 21, 2022 09:27

#70 made explicit that the readed geometry is not valid

01ccb8d

DGuidi marked this pull request as draft January 21, 2022 13:17

#70 polygon creation based on area, ignoring shell/holes orientation

3af1eac

DGuidi added 2 commits January 24, 2022 09:11

added a test that fails

15499f6

modified the polygon builder logic

603a9c1

when a potential hole of a shell is found, we check also the shell holes, to see if any hole contains potential hole. If so, the potential hole can not be considered a hole for the shell, but a separate geometry.

minor changes to read the code a bit more readable

6dd104a

DGuidi mentioned this pull request Jan 25, 2022

Wip/redo everything #69

Open

DGuidi marked this pull request as ready for review February 1, 2022 13:09

DGuidi added the help wanted Extra attention is needed label Feb 3, 2022

DGuidi marked this pull request as draft February 7, 2022 08:40

introduced conditional flag to handle new polygon builder logic

2d9b34c

TODO: flag needs to be handled differently, suggestions well accepted

added tests for ShapeDataReader (too many readers!) and some performa…

2739c7e

…nce checks (not good vibes...)

Update Issue70Fixture.cs

faf29de

moved flag to a static class, and added some docs

b4c4814

DGuidi marked this pull request as ready for review February 11, 2022 08:17

DGuidi added enhancement New feature or request and removed help wanted Extra attention is needed labels Feb 11, 2022

DGuidi and others added 8 commits February 11, 2022 09:26

Delete shell_bad_ccw.prj

90ff2a1

added some performance tests

398140a

Do sort on Polygon

389593b

removed unnecessary (and, actually, broken) check

0e2db23

added perftest stub

f36409a

updated perf test to use compiled tests instead of nuget packages

61232a5

minor changes to tests

14879f4

DGuidi mentioned this pull request Feb 23, 2022

Cache results of Geometry.Area and Geometry.Length computation NetTopologySuite/NetTopologySuite#595

Closed

DGuidi marked this pull request as draft March 21, 2022 13:45

DGuidi marked this pull request as ready for review March 21, 2022 13:45

DGuidi mentioned this pull request Nov 4, 2022

Add Test project from NetTopologySuite.IO.ShapeFile repository NetTopologySuite/NetTopologySuite.IO.Esri#7

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Read a polygon with a hole from a ShapeFile #80

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