Add Douglas-Peucker poly simplification algorithm as PolyUtil.simplify() #201

barbeau · 2015-09-16T18:51:56Z

Douglas-Peucker implementation ported from the Google MyTracks project (https://code.google.com/p/mytracks/source/browse/MyTracks/src/com/google/android/apps/mytracks/util/LocationUtils.java#81), (c) Google 2008, licensed under Apache v2.0.
Add demo activity for PolyUtil.simplify() - shows the original line in black, as well as simplified versions of the line with multiple tolerances in different colors
Add unit tests for PolyUtil.simplify() and PolyUtil.distanceToLine() in PolyUtilTest

As discussed with @broady in #144 (comment).

@broady A few unrelated white space changes snuck into this commit via Android Studio - seems this happens on save, and I haven't found a way to disable it. Let me know if this is an issue - I'm using the official Android code template.

broady · 2015-09-16T18:56:22Z

Whitespace changes LGTM. Rest LGTM, too.

pinging @googlebot for CLA ... (I don't think this actually does anything...)

@jfschmakeit @markmcd any thoughts?

broady · 2015-09-16T19:23:07Z

CLA checked manually. 👍

markmcd · 2015-09-16T21:45:55Z

library/src/com/google/maps/android/PolyUtil.java

+     * @param tolerance in meters.  Increasing the tolerance will result in fewer points in the
+     *                  simplified line.
+     */
+    public static void simplify(List<LatLng> line, List<LatLng> simplifiedLine, double tolerance) {


Why are you using out-parameters (and void return type), instead of returning a LatLng array or List?

I kept the same method signature and return type from the original MyTracks implementation. However, I'd actually prefer returning a LatLng List, so if you agree I'd be happy to change it.

+1, that's preferable. Also, change the parameter name to polyline to be consistent with the other functions in this file.

Actually - does this work for closed loops (i.e. polygon simplification)?

That's a good question - I think so, although I'll run some tests to see for sure.

Looks like it doesn't modify the lat/lons themselves, so we just need to be sure that the first and last points are always included. I presume that they are.

I can't tell just by reading the code.

Yes, assertEndPoints() test checks that the first and last points are unmodified.

barbeau · 2015-09-17T00:04:14Z

@broady @markmcd Ok, just force-pushed a new commit into this PR to replace the old one. This changes to the method signature and return type to:
public static List<LatLng> simplify(List<LatLng> polyline, double tolerance) {

Don't know if I'll get to testing if this works for polygons tonight yet.

markmcd · 2015-09-17T00:37:07Z

LGTM. Thanks for this - it's a valuable contribution.

Let us know when you're done so we can merge.

barbeau · 2015-09-17T15:38:23Z

Looks like this will work for polygons too, but there are a few caveats:

The polygon we run the algorithm on can't be closed (i.e., first and last LatLng in the List can't be exactly the same) - this breaks Douglas-Peuker, as it requires a positive distance measurement to start the algorithm.
If we remove the last coordinate duplicate coordinate to open the polyline, it doesn't fully simplify the polygon (there is one extra point) - you can see this in the screenshots below with a simplified triangle. The polyline starts with the lower-left corner - blue is the original triangle, and yellow is the simplified version. In the left screenshot, the bottom edge of the triangle still has a point in the middle (the last point prior to closing the polygon) that should have been removed based on the tolerance. The right screenshot shows the correctly simplified yellow triangle, with the middle point in the bottom edge removed.

I produced the yellow polygon on the right by adding a very small offset to the final "duplicate" point that closes the polygon. So, it looks like:

ArrayList<LatLng> triangle = new ArrayList<>();
triangle.add(new LatLng(28.06025,-82.41030));  // First point
...
triangle.add(new LatLng(28.06025000001, -82.41030000001));  // Closing "duplicate" point

This seems to work without causing problems - here's the same result for an oval:

So, as far as handling polygons with the simplify() method, I think we have two options:

Require developers to pass in a closed polygon to the simplify() method - Inside the method, we'd check to see if the first and last point were the same, and if so, add the offset to the last point. We'd add the requirement to the Javadocs for passing in a closed polygon.
Add a new simplifyPolygon() method - We'd allow developers to pass in closed or open polygons into this method. If its closed, we'd add the offset to the last point, and if its open, we'd close it with the offset point - then we'd call simplify().

For the output of either option above, we would swap out the final point w/ the offset for the original final point before returning the list.

Thoughts? I think the first option is my preference, as it keeps the library simpler - worst case scenario, a developer passes in an unclosed polygon, and they get a polygon back that's not totally simplified. This won't even be noticeable in most cases, as it's only a single extra point.

barbeau · 2015-09-17T15:41:19Z

Also, if we handle both polylines and polygons in the same simplify() method, I propose we change the parameter name to shape:
public static List<LatLng> simplify(List<LatLng> shape, double tolerance)

barbeau · 2015-09-17T21:16:16Z

@broady @markmcd I went ahead and implemented the Option 1 above ("Require developers to pass in a closed polygon to the simplify() method"), and force-pushed a new commit for this into this PR to replace the old ones. So, simplification for polylines and polygons are now both fully supported. I also included a number of new unit tests to cover polygon simplification as well.

So, it should be ready for your final review, as long as you're good with the above design decisions.

broady · 2015-09-17T21:18:27Z

library/src/com/google/maps/android/PolyUtil.java

+     *                  simplified shape.
+     * @return a simplified shape produced by the Douglas-Peucker algorithm
+     */
+    public static List<LatLng> simplify(List<LatLng> shape, double tolerance) {


s/shape/poly/

broady · 2015-09-17T21:19:26Z

Thank you for the diligent work on this.

LGTM. Just one small nit: I think shape should be renamed to poly.

barbeau · 2015-09-17T21:20:08Z

@broady Ok, np, I'll push a new commit shortly.

* Douglas-Peucker line simplification implementation ported from the Google MyTracks project (https://code.google.com/p/mytracks/source/browse/MyTracks/src/com/google/android/apps/mytracks/util/LocationUtils.java#81), (c) Google 2008, licensed under Apache v2.0. * Add support for simplifying polygons to the PolyUtil.simplify() implementation * Add demo activity for PolyUtil.simplify() - shows the original line in black, as well as simplified versions of the line with multiple tolerances in different colors. Also shows origin polygons in blue, with simplified polygons in yellow * Add unit tests for PolyUtil.simplify() (covering polylines and polygons), PolyUtil.distanceToLine(), and PolyUtil.isClosedPolygon() in PolyUtilTest

barbeau · 2015-09-17T21:24:17Z

Ok, done. I changed a few other shape references to poly as well.

Add Douglas-Peucker line simplification algorithm as PolyUtil.simplify()

barbeau · 2015-09-17T21:29:29Z

Awesome, thanks @broady and @markmcd for the review/merge!

Would it be possible to get a new library release out with this included? New demo app release would be great too.

broady · 2015-09-17T21:31:39Z

yup - I'm pretty busy with a bunch of other stuff, so I could only do a release next week. Maybe @markmcd will get to it first. We'll coordinate.

barbeau · 2015-09-18T00:57:50Z

👍

barbeau · 2015-09-24T16:52:42Z

Just to make sure this doesn't fall through the cracks - before this is released, PR #205 should be reviewed/merged. That PR fixes an issue with polygon simplification.

barbeau · 2015-10-30T03:44:00Z

Friendly ping - @broady and @markmcd do you think you could get a new release out soon?

On an unrelated note - I'm prepping a release of a major app overhaul (including port to Android Maps v2), but seeing a bizarre issue with markers jumping around on the map that's bad enough to postpone release - here's a video:
https://youtu.be/gYBunJYZTMo

So far no traction with an gmaps-api issue I filed in August. Any ideas for next steps?

barbeau · 2015-11-18T15:52:56Z

Another friendly ping - @broady and @markmcd could we get a new release that includes PolyUtil.simplify()?

barbeau · 2016-02-10T01:54:43Z

Yet another friendly ping - @broady and @markmcd could we get a new release that includes PolyUtil.simplify()?

markmcd · 2016-02-15T00:07:18Z

Sure thing. I was hoping to have contributed a little more myself to the next release, but I've pushed 0.4.1 anyways.

barbeau · 2016-02-15T01:59:04Z

Awesome, thanks!!

barbeau mentioned this pull request Sep 16, 2015

Add Douglas-Peucker algorithm #144

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markmcd reviewed Sep 16, 2015
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barbeau force-pushed the douglas-peucker2 branch from 8d1ab4c to 2182d07 Compare September 17, 2015 00:00

barbeau force-pushed the douglas-peucker2 branch from 2182d07 to 919f81e Compare September 17, 2015 21:13

broady reviewed Sep 17, 2015
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barbeau force-pushed the douglas-peucker2 branch from 919f81e to d724493 Compare September 17, 2015 21:24

broady added a commit that referenced this pull request Sep 17, 2015

Merge pull request #201 from barbeau/douglas-peucker2

7b1481e

Add Douglas-Peucker line simplification algorithm as PolyUtil.simplify()

broady merged commit 7b1481e into googlemaps:master Sep 17, 2015

barbeau changed the title ~~Add Douglas-Peucker line simplification algorithm as PolyUtil.simplify()~~ Add Douglas-Peucker poly simplification algorithm as PolyUtil.simplify() Sep 17, 2015

barbeau mentioned this pull request Sep 18, 2015

Reference for last point input to PolyUtil.simplify() can change #204

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barbeau deleted the douglas-peucker2 branch September 24, 2015 18:16

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Add Douglas-Peucker poly simplification algorithm as PolyUtil.simplify() #201

Add Douglas-Peucker poly simplification algorithm as PolyUtil.simplify() #201

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Add Douglas-Peucker poly simplification algorithm as PolyUtil.simplify() #201

Add Douglas-Peucker poly simplification algorithm as PolyUtil.simplify() #201

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