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BF: for degenerate polygons, add CLOSEPOLY vertex#17982

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brunobeltran:closepoly-clobbers-poly-vertex
Jul 22, 2020
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BF: for degenerate polygons, add CLOSEPOLY vertex#17982
QuLogic merged 1 commit intomatplotlib:masterfrom
brunobeltran:closepoly-clobbers-poly-vertex

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@brunobeltran brunobeltran commented Jul 20, 2020 •
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PR Summary

Fixes #17975. New iter_bezier code exposed an existing bug in patches.Polygon.set_xy.

The check xy[0] != xy[-1] was meant to check if the user had already included an extra CLOSEPOLY vertex for us in the list of vertices (shape(xy) = (N, 2)). But for single-vertex (degenerate) Polygons, it produced a false positive (since xy[0] == xy[-1] by definition), leading to malformed Paths that look like:

In [1]: from matplotlib.patches import Polygon
In [2]: pp = Polygon([[0, 0]])
In [3]: pp.get_path()
Out[3]: Path(array([[0., 0.]]), array([79], dtype=uint8))

PR Checklist

  • Has Pytest style unit tests
  • Code is Flake 8 compliant
  • New features are documented, with examples if plot related
  • Documentation is sphinx and numpydoc compliant
  • Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
  • Documented in doc/api/next_api_changes/* if API changed in a backward-incompatible way

@brunobeltran brunobeltran added topic: path handling Release critical For bugs that make the library unusable (segfaults, incorrect plots, etc) and major regressions. labels Jul 20, 2020
@brunobeltran brunobeltran mentioned this pull request Jul 20, 2020
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QuLogic
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Comment thread lib/matplotlib/patches.py Outdated
@brunobeltran brunobeltran force-pushed the closepoly-clobbers-poly-vertex branch from 33cc440 to 787bf03 Compare July 20, 2020 23:00
QuLogic
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Add a test?

@QuLogic QuLogic added this to the v3.3.1 milestone Jul 20, 2020
@brunobeltran brunobeltran force-pushed the closepoly-clobbers-poly-vertex branch from 787bf03 to e551a27 Compare July 20, 2020 23:13
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brunobeltran commented Jul 20, 2020

Add a test?

Couldn't decide what to test initially. Settled on testing that for a single point (x, y), the extents of a degenerate polygon with only that point as its vertices has the extents Bbox([[x, y], [x, y]]), which is the behavior where we actually differ from the (buggy) 3.2 behavior.

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jklymak commented Jul 20, 2020

I'm sure this was covered in the other PR, but what happens if x=np.Nan?

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brunobeltran commented Jul 21, 2020

tl;dr: NaN works.

The Polygon class doesn't complain about NaN's or clean them explicitly (currently. You might argue that it should).

So Polygon([[np.nan, 0]]) creates a path that would be correct if there were no NaN's there: Path([[np.nan, 0], [np.nan, 0]], [MOVETO, CLOSEPOLY]). The new Path.get_extents blindly adds all possible extremal values to a Bbox.null() (which in this case are all NaN). Adding a NaN to a Bbox is a no-op, (the call chain ends in _path.update_path_extents, which works on a PathNaNRemover<agg:conv_transform<PathIterator>>), so the Bbox stays null, correctly indicating there are no non-NaN points in the polygon.

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brunobeltran commented Jul 21, 2020

CI fail is due to "error allocating resources" on our Mac box?

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jklymak commented Jul 21, 2020

... the mac CI on azure seems to have issues

dstansby
dstansby approved these changes Jul 21, 2020
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Comment thread lib/matplotlib/patches.py Outdated
N, _ = xy.shape
if self._closed:
if len(xy) and (xy[0] != xy[-1]).any():
if N == 1 or N > 1 and (xy[0] != xy[-1]).any():
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Could you add a quick comment about the if statements you've changed just to say what they're doing? And maybe nverts is a better variable name than just N?

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@brunobeltran brunobeltran Jul 21, 2020 •
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I was reticent to add these comments, because technically the checks here are very brittle as-written, so I didn't want to give people the impression this is something which is okay to do in other code.

The checks are bad because we don't guarantee anything about the coordinates of a CLOSEPOLY vertex, but this code uses the assumption that these vertices will "correctly" match the start point of the curve.

In particular, both Python (e.g. Path.iter_bezier) and C++ code (i.e. handlers for agg:path_flags_close) actively ignore this vertex's value, because it's known to hold "trash" sometimes, especially after cleaning NaN arrays, etc (weird stuff like #17885 and #17914).

But, I added comments that make it more clear what the if statements are nominally doing, since you think it's a good idea. I personally like N since it matches what all the Polygon docstrings use, but I agree that nverts is easier to read if you're skimming the code only.

@brunobeltran brunobeltran force-pushed the closepoly-clobbers-poly-vertex branch from e551a27 to 1a1c288 Compare July 21, 2020 14:51
@brunobeltran brunobeltran force-pushed the closepoly-clobbers-poly-vertex branch from 1a1c288 to 0d5c018 Compare July 21, 2020 15:02
@QuLogic QuLogic merged commit 21c3e3f into matplotlib:master Jul 22, 2020
meeseeksmachine pushed a commit to meeseeksmachine/matplotlib that referenced this pull request Jul 22, 2020
@QuLogic @meeseeksmachine
Backport PR matplotlib#17982: BF: for degenerate polygons, add CLOSEP…
07847dd 
…OLY vertex
@meeseeksmachine meeseeksmachine mentioned this pull request Jul 22, 2020
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QuLogic added a commit that referenced this pull request Jul 22, 2020
@QuLogic
Merge pull request #18009 from meeseeksmachine/auto-backport-of-pr-17…
5c8392b 
…982-on-v3.3.x

Backport PR #17982 on branch v3.3.x (BF: for degenerate polygons, add CLOSEPOLY vertex)
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Release critical For bugs that make the library unusable (segfaults, incorrect plots, etc) and major regressions. topic: path handling

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Computing the bounding box of a degenerate polygon throws an error

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