Switch to using pillow for png as well. #15193

anntzer · 2019-09-05T12:44:11Z

PR Summary

Probably needs discussion as to whether the loss of precision for reading 16-bit RGB(A) pngs is a blocking issue or not.

The version check in _has_pil (pillow>=3.4) was dropped as the oldest
Pillow that supports Py3.6 (the oldest Python supported by Matplotlib)
is 4.0 anyways.

Some complications arise from an unfortunate design decision that
imread() returns pngs in 0-1 float dtype but others formats in integer
dtype (via Pillow), and from Pillow having not-so-great support for
16-bit images (e.g. Pillow#3041).

This makes reading of 16-bit RGB(A) PNGs (which are exposed as 0-1
floats by imread()) slightly less accurate, because Pillow coarsens them
to 8-bit first (by truncation); hence the slight change in the
pngsuite.png baseline image. (Note that the renderers only have 8-bit
precision anyways (unless using mplcairo with cairo master...), but the
discrepancy comes from differences in rounding between Pillow and
Matplotlib.)

I have a followup PR that makes it possible to build Matplotlib on Windows on a clean python.org python with just MSVC installed (by setting MPLLOCALFREETYPE=1).

Edit: This previously depended on #15175, but is now independent of it.

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/api_changes.rst if API changed in a backward-incompatible way

tacaswell

Modulo the 16bit read-back being document as an API change.

efiring · 2019-09-24T19:19:39Z

Looks good to me, in general terms.

anntzer · 2019-09-24T19:23:19Z

API change note about precision loss added.

lib/matplotlib/backends/backend_svg.py

lib/matplotlib/backends/backend_webagg_core.py

QuLogic · 2019-09-26T10:33:27Z

lib/matplotlib/backends/backend_wx.py

-            return self._print_image(filename, wx.BITMAP_TYPE_JPEG,
-                                     *args, **kwargs)
-        print_jpg = print_jpeg
+    def print_jpeg(self, filename, *args, **kwargs):


Doesn't the condition mean this function should be removed?

no, the wx backend (not wxagg!) has always relied on wx's native handling of png/tiff/jpeg/...

That's not what the removed code says.

If I understand this correctly, @QuLogic is right, but the API change is now documented.

QuLogic · 2019-09-30T20:56:47Z

lib/matplotlib/backends/backend_wx.py

-            return self._print_image(filename, wx.BITMAP_TYPE_TIF,
-                                     *args, **kwargs)
-        print_tif = print_tiff
+    def print_tiff(self, filename, *args, **kwargs):


And this one too.

QuLogic · 2019-09-30T23:51:28Z

lib/matplotlib/image.py

-            return pil_to_array(image)
-    from matplotlib import _png
+    img_open = (
+        PIL.PngImagePlugin.PngImageFile if ext == 'png' else PIL.Image.open)


Do we even need this, or can we just do everything through plain open? (I understand that the _pil_png_to_float_array bit is for backwards compatibility.)

No, the point is to force pillow to open the image as png if ext == "png", regardless of what pillow may think the image is.

anntzer · 2019-10-03T14:03:12Z

rebased

timhoffm · 2019-10-03T16:04:56Z

lib/matplotlib/backends/backend_webagg_core.py

+            buf = BytesIO()
+            (Image.fromarray(
+                output.view(dtype=np.uint8).reshape((*output.shape, 4))
+             .save(buf, format="png")))


Just style but rather:

data = output.view(dtype=np.uint8).reshape((*output.shape, 4)) Image.fromarray(data).save(buf, format="png")

timhoffm · 2019-10-03T16:25:13Z

lib/matplotlib/backends/backend_wx.py

-            return self._print_image(filename, wx.BITMAP_TYPE_JPEG,
-                                     *args, **kwargs)
-        print_jpg = print_jpeg
+    def print_jpeg(self, filename, *args, **kwargs):


If I understand this correctly, @QuLogic is right, but the API change is now documented.

timhoffm · 2019-10-03T16:25:47Z

doc/api/next_api_changes/2019-09-04-AL.rst

@@ -0,0 +1,10 @@
+Matplotlib now uses Pillow to save and read pngs


Please adapt to the new way of documentation API changes.

The version check in _has_pil (pillow>=3.4) was dropped as the oldest Pillow that supports Py3.6 (the oldest Python supported by Matplotlib) is 4.0 anyways. Some complications arise from an unfortunate design decision that imread() returns pngs in 0-1 float dtype but others formats in integer dtype (via Pillow), and from Pillow having not-so-great support for 16-bit images (e.g. Pillow#3041). This makes reading of 16-bit RGB(A) PNGs (which are exposed as 0-1 floats by imread()) slightly less accurate, because Pillow coarsens them to 8-bit first (by truncation); hence the slight change in the pngsuite.png baseline image. (Note that the *renderers* only have 8-bit precision anyways (unless using mplcairo with cairo master...), but the discrepancy comes from differences in rounding between Pillow and Matplotlib).

timhoffm

Anybody can merge after CI pass.

anntzer · 2019-10-04T08:34:49Z

self-merging per the above.

anntzer added the status: waiting for other PR label Sep 5, 2019

anntzer force-pushed the pillowpng branch 2 times, most recently from bcd1d5d to f41b156 Compare September 9, 2019 12:39

anntzer mentioned this pull request Sep 9, 2019

Simplify pdf image output. #15175

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anntzer force-pushed the pillowpng branch from f41b156 to f2f5032 Compare September 9, 2019 19:59

anntzer removed the status: waiting for other PR label Sep 9, 2019

anntzer force-pushed the pillowpng branch from f2f5032 to 3db30be Compare September 17, 2019 19:49

tacaswell added this to the v3.3.0 milestone Sep 24, 2019

tacaswell approved these changes Sep 24, 2019

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anntzer force-pushed the pillowpng branch from 3db30be to 92ddb3d Compare September 24, 2019 19:23

anntzer force-pushed the pillowpng branch from 92ddb3d to 5f9d583 Compare September 30, 2019 08:27

QuLogic reviewed Oct 1, 2019

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anntzer force-pushed the pillowpng branch 3 times, most recently from 113fbe9 to 9f9f93f Compare October 1, 2019 13:52

timhoffm added the status: needs rebase label Oct 3, 2019

anntzer force-pushed the pillowpng branch from 9f9f93f to 3d0cb93 Compare October 3, 2019 14:03

anntzer removed the status: needs rebase label Oct 3, 2019

anntzer force-pushed the pillowpng branch 2 times, most recently from 3a7407b to f0bcdff Compare October 3, 2019 14:53

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anntzer force-pushed the pillowpng branch from f0bcdff to 370e9a2 Compare October 3, 2019 20:40

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anntzer merged commit be978a3 into matplotlib:master Oct 4, 2019

anntzer deleted the pillowpng branch October 4, 2019 08:34

anntzer mentioned this pull request Oct 15, 2019

Use correct pip/pytest on azure #15418

Merged

timhoffm mentioned this pull request Oct 29, 2019

Cleanup following using Pillow as universal image reader #15571

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jklymak mentioned this pull request Jul 30, 2020

Error reading png image from URL with imread in matplotlib 3.3 #18129

Closed

kinshukdua mentioned this pull request Oct 4, 2021

ENH: Add support to save images in WebP format #21274

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Switch to using pillow for png as well. #15193

Switch to using pillow for png as well. #15193

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anntzer commented Oct 4, 2019

		@@ -0,0 +1,10 @@
		Matplotlib now uses Pillow to save and read pngs

Switch to using pillow for png as well. #15193

Switch to using pillow for png as well. #15193

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anntzer commented Sep 5, 2019 • edited Loading

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