WIP: switch to using pillow for png as well.

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.

- PIL returns uint16 images as int32
  (python-pillow/Pillow#3041); we could adopt
  imageio's dtype correcting code if desired.

.appveyor.yml

@@ -63,7 +63,7 @@ install:
   # https://github.com/conda-forge/conda-forge.github.io/issues/157#issuecomment-223536381
   #
   - conda create -q -n test-environment python=%PYTHON_VERSION%
-    msinttypes freetype=2.6 "libpng>=1.6.21,<1.7" zlib=1.2 tk=8.5
+    msinttypes freetype=2.6 zlib=1.2 tk=8.5
     pip setuptools numpy sphinx tornado
   - activate test-environment
   - echo %PYTHON_VERSION% %TARGET_ARCH%
@@ -85,11 +85,9 @@ test_script:
   # Now build the thing..
   - set LINK=/LIBPATH:%cd%\lib
   - pip install -ve .
-  # these should show no z, png, or freetype dll...
+  # these should show no z or freetype dll...
   - set "DUMPBIN=%VS140COMNTOOLS%\..\..\VC\bin\dumpbin.exe"
   - 'if x%MPLSTATICBUILD% == xTrue "%DUMPBIN%" /DEPENDENTS lib\matplotlib\ft2font*.pyd | findstr freetype.*.dll && exit /b 1 || exit /b 0'
-  - 'if x%MPLSTATICBUILD% == xTrue "%DUMPBIN%" /DEPENDENTS lib\matplotlib\_png*.pyd | findstr z.*.dll && exit /b 1 || exit /b 0'
-  - 'if x%MPLSTATICBUILD% == xTrue "%DUMPBIN%" /DEPENDENTS lib\matplotlib\_png*.pyd | findstr png.*.dll && exit /b 1 || exit /b 0'
 
   # this are optional dependencies so that we don't skip so many tests...
   - if x%TEST_ALL% == xyes conda install -q ffmpeg inkscape miktex pillow

INSTALL.rst

@@ -99,7 +99,7 @@ toolchain is prefixed. This may be used for cross compiling. ::
   export PKG_CONFIG=x86_64-pc-linux-gnu-pkg-config
 
 Once you have satisfied the requirements detailed below (mainly
-Python, NumPy, libpng and FreeType), you can build Matplotlib.
+Python, NumPy, and FreeType), you can build Matplotlib.
 ::
 
   cd matplotlib
@@ -128,7 +128,6 @@ Matplotlib requires the following dependencies:
 
 * `Python <https://www.python.org/downloads/>`_ (>= 3.6)
 * `FreeType <https://www.freetype.org/>`_ (>= 2.3)
-* `libpng <http://www.libpng.org>`_ (>= 1.2)
 * `NumPy <http://www.numpy.org>`_ (>= 1.11)
 * `setuptools <https://setuptools.readthedocs.io/en/latest/>`_
 * `cycler <http://matplotlib.org/cycler/>`_ (>= 0.10.0)
@@ -184,8 +183,8 @@ etc., you can install the following:
    .. _pkg-config: https://www.freedesktop.org/wiki/Software/pkg-config/
 
    If not using pkg-config (in particular on Windows), you may need to set the
-   include path (to the FreeType, libpng, and zlib headers) and link path (to
-   the FreeType, libpng, and zlib libraries) explicitly, if they are not in
+   include path (to the FreeType and zlib headers) and link path (to
+   the FreeType and zlib libraries) explicitly, if they are not in
    standard locations.  This can be done using standard environment variables
    -- on Linux and OSX:
 
@@ -202,8 +201,8 @@ etc., you can install the following:
       set LINK=/LIBPATH:C:\directory\containing\freetype.lib ...
 
    where ``...`` means "also give, in the same format, the directories
-   containing ``png.h`` and ``zlib.h`` for the include path, and for
-   ``libpng.so``/``png.lib`` and ``libz.so``/``z.lib`` for the link path."
+   containing ``zlib.h`` for the include path, and for
+   ``libz.so``/``z.lib`` for the link path."
 
 .. note::
 
@@ -244,20 +243,20 @@ Building on macOS
 -----------------
 
 The build situation on macOS is complicated by the various places one
-can get the libpng and FreeType requirements (MacPorts, Fink,
+can get FreeType (MacPorts, Fink,
 /usr/X11R6), the different architectures (e.g., x86, ppc, universal), and
 the different macOS versions (e.g., 10.4 and 10.5). We recommend that you build
 the way we do for the macOS release: get the source from the tarball or the
 git repository and install the required dependencies through a third-party
 package manager. Two widely used package managers are Homebrew, and MacPorts.
-The following example illustrates how to install libpng and FreeType using
+The following example illustrates how to install FreeType using
 ``brew``::
 
-  brew install libpng freetype pkg-config
+  brew install freetype pkg-config
 
 If you are using MacPorts, execute the following instead::
 
-  port install libpng freetype pkgconfig
+  port install freetype pkgconfig
 
 After installing the above requirements, install Matplotlib from source by
 executing::
@@ -281,7 +280,7 @@ https://packaging.python.org/guides/packaging-binary-extensions/#setting-up-a-bu
 for how to set up a build environment.
 
 Since there is no canonical Windows package manager, the methods for building
-FreeType, zlib, and libpng from source code are documented as a build script
+FreeType and zlib from source code are documented as a build script
 at `matplotlib-winbuild <https://github.com/jbmohler/matplotlib-winbuild>`_.
 
 There are a few possibilities to build Matplotlib on Windows:
@@ -305,9 +304,9 @@ the list of conda packages.
 
 ::
 
-  conda create -n "matplotlib_build" python=3.7 numpy python-dateutil pyparsing tornado cycler tk libpng zlib freetype msinttypes
+  conda create -n "matplotlib_build" python=3.7 numpy python-dateutil pyparsing tornado cycler tk zlib freetype msinttypes
   conda activate matplotlib_build
-  # force the build against static libpng and zlib libraries
+  # force the build against static zlib libraries
   set MPLSTATICBUILD=True
   python setup.py bdist_wheel
 

ci/azure-pipelines-steps.yml

@@ -21,12 +21,8 @@ steps:
     displayName: 'Use latest available Nuget'
 
   - script: |
-      nuget install libpng-msvc14-x64 -ExcludeVersion -OutputDirectory "$(build.BinariesDirectory)"
       nuget install zlib-msvc14-x64 -ExcludeVersion -OutputDirectory "$(build.BinariesDirectory)"
-      echo ##vso[task.prependpath]$(build.BinariesDirectory)\libpng-msvc14-x64\build\native\bin_release
       echo ##vso[task.prependpath]$(build.BinariesDirectory)\zlib-msvc14-x64\build\native\bin_release
-      echo ##vso[task.setvariable variable=CL]/I$(build.BinariesDirectory)\libpng-msvc14-x64\build\native\include /I$(build.BinariesDirectory)\zlib-msvc14-x64\build\native\include
-      echo ##vso[task.setvariable variable=LINK]/LIBPATH:$(build.BinariesDirectory)\libpng-msvc14-x64\build\native\lib_release /LIBPATH:$(build.BinariesDirectory)\zlib-msvc14-x64\build\native\lib_release
 
     displayName: 'Install dependencies with nuget'
 

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

@@ -0,0 +1,5 @@
+Matplotlib now uses Pillow to save and read pngs
+````````````````````````````````````````````````
+
+The builtin png encoder and decoder has been removed, and Pillow is now a
+dependency.

doc/devel/min_dep_policy.rst

@@ -58,7 +58,7 @@ minimum Python and numpy.
 System and C-dependencies
 =========================
 
-For system or c-dependencies (libpng, freetype, GUI frameworks, latex,
+For system or c-dependencies (freetype, GUI frameworks, latex,
 gs, ffmpeg) support as old as practical.  These can be difficult to
 install for end-users and we want to be usable on as many systems as
 possible.  We will bump these on a case-by-case basis.

lib/matplotlib/backend_bases.py

@@ -53,42 +53,37 @@
 from matplotlib.transforms import Affine2D
 from matplotlib.path import Path
 
-try:
-    from PIL import PILLOW_VERSION
-    from distutils.version import LooseVersion
-    if LooseVersion(PILLOW_VERSION) >= "3.4":
-        _has_pil = True
-    else:
-        _has_pil = False
-    del PILLOW_VERSION
-except ImportError:
-    _has_pil = False
 
 _log = logging.getLogger(__name__)
-
 _default_filetypes = {
-    'ps': 'Postscript',
     'eps': 'Encapsulated Postscript',
+    'jpg': 'Joint Photographic Experts Group',
+    'jpeg': 'Joint Photographic Experts Group',
     'pdf': 'Portable Document Format',
     'pgf': 'PGF code for LaTeX',
     'png': 'Portable Network Graphics',
+    'ps': 'Postscript',
     'raw': 'Raw RGBA bitmap',
     'rgba': 'Raw RGBA bitmap',
     'svg': 'Scalable Vector Graphics',
-    'svgz': 'Scalable Vector Graphics'
+    'svgz': 'Scalable Vector Graphics',
+    'tif': 'Tagged Image File Format',
+    'tiff': 'Tagged Image File Format',
 }
-
-
 _default_backends = {
-    'ps': 'matplotlib.backends.backend_ps',
     'eps': 'matplotlib.backends.backend_ps',
+    'jpg': 'matplotlib.backends.backend_agg',
+    'jpeg': 'matplotlib.backends.backend_agg',
     'pdf': 'matplotlib.backends.backend_pdf',
     'pgf': 'matplotlib.backends.backend_pgf',
     'png': 'matplotlib.backends.backend_agg',
+    'ps': 'matplotlib.backends.backend_ps',
     'raw': 'matplotlib.backends.backend_agg',
     'rgba': 'matplotlib.backends.backend_agg',
     'svg': 'matplotlib.backends.backend_svg',
     'svgz': 'matplotlib.backends.backend_svg',
+    'tif': 'matplotlib.backends.backend_agg',
+    'tiff': 'matplotlib.backends.backend_agg',
 }
 
 
@@ -1604,17 +1599,6 @@ class FigureCanvasBase:
     fixed_dpi = None
 
     filetypes = _default_filetypes
-    if _has_pil:
-        # JPEG support
-        register_backend('jpg', 'matplotlib.backends.backend_agg',
-                         'Joint Photographic Experts Group')
-        register_backend('jpeg', 'matplotlib.backends.backend_agg',
-                         'Joint Photographic Experts Group')
-        # TIFF support
-        register_backend('tif', 'matplotlib.backends.backend_agg',
-                         'Tagged Image File Format')
-        register_backend('tiff', 'matplotlib.backends.backend_agg',
-                         'Tagged Image File Format')
 
     @cbook._classproperty
     def supports_blit(cls):