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.

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).

Author: anntzer

.appveyor.yml

@@ -60,7 +60,7 @@ install:
   # if conda-forge gets a new pyqt, it might be nice to install it as well to have more backends
   # https://github.com/conda-forge/conda-forge.github.io/issues/157#issuecomment-223536381
   - conda create -q -n test-environment python=%PYTHON_VERSION%
-    freetype=2.6 "libpng>=1.6.21,<1.7" zlib=1.2 tk=8.5
+    freetype=2.6 zlib=1.2 tk=8.5
     pip setuptools numpy sphinx tornado
   - activate test-environment
   - echo %PYTHON_VERSION% %TARGET_ARCH%
@@ -81,11 +81,9 @@ 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
@@ -121,7 +121,6 @@ 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)
@@ -177,8 +176,8 @@ 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:
 
@@ -195,8 +194,8 @@ 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::
 
@@ -237,20 +236,20 @@ 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::
@@ -274,7 +273,7 @@ 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:
@@ -290,17 +289,16 @@ Wheel builds using conda packages
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 This is a wheel build, but we use conda packages to get all the requirements.
-The binary requirements (png, FreeType,...) are statically linked and therefore
-not needed during the wheel install.
+FreeType is statically linked and therefore not needed during the wheel install.
 
 Set up the conda environment. Note, if you want a qt backend, add ``pyqt`` to
 the list of conda packages.
 
 ::
 
-  conda create -n "matplotlib_build" python=3.7 numpy python-dateutil pyparsing tornado cycler tk libpng zlib freetype
+  conda create -n "matplotlib_build" python=3.7 numpy python-dateutil pyparsing tornado cycler tk zlib freetype
   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/development.rst

@@ -1,2 +1,13 @@
 Development changes
 -------------------
+
+Matplotlib now uses Pillow to save and read pngs
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The builtin png encoder and decoder has been removed, and Pillow is now a
+dependency.  Note that when reading 16-bit RGB(A) images, Pillow truncates them
+to 8-bit precision, whereas the old builtin decoder kept the full precision.
+
+The deprecated wx backend (not wxagg!) now always uses wx's builtin jpeg and
+tiff support rather than relying on Pillow for writing these formats; this
+behavior is consistent with wx's png output.

doc/devel/min_dep_policy.rst

@@ -60,7 +60,7 @@ 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.

examples/misc/image_thumbnail_sgskip.py

@@ -4,10 +4,10 @@
 ===============
 
 You can use matplotlib to generate thumbnails from existing images.
-matplotlib natively supports PNG files on the input side, and other
-image types transparently if your have PIL installed
-
+Matplotlib relies on Pillow_ for reading images, and thus supports all formats
+supported by Pillow.
 
+.. _Pillow: http://python-pillow.org/
 """
 
 # build thumbnails of all images in a directory

lib/matplotlib/animation.py

@@ -543,10 +543,6 @@ def cleanup(self):
 class PillowWriter(MovieWriter):
     @classmethod
     def isAvailable(cls):
-        try:
-            import PIL
-        except ImportError:
-            return False
         return True
 
     def __init__(self, *args, **kwargs):

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):