Re: [Matplotlib-users] plot() not using alpha value from RGBA tuple

[Eric F. <ef...@ha...>]

On 07/25/2011 08:21 AM, Ben Breslauer wrote:
> I think that I have found the problem here.  Line2D.draw() (and I
> presume other Artist subclasses) calls
>
> gc.set_foreground(self._color)
> ...
> gc.set_alpha(self._alpha)
>
> self._color is defined by the color kwarg, whether it be a hex value,
> 3-tuple, 4-tuple, or something else.  self._alpha is defined by the
> alpha kwarg, but if the alpha kwarg is None, it is not overwritten with
> color[3].  Therefore, using color=(R,G,B,A) does not set alpha
> correctly.  I'm not sure the best (i.e. most matplotlib-like) way to
> change this so that the A value gets used iff alpha is None, but I've
> attached a patch that does it one way (diff'ed against the github
> master, commit 67b1cd650f574f9c53ce).  I know the patch is less than
> ideal, as it adds kwarg-specific handling into a place where none had
> previously been done, and it's also in a for loop. Maybe it would be
> better to do the checking along with the scalex and scaley pops?


Alpha handling is a real can of worms; it has gotten better, but as you 
note, there are still problems.  Unfortunately, I don't think your 
proposed solution will work in general, because self._color could be a 
4-letter string, like "gray", in which case alpha would be set to "y", 
and that would not be good at all.  I suspect the right place to solve 
the problem is down in the GraphicsContext--although I thought I had 
already straightened that out some time ago.

As Ben notes, you should file a bug report.
Given that the handling of colors and alpha is inherently tricky (and 
backend-dependent--ps doesn't support alpha at all), solving the problem 
well is going to take a bit of time and care, most likely--and testing 
to make sure that a fix in one place doesn't cause a bug in another.  I 
think we should leave it alone until after the next release.

Eric

>
>
> Separately, I noticed that
> backend_bases.GraphicsContextBase.set_foreground claims to only expect
> an RGB definition, and not an RGBA definition.  As such, I think that it
> should call
>
> self._rgb = colors.colorConverter.to_rgb(fg)
>
> instead of
>
> self._rgb = colors.colorConverter.to_rgba(fg)
>
> since that will make self._rgb a 3-tuple instead of a 4-tuple.
>
> Ben
>
> On Sat, Jul 23, 2011 at 3:13 PM, Ben Breslauer <bbr...@gm...
> <mailto:bbr...@gm...>> wrote:
>
>     Hi,
>
>     I'm trying to fade some data, using alpha values, that I am plotting
>     with Axes.plot().  I can recreate this problem with 1 line of
>     pylab.plot.  If I use
>
>     pylab.plot([1,2,3],[1,4,9], color=(1,0,0,.2), linewidth=7)
>
>     then I get the equivalent of
>
>     pylab.plot([1,2,3],[1,4,9], color=(1,0,0), linewidth=7)
>
>     i.e. it does not use the alpha value.  However, if instead I use
>
>     pylab.plot([1,2,3],[1,4,9], color=(1,0,0), linewidth=7, alpha=.2)
>
>     then the line is faded out appropriately.  The plot documentation
>     indicates that RGBA tuples should be valid, so I'm wondering if this
>     is a bug.  Maybe alpha is defaulting to 1 or None and then not being
>     overwritten if color is a 4-tuple?
>
>     I'm using Arch Linux with kernel 2.6.39, matplotlib 1.0.1 from the
>     Arch repo, and the Qt4 backend.  My installed Qt version is 4.7.3,
>     and I am using KDE 4.6.5.  Below is verbose-debug output.  Thanks
>     for any help!
>
>     Ben
>
>
>     $HOME=/home/ben
>     CONFIGDIR=/home/ben/.matplotlib
>     matplotlib data path
>     /usr/lib/python2.7/site-packages/matplotlib/mpl-data
>     loaded rc file
>     /usr/lib/python2.7/site-packages/matplotlib/mpl-data/matplotlibrc
>     matplotlib version 1.0.1
>     verbose.level debug
>     interactive is False
>     units is False
>     platform is linux2
>     loaded modules: ['heapq', 'numpy.core.info
>     <http://numpy.core.info>', 'distutils', 'numpy.lib.format',
>     'functools', 'pylab', '_bisect', 'subprocess', 'sysconfig', 'gc',
>     'matplotlib.tempfile', 'distutils.sysconfig', 'ctypes._endian',
>     'encodings.encodings', 'pyparsing', 'matplotlib.colors',
>     'numpy.core.numerictypes', 'numpy.testing.sys',
>     'numpy.lib.__future__', 'numpy.fft.types', 'numpy.ma.cPickle',
>     'struct', 'numpy.matrixlib.defmatrix', 'numpy.random.info
>     <http://numpy.random.info>', 'tempfile', 'numpy.compat.types',
>     'pprint', 'numpy.linalg', 'matplotlib.threading',
>     'numpy.core.machar', 'numpy.testing.types', 'numpy.testing',
>     'collections', 'numpy.polynomial.sys', 'unittest.sys',
>     'numpy.core.umath', 'unittest.main', 'distutils.types',
>     'numpy.testing.operator', 'numpy.core.scalarmath', 'numpy.ma.sys',
>     'zipimport', 'string', 'matplotlib.subprocess', 'numpy.testing.os',
>     'unittest.functools', 'numpy.lib.arraysetops',
>     'numpy.testing.unittest', 'numpy.lib.math', 'encodings.utf_8',
>     'matplotlib.__future__', 'unittest.types', 'unittest.util',
>     'numpy.testing.re <http://numpy.testing.re>', 'numpy.version',
>     'numpy.lib.re <http://numpy.lib.re>', 'distutils.re
>     <http://distutils.re>', 'numpy.matrixlib.sys', 'ctypes.os',
>     'numpy.core.os', 'numpy.lib.type_check', 'numpy.compat.sys',
>     'unittest.StringIO', 'bisect', 'numpy.core._internal', 'signal',
>     'numpy.lib.types', 'numpy.lib._datasource', 'random',
>     'numpy.lib.__builtin__', 'numpy.fft.fftpack_lite',
>     'matplotlib.cbook', 'textwrap', 'numpy.core.multiarray',
>     'numpy.polynomial.string', 'distutils.version', 'cStringIO',
>     'numpy.polynomial', 'numpy.numpy', 'matplotlib.StringIO',
>     'unittest.time', 'locale', 'numpy.add_newdocs', 'unittest.difflib',
>     'numpy.core.getlimits', 'base64', '_ssl', 'numpy.lib.sys',
>     'encodings', 'numpy.ma.itertools', 'unittest.pprint', 'unittest.re
>     <http://unittest.re>', 'abc', 'numpy.matrixlib', 'numpy.ctypes',
>     'numpy.testing.decorators', 'matplotlib.warnings', 'rfc822',
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>     'numpy.lib._compiled_base', 'numpy.polynomial.legendre',
>     'threading', 'new', 'numpy.ma.warnings', 'numpy.random.mtrand',
>     'urllib2', 'matplotlib.cPickle', 'math', 'numpy.fft.helper',
>     'fcntl', 'unittest.case', 'matplotlib.numpy', 'UserDict',
>     'unittest.suite', 'numpy.lib.function_base', 'distutils.os',
>     'matplotlib', 'numpy.fft.numpy', 'exceptions', 'numpy.lib.info
>     <http://numpy.lib.info>', 'ctypes', 'numpy.lib.warnings',
>     'ctypes.struct', 'codecs', 'numpy.core._sort', 'numpy.os',
>     'unittest.loader', '_functools', '_locale', 'numpy.__builtin__',
>     'matplotlib.os', 'thread', 'StringIO', 'numpy.core.memmap',
>     'traceback', 'numpy.testing.warnings', 'weakref', 'itertools',
>     'numpy.fft.fftpack', 'numpy.linalg.lapack_lite', 'numpy.ma
>     <http://numpy.ma>', 'distutils.sys', 'os', 'marshal',
>     'numpy.lib.itertools', '__future__', '_collections', 'urllib',
>     'matplotlib.traceback', '_sre', 'unittest', 'numpy.core.sys',
>     'numpy.random', 'numpy.linalg.numpy', '__builtin__',
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>     <http://matplotlib.re>', 'numpy.core.cPickle', 'unittest.runner',
>     'operator', 'numpy.polynomial.polytemplate',
>     'numpy.core.arrayprint', 'distutils.string',
>     'numpy.lib.arrayterator', 'select', 'ctypes._ctypes', '_heapq',
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>     'sre_constants', 'datetime', 'numpy.core.shape_base', 'os.path',
>     'numpy.core.function_base', 'numpy.compat.py3k',
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>     'numpy.polynomial.chebyshev', 'matplotlib.types', 'cPickle',
>     'encodings.__builtin__', 'numpy.polynomial.warnings',
>     'matplotlib.new', '_codecs', 'numpy.lib.operator',
>     'unittest.fnmatch', 'numpy.polynomial.polynomial',
>     'numpy.__config__', 'pwd', 'matplotlib.pyparsing', 'httplib',
>     'numpy.lib.ufunclike', 'copy', 'numpy.core.re
>     <http://numpy.core.re>', '_struct', 'numpy.core.fromnumeric',
>     'hashlib', 'numpy.ctypeslib', 'keyword', 'array',
>     'numpy.lib.scimath', 'numpy.fft', 'numpy.lib', 'numpy.ma.numpy',
>     'numpy.random.numpy', 'matplotlib.urllib2', 'unittest.weakref',
>     'numpy.ma.extras', 'numpy.polynomial.__future__', 'posix',
>     'encodings.aliases', 'matplotlib.fontconfig_pattern', 'fnmatch',
>     'sre_parse', 'pickle', 'numpy.core.ctypes', 'mimetools',
>     'distutils.distutils', 'unittest.signal', 'copy_reg', 'sre_compile',
>     '_hashlib', '_random', '_ctypes', 'site', 'numpy.lib.polynomial',
>     'numpy.compat', 'numpy._import_tools', 'numpy.polynomial.hermite',
>     '__main__', 'numpy.fft.info <http://numpy.fft.info>',
>     'numpy.core.records', 'shutil', 'numpy.lib.cPickle', 'numpy.sys',
>     'numpy.polynomial.hermite_e', 'urlparse', 'unittest.result',
>     'strop', 'grp', 'linecache', 'numpy.polynomial.numpy',
>     'numpy.core.numeric', 'numpy.linalg.info
>     <http://numpy.linalg.info>', 'encodings.codecs', '_abcoll',
>     'numpy.core', 'matplotlib.locale', 'matplotlib.rcsetup',
>     'matplotlib.time', 'matplotlib.weakref', 'genericpath', 'stat',
>     'socket', 'numpy.lib._iotools', 'numpy.lib.index_tricks',
>     'numpy.testing.utils', 'warnings', 'numpy.lib.utils',
>     'numpy.core.defchararray', 'numpy.polynomial.polyutils',
>     'numpy.lib.shape_base', 'numpy.core.types', 'unittest.signals',
>     'sys', 'numpy.core.warnings', 'numpy.compat._inspect',
>     'numpy.core.__builtin__', 'ctypes.ctypes',
>     'numpy.testing.traceback', 'numpy.lib.os',
>     'numpy.testing.nosetester', 'types', 'numpy.lib.shutil',
>     'matplotlib.datetime', 'matplotlib.distutils', '_weakref',
>     'numpy.testing.numpytest', 'difflib', 'distutils.errors',
>     'numpy.matrixlib.numpy', 'matplotlib.matplotlib', '_weakrefset',
>     'matplotlib.shutil', 'numpy.lib.cStringIO', 'unittest.warnings',
>     'time', 'unittest.traceback', 'ssl', 'numpy.linalg.linalg',
>     'unittest.os', 'numpy.testing.numpy']
>     Using fontManager instance from /home/ben/.matplotlib/fontList.cache
>     backend Qt4Agg version 0.9.1
>     findfont: Matching
>     :family=sans-serif:style=normal:variant=normal:weight=normal:stretch=normal:size=medium
>     to Bitstream Vera Sans
>     (/usr/lib/python2.7/site-packages/matplotlib/mpl-data/fonts/ttf/Vera.ttf)
>     with score of 0.000000
>
>
>
>
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