Re: [matplotlib-devel] Is this a bug in LineCollection's handling of alpha values?

[John H. <jd...@gm...>]

On Fri, Jun 5, 2009 at 9:56 PM, Fernando Perez<fpe...@gm...> wrote:

> Hopefully the code below is illustrative and commented enough to
> clarify my question (also attached if you prefer to download it than
> to copy/paste).

Hey Fernando -- thanks for the report and test case.

I committed a change to svn which fixes this -- I'd like one of the
color gurus (Eric?) to take a look at this because the color handling
code is fairly complex as it handles a lot of different cases.  The
problem here was that the ColorConverter.to_rgba_array was applying
the alpha even though the input array was rgba already.  I special
case this and do not convert when the input is already an Nx4 array.
Are there any cases I am missing?  See the inline comment below:

    def to_rgba_array(self, c, alpha=None):
        """
        Returns a numpy array of *RGBA* tuples.

        Accepts a single mpl color spec or a sequence of specs.

        Special case to handle "no color": if *c* is "none" (case-insensitive),
        then an empty array will be returned.  Same for an empty list.
        """
        try:
            if c.lower() == 'none':
                return np.zeros((0,4), dtype=np.float_)
        except AttributeError:
            pass
        if len(c) == 0:
            return np.zeros((0,4), dtype=np.float_)
        try:
            result = np.array([self.to_rgba(c, alpha)], dtype=np.float_)
        except ValueError:
            if isinstance(c, np.ndarray):
                if c.ndim != 2 and c.dtype.kind not in 'SU':
                    raise ValueError("Color array must be two-dimensional")
                if len(c.shape)==2 and c.shape[-1]==4:
                    # looks like rgba already, nothing to be done; do
                    # we want to apply alpha here if
                    # (c[:,3]==1).all() ?
                    return c
            result = np.zeros((len(c), 4))
            for i, cc in enumerate(c):
                result[i] = self.to_rgba(cc, alpha)  # change in place
        return np.asarray(result, np.float_)

JDH