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DOC: Update color tutorial to explain alpha #9959

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Dec 27, 2017
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DOC: Update color tutorial to explain alpha #9959

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15 changes: 14 additions & 1 deletion tutorials/colors/colors.py
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Expand Up @@ -6,7 +6,7 @@
Matplotlib recognizes the following formats to specify a color:

* an RGB or RGBA tuple of float values in ``[0, 1]`` (e.g., ``(0.1, 0.2, 0.5)``
or ``(0.1, 0.2, 0.5, 0.3)``);
or ``(0.1, 0.2, 0.5, 0.3)``). RGBA is short for Red, Green, Blue, Alpha;
* a hex RGB or RGBA string (e.g., ``'#0F0F0F'`` or ``'#0F0F0F0F'``);
* a string representation of a float value in ``[0, 1]`` inclusive for gray
level (e.g., ``'0.5'``);
Expand All @@ -23,6 +23,19 @@
the indexing occurs at artist creation time and defaults to black if the
cycle does not include color.

"Red", "Green" and "Blue", are the intensities of those colors, the combination
of which span the colorspace.

How "Alpha" behaves depends on the ``zorder`` of the Artist. Higher
``zorder`` Artists are drawn on top of lower Artists, and "Alpha" determines
whether the lower artist is covered by the higher.
If the old RGB of a pixel is ``RGBold`` and the RGB of the
pixel of the Artist being added is ``RGBnew`` with Alpha ``alpha``,
then the RGB of the pixel is updated to:
``RGB = RGBOld * (1 - Alpha) + RGBnew * Alpha``. Alpha
of 1 means the old color is completely covered by the new Artist, Alpha of 0
means that pixel of the Artist is transparent.

All string specifications of color, other than "CN", are case-insensitive.

"CN" color selection
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