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pyplot.plot conflict: markerfacecolor ='none' and alpha #11104

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zsunpku opened this issue Apr 22, 2018 · 4 comments · Fixed by #11105
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pyplot.plot conflict: markerfacecolor ='none' and alpha #11104

zsunpku opened this issue Apr 22, 2018 · 4 comments · Fixed by #11105
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topic: color/alpha
Milestone
v2.2.3

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@zsunpku
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zsunpku commented Apr 22, 2018

Bug report

Bug summary
I tried to use a previously-works-fine code to make a plot but failed to reproduce the same one -- after updating matplotlib to 2.2. The hollow circles in plot (defined by markerfacecolor='None') are now in black facecolor. After manually locating the error, I found that the keyword 'alpha' seems conflict to markerfacecolor = 'None' which never happened before. This conflict will only affect facecolor set as 'None'.

Code for reproduction

import matplotlib.pyplot as plt
# This works properly
plt.figure()
plt.plot([1,2,3],[1,2,3],'o',c='g',markeredgecolor='g',markersize = 10,markerfacecolor='r',linewidth=2,alpha=1)
plt.show()
# markerfacecolor='none' conflicts with alpha keyword
plt.figure()
plt.plot([1,2,3],[1,2,3],'o',c='g',markeredgecolor='g',markersize = 10,markerfacecolor='none',linewidth=2,alpha=1)
plt.show()
# markerfacecolor='none' conflicts with alpha keyword, and this happens on different alphas
plt.figure()
plt.plot([1,2,3],[1,2,3],'o',c='g',markeredgecolor='g',markersize = 10,markerfacecolor='none',linewidth=2,alpha=0.5)
plt.show()
# remove alpha keyword and markerfacecolor='none' works well
plt.figure()
plt.plot([1,2,3],[1,2,3],'o',c='g',markeredgecolor='g',markersize = 10,markerfacecolor='none',linewidth=2) # no alpha here
plt.show()

Actual outcome

![1](https://user-images.githubusercontent.com/13745875/39099965-ee512c2c-4650-11e8-9cc4-969ebe75ac2a.png)
![2](https://user-images.githubusercontent.com/13745875/39099966-ee604bb2-4650-11e8-9245-d846bbed9523.png)
![3](https://user-images.githubusercontent.com/13745875/39099968-ee6aa2f6-4650-11e8-8420-bde423460955.png)
![4](https://user-images.githubusercontent.com/13745875/39099969-ee73fbbc-4650-11e8-9358-72e00202e539.png)

Matplotlib version

  • Operating system: Windows 10 x64
  • Matplotlib version: 2.2
  • Matplotlib backend (print(matplotlib.get_backend())): both terminal ipython (Qt5Agg) and jupyter notebook (module://ipykernel.pylab.backend_inline)
  • Python version: 3.6.2
  • Jupyter version (if applicable): 4.3.0
  • Other libraries: N/A

matplotlib and python installed by conda

channel by default

Thanks a lot!
@ImportanceOfBeingErnest
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ImportanceOfBeingErnest commented Apr 22, 2018 •
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This is reproducible on current master. print(line.get_markerfacecolor()) prints "none" correctly, so this might happen at draw time?!

@anntzer
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anntzer commented Apr 22, 2018

Bisects to #9911. Humpf...

@anntzer
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anntzer commented Apr 22, 2018

After some investigation, turns out that the signature

    def draw_markers(self, gc, marker_path, marker_trans, path,
                     trans, rgbFace=None):

indeed requires rgbFace to possibly be None. The face color is converted per

    if (!convert_face(faceobj, gc, &face)) {
        return NULL;
    }

and convert_face explicitly checks for None at

int convert_face(PyObject *color, GCAgg &gc, agg::rgba *rgba)
{
    if (!convert_rgba(color, rgba)) {
        return 0;
    }

    if (color != NULL && color != Py_None) {  // <-- HERE.
        if (gc.forced_alpha || PySequence_Size(color) == 3) {
            rgba->a = gc.alpha;
        }
    }

    return 1;
}

Looks like the "easy" fix is to manually revert the relevant parts of #9911.

@anntzer
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anntzer commented Apr 23, 2018

Actually I have a better fix coming.

@anntzer anntzer mentioned this issue Apr 23, 2018
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@tacaswell tacaswell added this to the v2.2.3 milestone Apr 30, 2018
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When drawing markers, don't set the GraphicsContext alpha. anntzer/matplotlib
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@tacaswell @anntzer @zsunpku @ImportanceOfBeingErnest