Errors in rendering boundaries between filled paths and within a quadmesh

[efiring]

This is the underlying problem raised in #1178.
It is illustrated by the test below; note that boundary anomalies are visible in all forms--agg on the screen, and pdf and svg displayed with a viewer--but in different places depending on the viewer and the size of the figure as rendered.
Note that the colorbar is rendered using pcolormesh, which has its own renderer with agg but otherwise is handled by draw_path_collection.

import numpy as np
import matplotlib.pyplot as plt

z = np.arange(150)
z.shape = (10,15)

fig, axs = plt.subplots(2,2)

ax = axs[0,0]
cs0 = ax.contourf(z, 20)
cbar0 = fig.colorbar(cs0, ax=ax)

ax = axs[0,1]
cs1 = ax.contourf(z, 20, alpha=0.3)
cbar1 = fig.colorbar(cs1, ax=ax)

ax = axs[1,0]
im2 = ax.imshow(z, interpolation='nearest')
cbar2 = fig.colorbar(im2, ax=ax)

ax = axs[1,1]
im3 = ax.imshow(z, interpolation='nearest', alpha=0.3)
cbar3 = fig.colorbar(im3, ax=ax)

plt.savefig("test1.pdf")
plt.savefig("test1.svg")
plt.show()

[dmcdougall]

If you start the code block with ```python you'll get some syntax highlighting.

[jenshnielsen]

To me it seems like the pdf backend draws them correctly without addition edges however some viewers render these incorrectly and adds some white space. In the agg backend however even without edges there is an overlap between the paths visible only when alpha is less than 1. I guess this is an unrelated bug in the agg drawing code. It is correctly called with no edges. The pngs from the cairo backends are drawn without any overlap. However it does seem like this backend does not support alpha for imshow. I will open a separate issue for this.

[jenshnielsen]

The pdf problem in evince is probably equivalent to this one
https://bugs.launchpad.net/ubuntu/+source/poppler/+bug/318130
which was reported in 2009

[jenshnielsen]

Indeed the pdf problem in evince is fixed by applying the patch to poppler linked to in that bug report (0001-Align-fills-to-device-integer-coordinates.patch) so it seems like an aliasing problem.

[dmcdougall]

Did we decide that this is a viewer problem? If so, should this be closed as it requires an upstream fix?

[jenshnielsen]

No there is another issue. The agg backend still renders the colorbar with overlap like the pdf did
before meaning that the colorbar with alpha < 1 will look wrong in png. Furthermore alpha is ignored for images (#1193) in the cairo backend so it is not actually possible to render png colorbars with alpha < 1 correctly.