Joseph Smidt wrote:
> I am making contourf plots with some transparency as I want to
> eventually overlay such plots. When I plot such plots I see artifacts
> or lines that shouldn't be there as seen here:
> http://josephsmidt.googlepages.com/ex.png
>
> Is this a bug or am I plotting incorrectly? My script is here:
> http://josephsmidt.googlepages.com/plotex.py and the data needed for
> the plot is here: http://josephsmidt.googlepages.com/exCHI.txt
>
> However, the relevant part of the code for plotting I'll just cut and
> paste here:
>
> levels = [x0, x1, x2, x3]
> contourf(Y,X,transpose(CHI),levels,alpha=0.7)
> show()
>
> Thanks.
>
> Joseph Smidt
>
>
This is a problem with two parts: first, the filled-contour algorithm
creates singly-connected patches, so cuts are required (the horizontal
lines); second, depending on whether edges are stroked and on the
antialias setting, these cuts, and the boundaries between regions, can
show up as artifacts when rendered. The rendering problem can be
backend-dependent.
When I make a contourf plot using the agg backend (which is presumably
what you are using to make the png file--either directly or indirectly
via an interactive backend), I am not seeing the artifacts, so I think
the default settings in svn are good for agg rendering with alpha. My
guess is that you are using a version of mpl with different settings. We
have struggled with this problem, trying different combinations of
settings, for a long time. There doesn't seem to be a formula that works
for all backends and all alpha values.
I suspect that in the version of mpl you are using, the patch edges are
being stroked. As a workaround, try this:
cs = contourf(Y,X,transpose(CHI),levels,alpha=0.7)
for c in cs.collections:
c.set_edgecolors(['none'])
Eric
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