#9529: in graph_plot.py, sometimes get "libpng error: Image width or height is
zero in IHDR"
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Reporter: rlm | Owner: AlexGhitza
Type: defect | Status: new
Priority: major | Milestone:
Component: algebra | Keywords:
Author: | Upstream: N/A
Reviewer: | Merged:
Work_issues: |
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Comment(by cwitty):
To reproduce this fairly quickly, type:
{{{
sage.plot.plot.DOCTEST_MODE = True
while True:
d = DiGraph({}, loops=True, multiedges=True, sparse=True)
d.add_edges([(0,0,'a'),(0,0,'b'),(0,1,'c'),(0,1,'d'),
(0,1,'e'),(0,1,'f'),(0,1,'f'),(2,1,'g'),(2,2,'h')])
GP = d.graphplot(vertex_size=100, edge_labels=True,
color_by_label=True, edge_style='dashed')
GP.set_edges(edge_style='solid')
GP.plot()
}}}
at a sage: prompt.
I spent some time looking into this; here are the results I found before I
gave up.
We draw our plots with bbox_inches='tight', which means:
{{{
Bbox in inches. Only the given portion of the figure is
saved. If 'tight', try to figure out the tight bbox of
the figure.
}}}
There's also a parameter pad_inches:
{{{
Amount of padding around the figure when bbox_inches is
'tight'.
}}}
matplotlib gets its "tight bbox" with the appropriately-named
get_tightbbox() function, in src/lib/matplotlib/figure.py. Its docstring
proclaims,
{{{
It only accounts axes title, axis labels, and axis
ticklabels. Needs improvement.
}}}
and it's true, it does need improvement.
Our graph_plot figure has "empty" axes (no title, labels, or ticklabel),
but it does have an axes object with a size, which gets used for the tight
bbox. I believe that size comes indirectly from get_minmax_data in
sage/plot/plot.py, but I'm not sure.
I can think of a few possibilities to fix the problem, ordered from best
to worst.
1) Fix matplotlib get_tightbbox() to actually be a bounding box for the
drawing, not just for the "axes". This would definitely be best... it
would also fix problems like graphs with loops going off the edge of the
plot, etc.
2) Adjust our get_minmax_data functions to be closer to a correct bounding
box. For instance, currently get_minmax_data on a Text object only looks
at the center of the text, and doesn't take the size into account.
There's a limit to how good a job we could do, though... we don't want to
start digging through font metrics, etc.
3) Set pad_inches to make all our plots "a little bit bigger". The
downside, of course, is that this simple approach would affect *all* our
plots.
Like I said, I'm giving up now.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/9529#comment:3>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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