I recently noticed that setting the dpi for savefig doesn't work as expected
when saving to pdf. Take the following code, for example:
>>> import matplotlib.pyplot as plt
>>>
>>> plt.figure(figsize=(8,6))
>>> plt.plot([1,2])
>>> plt.savefig('test.png', dpi=100)
>>> plt.savefig('test.pdf', dpi=100)
The resulting png file is 800 x 600 (as expected), while the pdf file is 576 x
432 [which is (800 x 600) * 72/100]. I found an old thread suggesting that a
dpi of 72 should be hard coded into the PDF renderer for compatibility with
older versions of the PDF spec. That makes sense; however, it'd be nice if the
docstring for savefig told users about his behavior.
Below, is a patch to the savefig docstring. I'm sure someone else could word
this better, but I thought I'd at least try.
Best,
-Tony
P.S. maybe enough time has passed that most people have adopted PDF
viewers/parsers using PDF >= 1.6, and this hard-coded dpi could be removed?
Just a thought.
Index: lib/matplotlib/figure.py
===================================================================
--- lib/matplotlib/figure.py (revision 8561)
+++ lib/matplotlib/figure.py (working copy)
@@ -1018,7 +1018,10 @@
*dpi*: [ None | scalar > 0 ]
The resolution in dots per inch. If *None* it will default to
- the value ``savefig.dpi`` in the matplotlibrc file.
+ the value ``savefig.dpi`` in the matplotlibrc file. NOTE: when
+ saving to pdf, the dpi will not affect the page dimensions (which
+ is always 72 dpi), but it will affect the resolution of rasterized
+ elements in the plot.
*facecolor*, *edgecolor*:
the colors of the figure rectangle
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