[matplotlib-devel] a patch to have a correct baseline when usetex=True

[Jae-Joon L. <lee...@gm...>]

Hello,

AFAIK, current backends (I only tested agg, pdf, and ps) do not
properly respect the text baseline when text is rendered using TeX.
The get_text_width_height_descent() method in Agg and PS backends
simply return 0 for the descent value. While PDF backend uses the
dviread module to figure out correct descent values, there are cases
this does not work well (e.g. $\frac{1}{2}\pi$).

As an example, the attached figure shows the result for the Agg
backend. In all cases, the texts are placed at (0,0) with
baseline-alignment. Leftmost one is when usetex=False, which has a
correct baseline. The middle one is when usetex=True. It is bottom
aligned, which is not intended. The rightmost one is also when
usetex=True but after the patch I describe below.

First of all, I borrowed this idea from the PyX which is in GPL.
Although there is little of copying, other than the basic idea, I'm
not 100% sure if this could be BSD-compatible.

Anyhow, the idea is that you can have LateX to print out the width,
height, and descent (=depth) of a given text by enclosing the text in
a box. For example,

 \newbox\MatplotlibBox%
 \setbox\MatplotlibBox=\hbox{$\frac{1}{2}\pi$}%
 \copy\MatplotlibBox
 \immediate\write16{MatplotlibBox:\the\wd\MatplotlibBox,\the\ht\MatplotlibBox,\the\dp\MatplotlibBox}%

I define a newbox (called MatplotlibBox) which encloses
$\frac{1}{2}\pi$. And then print out the width, height and depth of
the box.

Attached is a patch of a texmanager.py which utilize above method to
figure out the dimension of the text. The template string to generate
a  ".tex" file is slightly modified. After latex is run, the
dimensional information  of the text is extracted and saved in
".baseline" file and get_text_width_height_descent() method is added
under the TexManager class, which reads in the ".baseline" file and
return its content. (you need to empty out the tex.cache directory for
this work correctly).

A backend can simply call the get_text_width_height_descent() of
texmanager (a simple patch for the Agg backend is attached). I also
tested this with PS and PDF backends and they worked out fine.

So if the license issue is okay, I wonder if this patch can be
reviewed and applied (after any necessary modifications) to improve
the baseline handling in matploltib.

Regards,

-JJ