[matplotlib-devel] Font issues

[Paul B. <ba...@st...>]

Hi John,

I'm trying to finish off the basic features of the font manager module, so we 
can get the next release out.  I'm currently working on the fonts_demo.py 
example, which has brought to light some deficiencies in the basic 
implementation of font_manager.  I would like to get your opinion on the 
following issues:

1. Currently, the relative font size strings ('smaller' and 'larger') are not 
implemented.  This is because these sizes depend on the parent font size: a 
feature which is not yet implemented.  Note that the font_manager only has a 
default_size, which is used by the absolute font size strings (xx-small, 
x-small, small, medium, etc.).

For this to work, it would seem that each FontProperties instance would need a 
pointer to its parent FontProperties instance.  When get_size_in_points() is 
called, it would asks its parent font what size it is in order to determine its 
absolute size.  If the parent is also a relative size, then the parent would ask 
its parent what size it is, etc.  This would handle the case where set_size() is 
called twice using a relative font size.

What do you think of this approach?  It has its problems, mainly with the 
possibility of circular references, but checks for such situations could be 
added in later.  If this feature (of relative font sizes) isn't that important, 
then we could defer it until after the release.

2. The above problem has also highlighted an issue with the current font manager 
design.  Currently, the fontManager class has attributes of default_size and 
default_weight that are set at initialization.  If we pulled these attributes 
out of the class and replaced them with a default FontProperties instance, then 
this instance could be used as the root parent FontProperties instance.  What do 
you think?

Also, since there is often only one instance of the fontManager class, this 
class could be eliminated and be replace with some initialization code and a 
findfont() function, unless you think that several instances of this class might 
be used in future versions of matplotlib.

3. Finally, Perry and I were discussing how one might use Traits in matplotlib. 
  The Traits module is being used in the Chaco plotting package for checking the 
type and value of class attributes, such as the color and size of fonts or the 
color and width of lines.  (See the Chaco page at the ScyPy web site for an 
overview.)  It would appear that matplotlib's rcParams has a similar purpose, 
though on a more limited scale than Traits.  I think it should still be possible 
to have the matplotlibrc file for customization, and to feed this information to 
the Traits module on start up.  Traits may not be able to handle the link 
attribute problem discussed above, but this can probably be implemented 
separately if necessary.

Note that all these issues are not critical and could be deferred until after 
the release.

Cheers,
Paul

-- 
Paul Barrett, PhD      Space Telescope Science Institute
Phone: 410-338-4475    ESS/Science Software Branch
FAX:   410-338-4767    Baltimore, MD 21218