matplotlib-devel Mailing List for matplotlib

When using a vectorial backend like PS or PDF, I noticed tick marks
and grid lines are not perfectly aligned.
Here is a little script revealing the problem:
 import os
 from pylab import *
 rc('axes', linewidth=0.5, axisbelow=True)
 rc('xtick', direction='out')
 rc('ytick', direction='out')
 t = arange(0.0, 2.0, 0.01)
 s = sin(2*pi*t)
 plot(t, s, linewidth=1.0)
 title('Tick marks, grid lines and other marks should be aligned')
 xlabel('time (s)')
 ylabel('voltage (mV)')
 grid(True, linestyle='-', color='#d3d3d3')
 axhline(0.5, linewidth=0.5)
 plot([0.5], [0.5], '+')
 plot([1.0], [0.5], marker=0)
 savefig('testchart.eps')
 savefig('testchart.png')
 show()
The attached file testchart-misaligned.eps shows the result with the
PS backend: tick marks and grid lines are not perfectly aligned, which
is bad; but the "+" mark placed in the plot is correctly aligned with
the grid lines, as expected.
In the attached file testchart-misaligned.png, produced with the Agg
backend, this is the opposite: tick marks and grid lines are perfectly
aligned, but the "+" mark is not exactly over the grid lines.
I think this is caused by a workaround used in lines.py, in methods
_draw_tickleft, _draw_tickright, _draw_tickup and _draw_tickdown,
consisting in adding 0.5 to marks coordinates in order to place them
in pixel center. This is good for bitmap backends like Agg, but it's
bad for vectorial backends like PS, PDF and SVG. I propose to remove
this behavior from lines.py and move it in _backend_agg.cpp.
The attached file alignment.patch is a patch implementing this proposition.
The files testchart_aligned.eps and testchart_aligned.png show the
results with the PS and Agg backends after applying the patch:
everything is aligned.
Do you agree to apply this patch, or have I misunderstood something? :-)
Cheers,
-- Nicolas Grilly