Re: [matplotlib-devel] Unavoidable gridlines in pcolor pdf output

[Jouni K. S. <jk...@ik...>]

Benjamin Root <ben...@ou...> writes:

>> It seems that savig a pcolor plot to a pdf format always includes
>> gridlines, which isn't true for other output formats like png.  The
>> attached example demonstrates this problem.  I've only tested this on
>> version 1.1.1rc1.
>
> I have reported something like this before, and it seems that it is
> dependent upon the PDF viewer and i's settings.  Particularly, the
> antialiasing settings.  I have seen this with gs-based viewers. Have you
> tried others?

I see gridlines in the resulting image in gs, xpdf Preview.app but not
in Adobe Reader. When I zoom in Preview, the lines jump around a bit,
and are always the same width on the screen regardless of zoom level.

What gets drawn in this example is a lot of polygons, so that adjacent
polygons share an edge with the exact same coordinates. The code fills
the inside of each polygon, and apparently some rendering algorithms
leave a minimal-width line between polygons.

Making the polygons overlap by one pdf unit (1/72 of an inch) seems to
make the lines go away at least in Preview. Making them overlap by 0.1
units does not help.

I don't think this can be fixed purely in the pdf backend; it just gets
a path collection and draws each polygon. I suppose pcolor would need to
pad the polygons a little, but could it cause problems with other
backends or use cases? (E.g. if you use alpha blending, neighboring
rectangles might get gridlines of a darker color where they overlap.)

I wonder if drawing zero-width lines between neighboring rectangles,
e.g. taking the color always from the left or the upper rectangle, would
work. In matplotlib a line of zero width means no line at all, but in
the pdf file format it means a line of the smallest possible width,
which might work well for filling in a missing line of that same
width. This would need some kind of special-casing in the graphics
context (because linewidth=0 means to not draw a line) so it's not
completely trivial to try it out.

A different approach would be to output a raster image from pcolor and
render it with large pixels. I don't know how well that would work with
Agg, though.

-- 
Jouni K. Seppänen
http://www.iki.fi/jks