Re: [matplotlib-devel] contourf linewidth=1 help request

[Eric F. <ef...@ha...>]

Jordan,

I have looked into various aspects of the problem you reported.  It 
seems to be essentially a problem with antialiasing; all artifacts go 
away in the AGG backend with antialiasing turned off, regardless of the 
linewidth, right down to zero.  With antialiasing on, however, it seems 
that a linewidth of about 0.5 with AGG (on my machine, with everything 
default) is needed to prevent the antialiasing from leaving light lines 
at polygon boundaries, regardless of whether they are the straight 
boundaries caused by chunking or the curved boundaries of the contours 
themselves.  With Postscript, as rendered by ghostscript via gv, I see 
artifacts only with a much smaller linewidth, and only at some 
magnifications.  Your ps viewers may be doing more antialiasing, and 
therefore showing artifacts over a larger range of linewidths.  There is 
probably more going on, and more to be understood, but this got me far 
enough to manage the problem for now.

It looks like there is no problem with simply turning off the chunking. 
  I have made a bunch of changes in CVS, including turning off the 
chunking by default, and allowing one to specify chunking, antialiasing, 
and linewidth explicitly for contourf.  So, I think you will find that 
if you use the CVS version, the problems you reported will go away.  If 
you haven't done it before, you will find that downloading and 
installing from CVS is very quick and easy; instructions are given on 
the sourceforge download site.  (As John notes, there may be a lag 
between developer CVS and the mirrors; make sure that what you download 
includes my changes, committed a few minutes ago.)  Alternatively, if 
you simply want to turn off chunking in 0.84, you can do it by editing 
cntr.c, approximately line 1384, change

     long nchunk = 30; /* hardwired for now */
to
     long nchunk = 0; /* hardwired for now */

and recompile/reinstall.


If you use the CVS version (or the next mpl release, whenever that comes 
out), note that the APIs for contour, contourf, and clabel have changed 
as described in API_CHANGES and as illustrated in 
examples/contour_demo.py and contourf_demo.py.

Eric

Jordan Dawe wrote:
> Eric Firing wrote:
> 
>> Jordan,
>>
>> Thanks, now I see the problem and where it is coming from.  In 
>> contour.py, near the bottom, in the contourf method, change the 
>> linewidth from 1 to 0.01, so the line reads:
>>             col = PolyCollection(nlist, linewidths=(0.01,))
>>
>> I think this will be OK; based on the comment below the line, I think 
>> I found that using a linewidth of zero was problematic, so I probably 
>> simply used 1 thinking it was small enough not to matter.
>>
>> I will need to make a corresponding change in the CVS version so that 
>> it will get into the next release.  I'll do that this evening.
>>
> Sorry about this, but that patch doesn't seem to work.  When I open the 
> eps files generated with this change implimented, I get the artifact 
> problem mentioned in the comment; fine lines at the boundaries of the 
> contouring engine's subdomains.  In fact, I get the artifacts at 
> linewidths as large as 0.5.
> 
> Jordan