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Eric Firing wrote:
> Ian,
>
> I have applied your patch and modified contourf_demo slightly to illustrate
> interior masking. Thanks very much for the beautiful work! These contour
> bugs that you fixed were major mpl problems--general embarrassments, and
> specific impediments to my own applications, since the data sets I work with
> often have masked interior regions. (And, I also use line contours on top
> of filled contours, so the saddle-point decision fix helps as well.)
I'm glad my contribution passes the quality control checks!
> It occurs to me that there might be a nice refinement: when following a
> masked boundary, how hard would it be to cross the single-cell gap
> diagonally instead of proceeding step-wise along the boundary? In the case
> of the circular masked region that I added to the contourf_demo, this would
> simply smooth out the boundary of that region.
>
> Eric
I think it would be fairly easy to do half a solution to this, but
difficult to do it properly. It would be easy to change the
edge_walker function to miss out a grid point when, for example,
moving clockwise around a masked region from an i-edge to a j-edge.
But what should happen in the situations when a contour level
intersects one of those two edges: either (a) do nothing, or (b) still
do the diagonal cut-off. The do nothing option (a) is easy (!) but
means that there will be a mixture of diagonal cut-offs and stepwise
changes, which won't look particularly elegant, whereas (b) will mean
some pretty serious rewriting as the contouring code will have to deal
with these diagonal edges for both contour lines and filled contours,
and there will have to be some slightly arbitrary interpolation from
the grid z-values to these diagonal edges. So the answer to your
question is "difficult but doable".
My preference is to leave it as it is, as the current blocky solution
is what I expect to see. But I am happy to take a look at it if
you/others think it is a good idea.
On the subject of contouring masked grids, I sometimes want to specify
which grid squares are masked rather than which grid points, i.e. for
a grid of nx by ny points I want to specify a mask of (nx-1) by (ny-1)
squares. I've discovered that cntr.c uses such a square mask,
creating it from the incoming point mask. It would therefore be easy
to add support for such a grid by changing the python front end to
pass it in. Is this a good idea and would this be useful to others,
or am I being overly simplistic?
Ian
P.S. Eric, I see that you work with Kelvin Richards - he was my PhD
supervisor many years ago. Small world!

Neil Crighton wrote:
> Hi,
>
> I posted a patch that makes some small changes to minor tick autoscaling:
>
> http://sourceforge.net/tracker/?
> func=detail&aid=2924245&group_id=80706&atid=560722
>
> If someone could check it's ok and apply it, that would be great.
> 
I can't see the harm, so I applied this in r8082. Also, the patch did
two things. The second thing, "don't create minor ticks on top of
existing major ticks", I pulled out into a second patch and applied in
r8083.

Ian Thomas wrote:
> Hello all,
> 
> I think I have fixed two bugs in the contouring code (src/cntr.c):
> 1) inconsistent behaviour in how contour and contourf handle saddle
> grid squares, and
> 2) incorrect handling of masked regions in filled contour plots.
Ian,
I have applied your patch and modified contourf_demo slightly to 
illustrate interior masking. Thanks very much for the beautiful work! 
These contour bugs that you fixed were major mpl problems--general 
embarrassments, and specific impediments to my own applications, since 
the data sets I work with often have masked interior regions. (And, I 
also use line contours on top of filled contours, so the saddle-point 
decision fix helps as well.)
It occurs to me that there might be a nice refinement: when following a 
masked boundary, how hard would it be to cross the single-cell gap 
diagonally instead of proceeding step-wise along the boundary? In the 
case of the circular masked region that I added to the contourf_demo, 
this would simply smooth out the boundary of that region.
Eric
> 
> Attached is a gzipped tar file containing an explanation of the bugs
> and what I've done, as well as an svn diff against HEAD and small
> example scripts to demonstrate the bugs before and after the fixes.
> 
> I have tested the fixes for the cases I normally come up with, but
> this is by no means exhaustive.
> 
> Ideally it would be good if someone who is more familiar with cntr.c
> than I am could take a look at what I've done and see if it is OK.
> 
> Ian Thomas
> ian...@go...
> 
> 
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Hi,
I posted a patch that makes some small changes to minor tick autoscaling:
http://sourceforge.net/tracker/?
func=detail&aid=2924245&group_id=80706&atid=560722
If someone could check it's ok and apply it, that would be great.
Cheers,
Neil
_______________________________________
http://www.astro.dur.ac.uk/~nhmc