[Matplotlib-users] Pcolormesh vs contourf

Eric Firing efiring at hawaii.edu
Sat Jan 7 14:57:28 EST 2017

On 2017/01/07 9:24 AM, Sameer Grover wrote:
> On 6 January 2017 at 06:50, Eric Firing <efiring at hawaii.edu
> <mailto:efiring at hawaii.edu>> wrote:
>     Sameer,
>     They are fundamentally different in what they do; this is most clear
>     if you experiment with using both methods on a low-resolution field
>     (say 10 by 12 points), and with a small number of contours (5 or
>     10). Contouring is most typically done with a moderate number of
>     contours, maybe 30 or fewer.  It is most immediately appropriate for
>     data values at discrete points, since it is interpolating.
>     pcolormesh does not interpolate, but shows the value in each
>     quadrilateral as a block of solid color, so it is more like an image.
>     There are data sets and situations for which either of the two
>     methods can be appropriate, and other cases where one is clearly
>     better than the other.  For a somewhat noisy field, pcolormesh is
>     usually better; it allows one to see the signal and the noise, and
>     let one's eye pick out the former.  Isolated extreme values are
>     represented better with pcolormesh.  For smoother fields and for
>     seeing large-scale structure, contourf may be more appropriate.
>     What kind of data are you plotting?
>     Eric
>     On 2017/01/05 2:40 PM, Sameer Grover wrote:
>         Hello,
>         I'd like to know about the difference between contourf and
>         pcolormesh
>         and their intended uses. So far, I've been using contourf with a
>         large
>         number of levels (150 - 200) to plot two dimensional data. Are
>         their any
>         disadvantages of this and is pcolormesh better suited for this task?
>         As far as I can make out, contourf displays a "smoothened-out"
>         image and
>         pcolormesh is more "boxy".
>         Thanks.
>         Sameer
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> Hi Eric,
> The data is usually around 1000 X 1000 large or smaller, mainly
> experimental or theoretical data obtained by varying two independent
> parameters. With a large number of contours, the visual output from them
> doesn't look too different, especially if pcolormesh is used with
> gouraud shading. But I realize they do things differently.  Thank you
> for the clarification.
> My only comment is that contourf(x,y,z) expects z of shape(size(y),
> size(x)) whereas pcolormesh (x,y,z) expects z of shape (size(y)+1,
> size(x)+1), for obvious reasons.
> It would be nice if pcolormesh could adjust x and y so that the centres
> of each quadrilateral is at the value of x and y, so that both functions
> could be called in the same way.

If x and y are uniformly spaced, then imshow does that (and much 
faster), with suitable specification of extent, aspect, and interpolation.

I view the way pcolormesh handles x and y as fundamental, so I am 
reluctant to add an option to interpolate/extrapolate a pixel-centered 
grid to the the edge grid that pcolormesh absolutely requires.  In the 
simplest cases, such as a uniform rectangular grid, such a 
transformation is straightforward, but in the more general cases that 
pcolormesh handles, it is not; there is no single algorithm that would 
"do the right thing" in all cases.

There is scope for making it easier to do common things, but rather than 
fold it into functions like pcolormesh, I think it is better to break it 
out into standalone grid manipulation functions.

We also have a NonuniformImage class that handles irregular (and 
regular) rectangular pixel-centered grids, and can be used directly. 
That class needs an Axes method and a pyplot function.


> It's not hard to do but something that could be considered if there are
> enough use cases.
> Sameer

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