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Why does pcolor not display the full matrix?

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Darcy Cordell
Darcy Cordell on 3 Dec 2020
Commented: Paul on 23 Feb 2021
There are several questions on here asking why pcolor does not display the full matrix. Most answers say something along the lines of "use image/imagesc instead". And then other people jump in and say pcolor is more powerful (because it allows irregularly spaced grids). In my case, I use irregularly spaced grids to display geophysical modelling results so image/imagesc does not work. Some people suggest first interpolating your irregularly-spaced data and then using image/imagesc. But, for me, the actual x and y vectors matter a lot (e.g. the location of a given model cell is very important).
I understand that pcolor removes the right column and the top row.
My question is: Why does this happen? Is this a feature or a bug?
From the pcolor help:
"The grid covers the region X=1:n and Y=1:m, where [m,n] = size(C)."
Maybe I'm misunderstanding, but doesn't the grid cover the region X = 1:n-1 and Y=1:m-1? Why does it say it covers the full region if, in reality, it cuts off the top row and right column of the data matrix?
To me this is a bug that should be fixed and everyone seems to complain about it. But I figure there must be some deeper reason why it is not (or can't be) fixed.
Here's an example script:
%Some irregularly spaced vectors:
x = [0.5 0.8 1 1.6];
y = [1.2 1.4 2 2.2];
%Some data
r = ones(4,4); %Size = 4 by 5
r(1,1) = 3;
r(4,4) = 10;
r(2,2) = 10;
pcolor(x,y,r)
colorbar
% r(2,2) is in the correct spot
% and you can see r(1,1) as well.
% But you cannot see r(4,4) = 10

Answers (1)

Steven Lord
Steven Lord on 3 Dec 2020
This is not a bug. The perceived bug is the result of the 2-dimensional fencepost error. From the description of the C input argument on the documentation page for the pcolor function:
"The values in C map colors in the colormap array to the vertices surrounding each face. The color of a face depends on the color at one of its four vertices. Of the four vertices, the one that come first in X and Y determines the color of the face. If you do not specify X and Y, MATLAB uses X=1:n and Y=1:m, where [m,n] = size(C). Because of this relationship between the vertex colors and face colors, none of the values in the last row and column of C are represented in the plot."
The values in C do not represent the colors of the faces but represent the colors of the vertices. If you look at the picture included in that input argument description the corresponding C is 3-by-3 but only the four (2-by-2) elements with arrows pointing to the faces are used in determining the colors of the pcolor plot. Similarly, if you wanted to draw a 9-by-9 Sudoku grid on a piece of paper you need to draw 10 horizontal and 10 vertical lines to fully enclose the 81 squares where numbers would be placed.
  3 Comments
Paul
Paul on 23 Feb 2021
Coincidentally, I just ran into this exact pcolor v. imagesc issue yesterday. pcolor returns a surface, which gives control over some things that imagesc does not, like EdgeColor.

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