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Why can't pcolor show the entire matrix to plot?

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I am trying to plot a pcolor of the matrix Error3D which is size 300x300.
Error3D is mostly NaNs except for Error3D(:,100), Error3D(:,200) and Error3D(:,300).
When I try to plot the matrix is shows the correct values for Error3D(:,100) and Error3D(:,200) but the values for Error3D(:,300) do not appear.
(please see attached figure)
Here is my code:
load('Error3D.mat')
figure(1)
clf;
hold on;
pcolor(Error3D);
cb = colorbar;
Does anyone know how to fix this issue and make the values of Error3D(:,300) visible on my plot?
Thank you

Accepted Answer

DGM
DGM on 18 Aug 2021
Edited: DGM on 18 Aug 2021
The plot appears black because a pcolor() plot is basically like a surf() plot. The faces are all outlined by black edges. When the mesh is dense, all you see is edges.
After the pcolor() call, do this:
shading flat
or just set the EdgeColor property of the pcolor() plot to 'none'.
h = pcolor(Error3D);
set(h,'edgecolor','none');
Either one will basically do the same thing.
  5 Comments
DGM
DGM on 18 Aug 2021
That can be done using alphadata.
% ...
h = imagesc(Error3D);
set(h,'alphadata',~isnan(Error3D))
% ...
Since the default color of the underlying axes is white, that's what shows through.
FWIW, I run an inverted display, so that prior example is inverted as I normally see it. This example is flipped so that it's like you'd normally see it.

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More Answers (1)

Image Analyst
Image Analyst on 18 Aug 2021
pcolor() does not show the values as little square pixels like the imaging functions (imshow(), imagesc(), and image()) do.
m = randi(255, 3, 3); % Make 3x3 matrix
pcolor(m)
shading 'flat';
Basically it's because it's not the center of the little square that has the value, it's the edge of the square that has the value. Then it makes a plane tilted to the other edge with the colors changing along the way. However if you say shading flat, it doesn't change the color, it basically makes the square flat, not tilted, and so the color will be the color of one of the edges (I'm not sure which edge it uses to decide on the color).
So you can see from the above display of a 3x3 matrix, there are 3 edges but only 2 tiles/squares. For this annoying reason, I don't use pcolor() and use imshow() instead. You also might want to consider that if you just simply want to display a matrix. You can still apply a colormap if you want, and one of your own choosing instead of the one pcolor chooses for you, by calling colormap().
  3 Comments
Image Analyst
Image Analyst on 19 Aug 2021
Unfortunately your array is virtually all NaN's so there's virtually nothing to display. But if it weren't this is how to display a gray scale image:
% fileName = 'Error3D.mat'
% s = load(fileName)
% grayImage = s.Error3D;
grayImage = imread('cameraman.tif');
[rows, columns, numberOfColorChannels] = size(grayImage)
% Unfortunately the array is virtualyl all NaNs.
imshow(grayImage);
subplot(2, 1, 1);
cmap = hsv(256);
imshow(grayImage, 'Colormap', cmap);
axis('on', 'image');
colorbar;
For a surface plot, see attached demo.
A LL
A LL on 19 Aug 2021
Great! Thanks Image Analyst for your help!

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