MATLAB pcolor/surf bilinear interpolation (shading interp)

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Tom Holden
Tom Holden il 18 Nov 2017
Commentato: Walter Roberson il 30 Apr 2019
Consider the following MATLAB code:
C = [ 0 0 0 0 0
0 1 2 1 0
0 2 4 2 0
0 1 2 1 0
0 0 0 0 0 ];
pcolor( C );
shading interp;
axis square
Note that `C` is invariant under 90 degree rotations. Also note this sentence from the help for `pcolor`: "With shading interp, each cell is colored by bilinear interpolation of the colors at its four vertices, using all elements of C."
However, the plotted image is as follows:
Note that the image is not invariant under 90 degree rotations (consider e.g. the four corners). Now, unless I horribly misunderstand bilinear interpolation, this must be wrong. MATLAB seems to be interpolating on triangles, which is not the same as bilinear interpolation.
Is there any way of working round this MATLAB bug, and getting correct bilinear interpolation? (Other than manually interpolating additional points myself, which still would not cure the issue if one zoomed in far enough.)
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Image Analyst
Image Analyst il 21 Dic 2017
Joe, can you move this down to the "Answers" section with the rest of the answers? The comments section up here is reserved for requests for clarification of the original question. Sometimes when posters see comments up here, they think they are the answers and never scroll down to see the actual answers in the Answers section.
Tom Holden
Tom Holden il 21 Dic 2017
The solution on the linked page is doing manual interpolation. The question asked for a solution other than manual interpolation...

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Image Analyst
Image Analyst il 21 Dic 2017
pcolor() does NOT use all the elements in your array - it ignores the last row and last column. Just comment out the shading line of code and run it:
Is that what you want and were expecting? I'm guessing not.
So, don't use pcolor. Use imshow(). If you need higher resolution, then use imresize() to make bump up the resolution if you want to:
C2 = imresize(C, 100); % Blow up by a factor of 100;
imshow(C2, []);
axis on;
colorbar;
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marco riva
marco riva il 30 Apr 2019
Modificato: marco riva il 30 Apr 2019
I want to use this method, with imresize. However I want to keep axis values from 0 to 5 (not 0 to 500 due to the resize). Is there a way to enforce that?
Thank you
Walter Roberson
Walter Roberson il 30 Apr 2019
marco riva: you can provide XData and YData parameters to imshow() to specify the data units where the image is to be placed. Note that XData and YData are the coordinates of the center of the lower left and upper right pixels so the image edge will be at coordinates one half pixel beyond that.

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