How can I display all 'NaN' values as black in my image to differentiate them from lowest values?

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I am using imagesc to display matrices. Some of them are 'NaN' values, and I want those to be black in the display so that I can distinguish the 'NaN' values from the lowest values.

Accepted Answer

MathWorks Support Team
MathWorks Support Team on 27 Jun 2009
This can be done by writing code that will check for the NaN values in the image matrix and set it black. For example, create a 5X5 matrix 'a' containing some NaN values and some '0' values. Get minimum and maximum values of that matrix and also location of all 'NaN' values. Then replace all NaN with values which are less than minimum number present in that matrix. Then redraw the colormap and plot the image.
a =[
0 0.8843 NaN 0.3900 NaN
NaN 0.5880 0.8256 0.1117 0.4950
0.8620 0.1548 NaN 0.1363 0.7476
NaN 0.8999 0.3185 0.6787 NaN
0.5144 0 0.5341 0.4952 0.8507
];
minv = min(min(a));
maxv = max(max(a));
a(isnan(a)) = minv-((maxv-minv)/5);
ddd=[0 0 0;jet(10)];
colormap(ddd);
imagesc(a)
  1 Comment
Nitin Khola
Nitin Khola on 13 Jun 2016
Edited: MathWorks Support Team on 19 Apr 2021
Hi Tord,
The behavior of NaN values in "imagesc" is not defined currently as described in the following documentation: https://www.mathworks.com/help/matlab/ref/imagesc.html#input_argument_c
So that we can work around this behavior for colorbar and data cursor, it will require changing the ticklabels for the former and programming the callback (as per the position of the mouse pointer e.g. if on a NAN tile, display "NaN") for latter. Please refer to the following documentation which can help you implement this behavior:
Cheers!
Nitin

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