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

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.

Answer 1

[MathWorks Support Team]

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)

[Nitin Khola]

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:

https://www.mathworks.com/help/matlab/ref/matlab.graphics.illustration.colorbar-properties.html

https://www.mathworks.com/help/matlab/creating_plots/interactively-explore-plotted-data.html#f4-50382

Cheers!

Nitin