How to select the range of data based on threshold range of our own choice?

Dear MATLAB experts,

I want to use only the points corresponding to relatively high amplitudes? (e.g., try -10 to 0 dB or -20 to -10 dB or -30 to -20 on the normalized plots).

In attached imaging data output which veries from -45 to 0 dB.

I want to make 3 classes and plot data in with 3 classes and 3 colors.

.

I lookforward to receive your suggestions. I tried an approach but it gives logical output which is not suitable for my case.

Tahnk you!

[Image Analyst]

You can adapt the colormap to have only 3 colors. Please attach your original image varible in a .mat file if you want anybody to try anything. What do you want to be the color if the value is outside your ranges? Why do your three ranges (-10 to 0 dB, or -20 to 10 dB, or -30 to -10) overlap? For example a -15 : which class should it be in the second or the third???

[Amjad Iqbal]

@Image Analyst

Sorry, for my mistake.

I corrected the ranges (-10 to 0 dB or -20 to -10 dB or -30 to -20) in three classes now, as high, medium and low respectively.

Also the .mat file is attached for reference.

load('output_classes.mat'); % data

imagesc(x_vect, y_vect,20*log10(abs(S_VH)/max(max(abs(S_VH)))),[-45 0]),...

axis('equal'),axis('tight'),title('Output'),colormap('jet'),...

colorbar,set(gca,'FontSize',14),grid;

Please have a look, Thank you!

Answer 1

[Image Analyst]

Try this:

load('output_classes.mat'); % data
intensity = 20*log10(abs(S_VH)/max(max(abs(S_VH))));
imagesc(x_vect, y_vect, intensity,[-45 0]);
axis('equal');
axis('tight');
title('Output');
cmap = [0,0,0;
    1,0,0;
    0,1,0;
    0,0,1
    ]

cmap = 4×3

0 0 0 1 0 0 0 1 0 0 0 1

colormap(cmap);
colorbar;
caxis([-40, 0]);
set(gca,'FontSize',14)
grid; 

[Amjad Iqbal]

@Image Analyst

Much appreciated dear @Image Analyst

It's very fast and much accurate method. Thank you for your kind input and providing solution