Problem with colormap using imshow()

I have a 2048x2048 matrix which has values ranging from 0 to about 8000.

I want to display only the values that are above a threshold (in my case, threshold = 350) while retaining information about the different values above threshold in the colormap. I have tried this:

main_image = 'example1.tif';

A = double(imread(main_image)); % Converts image to matrix of doubles

figure(1) 
imshow(A.*(A>350)); 
colormap(copper)
colorbar

However this is my result:

Hopefully you can see that the image is now "binary", and all values above threshold have exactly the same color in the colormap (in spite their actual pixel value wildly varies between 350 and 8000). How can I avoid this? I want the "hotter" pixels to look clearly different from the "colder" ones.

Thank you very much!

Answer 1

[Walter Roberson]

imshow(A.*(A>350), []); 

or

imagesc(A.*(A>350))

Remember that your data values have been converted to double precision because you used double(), but imshow() and image() expect double precision values to be in the range 0 to 1 unless you tell it to use a different scaling.

[Andrea Angella]

imshow(A.*(A>350), []); 

This actually worked, thank you!

I didn't know about the 0 to 1 range, but this probably explains why the following also gave a meaningless result:

imshow(A);

Could you please explain what the [] square brackets do to my matrix A? Also, your code has worked perfectly, but... do you know any way of how I could increase the contrast? (I'm now using a different colormap).

Thank you!

[Walter Roberson]

https://www.mathworks.com/help/images/ref/imshow.html#bvmnrxi-1-lowhigh

Grayscale image display range, specified as a two-element vector. For more information, see the 'DisplayRange' name-value pair argument.

Answer 2

[yanqi liu]

yes，sir，may be set the max value to replace，such as

clc; clear all; close all;
main_image = 'cameraman.tif';
% Converts image to matrix of doubles
A = double(imread(main_image)); 
vmax = 229;
A(A > vmax) = vmax;
figure(1) 
imagesc(A); axis equal; axis off;
colormap(copper)
colorbar