Imagesc Color Assignment of Values

Hi I am currently trying to create a colormap for the function imagesc(); Currently my code looks like the following:

caxis manual
my_map=[0 0 0;1 1 1;1 0 1;1 0 0];
x=[0:100];
imagesc(x)
colormap(my_map)
colorbar

As of right now it will map values of 0:25 to white, 25:50 red and so forth. Is there any method I could use to change the assignment of values for the colors. Example: assigning values of 0:5 to white 6:80 black.

Thank you in advance.

Answer 1

[yanqi liu]

clc; clear all; close all;
% 0:25 to white, 25:50 red and so forth.
figure;
caxis manual
my_map=[0 0 0;1 1 1;1 0 1;1 0 0];
x=[0:100];
imagesc(x)
colormap(my_map)
colorbar

% 0:5 to white 6:80 black.
figure;
caxis manual
my_map=[repmat([1 1 1], length(0:5), 1)
    repmat([0 0 0], length(6:80), 1)
    repmat([1 0 0], length(81:100), 1)];
x=[0:100];
imagesc(x)
colormap(my_map)
colorbar

[Lucio De Pra]

Thank you for the response this is very helpful. Is it possible to make the values go from 0:.2 and 0.2:0.4 on the colorbar. I have tried limiting the caxis but worked to no avail.

Answer 2

[Walter Roberson]

NO.

Values below the first caxis value are always assigned the lowest color in the color map.

Values above the second caxis value are always assigned the highest color in the color map.

In-between, the ratio: (value - caxis(1))/(caxis(2)-caxis(1)) is calculated, and that gives you the linear portion of the way through the colormap that is used.

There is no way to say to use a particular colormap entry for a particular range of values, and a different colormap entry for a different range of values.

What you can do is create a colormap with 27 entries. Assign the same white color to the first two. Assign black to the remaining 25 entries. The span of values is 81, so the first three values (0, 1, 2) would use the "first" white slot, the second three values (3, 4, 5) would use the "second" white slot, and all other values would use some other slot that would happen to hold black.

But it is typically easier instead,

bin = discretize(x, [0 5 80 inf])
imagesc(bin)
colormap(my_map)

[Lucio De Pra]

Oh okay I understand so let's say i have an array of numbers with values between 0-1 and I want my threshold to be 0.1. I can set a colormap with 10 entries and basically black out 0:0.1 and if i want the color red for the range 0.1-0.5 i can just set the intervals of [0.1 0.2] [0.2 0.3] and [0.3 0.4] as the color red.

I appreciate the help regardless thank you!