Strange colormap behaviour in MATLAB 2013b

I want to use a colormap in a MATLAB GUI to review a large number of images from an earlier analysis. I have tested the following script and it gives me two slightly different displays:

clear all
close all
clc

fclose('all');

a = linspace(0, 255, 256);
b = imresize(a, [256, 256]);

% Smooth display here
f = figure;
imshow(b, [0, 255]);
colormap(jet);
click = waitforbuttonpress;
delete(f);

pause(0.25);

% Dark vertical stripes here
g = figure;
imshow(b, [0, 255], 'Colormap', jet);
click = waitforbuttonpress;
delete(g);

So, what is happening here, and which form is correct ?

Answer 1

[Image Analyst]

In the first case it uses a 256 index mapping while in the second case it uses 64. Actually 64 is the default as you can see just by typing cm=jet on the command line so I don't know why the first case puts up 256 colors instead of 64. Try this code:

clear all
close all
clc
workspace

fclose('all');

a = linspace(0, 255, 256);
b = imresize(a, [256, 256]);

% Smooth display here
figure;
imshow(b, [0, 255]);
c1 = colormap(jet) % Retrieve the colormap that is actually being used.
colorbar;
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 .5 1]);

% Dark vertical stripes here
figure;
imshow(b, [0, 255], 'Colormap', jet(256));
colorbar;
c2 = colormap() % Retrieve the colormap that is actually being used.
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [.50 0 .5 1]);

Notice how I made the second figure smooth by specifying 256 colors instead of the default 64:

imshow(b, [0, 255], 'Colormap', jet(256));

Look in the Workspace panel at the sizes for c1 and c2 as you pass different numbers into jet() as an argument. You can use jet, jet(16), jet(64), jet(256), etc. but you can't use a number more than 256.

Answer 2

[Pawel Tokarczuk]

Many thanks, that's clear and complete. I hadn't realized that 'jet' could take a parameter - I assumed it was a built-in function or a named global array, and always the same. Over and out.