Problem with colormap!

I have a CT image (a matrix 512 x 512) that has maximum value is 2062 and minimum ones is 64. I want to show it so that the maximum pixel get white and the minimum ones get black color. I tried to use function colormap(C), with C=[a a a], a=[0:1/2000:1]'.Then used function image (CT)to show it. I imagined a color bar that begin with black (at 1) and change along with shade value to white (at 2062 or less than that a little). But i got a figure with weird color bar. It's discrete and just black and gray like the attached image (hadn't used function image(CT) yet). I'm wondering what's wrong!

[Nguyen Tan Duoc]

Are there any error with Matlab? please help me!

Answer 1

[Walter Roberson]

This of course has nothing to do with the CT image; the question could have been posed without mentioning it.

On OS-X it looks fine:

My speculation is that you are using MS Windows and using a release before R2014b. Before R2014b, MATLAB on MS Windows only supported 256 entries in the color map. I am told that from R2014b onwards that the limit is substantially higher.

[Nguyen Tan Duoc]

Thanks for your answer. I'm using matlab R2013a on MS Windows 8.1 pro. I hasn't known that colormap just support 256 color. It doesn't have any notice in matlab helping center about that. You sure a bout that?

[Walter Roberson]

How about a direct statement from one of the developers? http://www.mathworks.com/matlabcentral/answers/47458#answer_58070

[Mike Garrity]

Yes, for many years the colormap was implemented on Windows using a GDI Palette, which meant a limit of 256 colors. Starting in R2014b, it is implemented on all platforms as a texturemap. This means that the max size is dependent on your graphics card, but 2,048 is pretty typical.

[Nguyen Tan Duoc]

Thanks everybody!that's great answer.

[Image Analyst]

You're welcome. Hopefully your final decision was to use imshow() and not image() with that colormap you tried to create.

Answer 2

[Image Analyst]

Don't use image(). Use imshow():

imshow(CT, []);  % Be sure to use [].

[Nguyen Tan Duoc]

Thanks for your answer. I had known this way. It gave a good image. But i want to know what's wrong with my command colormap(C), with C=[a a a],a=[0:1/2062:1]'. why i got a weird color bar like that?

[Image Analyst]

Was CT a floating point image (double)? That might be why. You might also need to fiddle with the caxis() function. With imshow() you don't have to worry about any of that - no need to do it manually since imshow() will do it automatically for you.

[Nguyen Tan Duoc]

CT is uint16, not double. I got that figure after command:

a=[0:1/2062:1]';

C=[a a a];

colormap(C)

colorbar

I hadn't use function image yet!

[Image Analyst]

Then how did you display the image?

[Nguyen Tan Duoc]

I would display CT. But it was too weir with that color bar.