Steve on Image Processing

In a comp.soft-sys.matlab newsgroup post on January 17, Kevin wanted to know how the brightness/contrast control in imtool worked. Image Processing Toolbox function imtool is an image viewing/navigation GUI.

In medical imaging the brightness/contrast control is called the "window/level" control, and there is a reasonably standard set of mouse motions for changing the window and level. These mouse motions are supported by imtool.

This diagram shows how the terms "window" and "level" are related to brightness and contrast adjustment. Making the window wider or narrower decreases or increases the display contrast; moving the level left or right changes the display brightness.

This window-level scaling function is just like the CLim scaling function I described previously. I replied to Kevin that the brightness/contrast control in imtool worked by adjusting the axes CLim property.

Here's an non-medical example of how adjusting the CLim can make previously unseen image details visible.

I = imread('cameraman.tif');
imshow(I)
text(256, 265, 'Image courtesy of Massachusetts Institute of Technology', ...
   'HorizontalAlignment', 'right', 'FontWeight', 'light', 'FontSize', 7, ...
   'Color', [.7 .7 .7])

Not much detail is visible in the coat or glove. But you can make out much more detail if you change the CLim property of the axes:

imshow(I)
set(gca, 'CLim', [6 21])  % caxis([6 21]) does the same thing.

If you want to see how to make this adjustment interactively, look here in the Image Processing Toolbox Users Guide.

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Published with MATLAB® 7.1