Adjusting CData to match CLim for grayscale Image

I have a grayscale image in a figure displayed through imshow. I adjust the axes CLim limits [a b] to improve it's contrast to desired levels. How can I adjust the image matrix (CData) to match the image displayed by my CLim limits? I've tried imadjust function i.e. Image2=imadjust(Image1,[a b],[]) , but the resultant Image2 contrast doesn't look like the one displayed by CLim limits [a b].

Answer 1

[Walter Roberson]

t = double(max(min(Image1, b),a));
adjusted_image = (t - a) / (b - a);

now adjusted_image is in the range 0 (corresponding to a) to 1 (corresponding to b). Remember to caxis([0 1])

[John]

Exactly what I was looking for. I had a brain freeze :)

Thanks

Answer 2

[Image Analyst]

What are you trying to do? If you want to stretch contrast, you can pass in the values you want for black and white into imshow(). Or else call caxis(). I don't understand why you would ask what you asked.

[John]

I'm making a GUI where user can adjust the contrast (just like imcontrast) by changing CLim limits and when they hit apply, they get the adjusted_image matrix matching the displayed image. The answer provided by Walter worked.

[Image Analyst]

Yeah it works but it's somewhat inefficient because it has to compute two intermediate images. Anyway, there's a function for doing that normalization - it's called mat2gray().

But John, I think you might have bee a little too quick to discard my answer. caxis just does a look up table modification so it's really fast and uses no extra memory. I think you just didn't know what I was suggesting so I give you an example:

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format compact;
fontSize = 20;
format short g;

% Read in low contrast image.
imshow('pout.tif');
colormap(gray(256));
colorbar;

% Set up figure properties:
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Get rid of tool bar and pulldown menus that are along top of figure.
set(gcf, 'Toolbar', 'none', 'Menu', 'none');
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off') 

promptMessage = sprintf('Click continue to adjust caxis');
titleBarCaption = 'Continue?';
buttonText = questdlg(promptMessage, titleBarCaption, 'Continue', 'Quit', 'Continue');
if strcmpi(buttonText, 'Quit')
  return;
end

for k = 1 : 5 : 200
  caxis([k, 255]);
  pause(0.1);
end

promptMessage = sprintf('Click continue to adjust caxis');
titleBarCaption = 'Continue?';
buttonText = questdlg(promptMessage, titleBarCaption, 'Continue', 'Quit', 'Continue');
if strcmpi(buttonText, 'Quit')
  return;
end
colormap(gray(256));

for k = 254 : -5 : 65
  caxis([0, k]);
  pause(0.1);
end

promptMessage = sprintf('Click continue to adjust caxis');
titleBarCaption = 'Continue?';
buttonText = questdlg(promptMessage, titleBarCaption, 'Continue', 'Quit', 'Continue');
if strcmpi(buttonText, 'Quit')
  return;
end
colormap(gray(256));

for k = 1 : 5 : 127
  caxis([k, 255-k]);
  pause(0.2);
end

[Walter Roberson]

The intermediate images probably would not be too large.

[John]

Thanks Robert. I had the GUI ready, just needed that part of adjusting the matrix. I've employed the mat2gray. I agree the caxis is more efficient, but I'm working with very small images (about 128x128pixel, 8bit) so in the end, the memory isn't an issue for now. I'll implement your suggestion if need be in the future.