Can anyone please answer why the pixel value changes from about 90 to about 0.23 when I apply ind2rgb function?

% I have true color image (looks gray though because it is ultrasound image). I read the image. 
im = imread('c1d7.JPG');
% converted the image into gray,
I = rgb2gray(im);
imshow(im);
% Then ind2rgb
rgbImage = ind2rgb(I, jet(256));
% This leads the value change as in my question from ~90 to ~0.23

What would be the solution?

Thanks

Surya

Answer 1

[Image Analyst]

Try this:

% I have true color image (looks gray though because it is ultrasound image). I read the image. 
im = imread('peppers.png');
subplot(2,2,1);
imshow(im);
% Convert the image into gray scale.
I = rgb2gray(im);
subplot(2,2,2);
imshow(I);
% Then ind2rgb
rgbImage = uint8(255 * ind2rgb(I, jet(256)));
subplot(2,2,3);
imshow(rgbImage);

% 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')

[Walter Roberson]

I have found that im2uint8 has a slightly different answer than uint8(255*x). I do not recall the details at the moment but I found that im2uint8 was better handling. I think it might have had to do with uint8 rounding so 0 and 255 could result with only half the probability of the other values if you use uint8 naively

[Surya Gnyawali]

Wonderful! It worked. Thanks a lot!

Answer 2

[Walter Roberson]

Color tables used with ind2rgb must have entries in the range 0 to 1, which jet() does have. If you want the uint8 version of the colored image apply im2uint8 to it.