I want to change the gray scale from [0 1] to [0 500]

I want to change the gray scale from [0 1] to [0 500] and have used the following functions

b= imadjust(a,[0; 1],[0; 500])

and

 %j = imadjust(a,stretchlim(a),[0 500]);

but they don't seem to work. There are errors when i plot them.

CODE:

 a = ones(256, 256, 3);
 a(10:110,40:100,:)=0;
 a(120:200,120:200,:)=0;
 %colormap('gray');
 image(a)
 figure();
 b= imadjust(a,[0; 1],[0; 500])
 %j = imadjust(a,stretchlim(a),[0 500]);
 image(b)

Answer 1

[Walter Roberson]

"By default, the CDataMapping property for the image is set to 'direct' so image interprets values in C as indices into the colormap. For example, the bottom right pixel corresponding to the last element in C, 22, uses the 22nd color of the colormap."

Do you have 500 colors or more in your colormap?

[Anvinder Singh]

Walter, I have only 3 colors . What I want to do is once I have the scale from 0 to 500 then the rectangles that I am creating using this code will have different intensities of gray color on this scale rather than 0 to 1.

[Walter Roberson]

imagesc(b) rather than image(b)

[Anvinder Singh]

Walter, the imagesc doesnt work. The error i get is "Error in imadjust (line 99) validateLowHigh(lowIn,highIn,lowOut,highOut);"

[Walter Roberson]

Then that was not errors when you plot them, that was errors when you did the adjustment. And you did not post the complete error message.

Answer 2

[Image Analyst]

How about using mat2gray():

b = 500 * mat2gray(a);

It scales min to max, whereas imadjust() scales based on percentage of counts in the histogram tails. Just depends on what you want to do.

[Anvinder Singh]

Image analyst, the mat2gray command works but with the following concern : The line a = ones(256, 256, 3); means the image should have value 1 as the background (white) but it no longer has that. The background is black. If i replace it with zeros it still remains black.

[Walter Roberson]

After the mat2gray(), how are you displaying b ?

[Anvinder Singh]

" imagesc(b)"

[Anvinder Singh]

For sometime i can give up on this problem coz i have another one which is more important. After i have plotted the image i want to add poisson random noise and then display that image also. I am able to add poisson noise but the noise shows colors where as i am expecting it to be grayscale. Here is the code. :

b = mat2gray(a, [0,500]);
a = ones(256, 256, 3);
a = ones(256, 256, 3);
 a(10:110,40:100,:)=250;
 a(120:200,120:200,:)=500;
 colormap('gray');
 figure(1);
 imagesc(b)
 colormap('gray');
 c = poissrnd(b);
figure(2);
imagesc(c);

I have attached the poisson noised up image also :

[Walter Roberson]

Unfortunately, there is no way with MATLAB images to have a maximum value of 500 map to full intensity for RGB images. It can be done for pseudocolor images, but not for RGB images.

To extend the range of accepted values past 255 for RGB images, you need to use uint16() images. But then 500 is only about 1 part in 131 of the way to the maximum value 65535, so if you used a colormap with 256 entries it would only be mapping to index 1 or index 2, which is essentially too dark to show up on any linear scale.

[Image Analyst]

Wait, I don't understand. If you want gray scale noise, then why are you creating a color image? Just create a gray scale image like I showed you.

And what's special about the value 500? Why do you want that to be your max value?

[Walter Roberson]

colormap() applies only to the current figure. When you figure(2) you are changing figures to one where you have not done the colormap('gray')

[Anvinder Singh]

Image analyst, I do not mean to create a color image, i need to change the scale of the grayscale images from [0 1] to [0 500] so 1 maps to 500 and the values between 0 and 500 are just different shades of gray. I have used mat2gray which now has scale of 0 to 255 instead of 500, it still doesnt show the grayscale noise. It is colored.

[Anvinder Singh]

Walter, I uncommented the colormap('gray') for the second image but it is still in color not in grayscale. I have changed 500 to 255 now so that i can map the [0 1] scale of grayscale images to [0 255]. But it doesn't still show what i am expecting. I have tried 'imagesc' and 'image' both of them. i need the noise generated by the image 'b' to be in grayscale but this grayscale color intensity i want to be in [0 255]. This is the reason i am using mat2gray and colormap('gray')but it doesnt seem to be working. What am i doing wrong ?

[Image Analyst]

then create a grayscale image, not a color image. DON'T DO THIS:

a = ones(256, 256, 3); % The badly-named a is really an RGB image.

DO THIS INSTEAD:

grayImage = ones(256, 256);

[Anvinder Singh]

Tried, it still shows the color image. I have "grayimage = ones(256, 256);" now followed by "b = mat2gray(grayimage, [0,255]);" and then rest of the code as shown above but the output is colored.

[Image Analyst]

Come on. It does not. Here's a full blown demo:

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 long g;
format compact;
fontSize = 20;

% Make all white.
grayImage = ones(256, 256);
% Make some rectangles black.
grayImage(10:110,40:100) = 0;
grayImage(120:200,120:200) = 0;
subplot(3, 1, 1);
imshow(grayImage);
axis on;
colormap(gray(500));
colorbar;
title('Original Image', 'FontSize', fontSize);

% Scale the image intensity to between 0 and 500
scaledImage = 500*mat2gray(grayImage);
subplot(3, 1, 2);
imshow(scaledImage, []);
axis on;
colormap(gray(500));
colorbar;
title('Scaled Image', 'FontSize', fontSize);

% Make noisy image where lambda is the value of the image at that location
noisyImage = poissrnd(scaledImage);
subplot(3, 1, 3);
imshow(noisyImage, []);
title('Noisy Image', 'FontSize', fontSize);

% 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')

[Anvinder Singh]

Thank you Image Analyst for the detailed demo. After implementing yours and comparing it with mine i am able to differentiate between the two codes. Have you notices that once you add poisson random noise either the rectangles get noisy or the rest of the area in the matrix. Both of them do not get noisy at the same time. I have attached a sample image wherein both rectangles and the background get noisy but the image is in RGB. appreciate your help !

[Image Analyst]

I don't know what that stuff is. Figure 1 on the left seems to be the original matrix/image, and Figure 2 on the right seems to be the colored matrix that you say you don't want. I have no idea why you posted them.

As for my code, you'll see that the gray area has noise while the zero regions do not. As you should know, Poisson noise depends on the value, so if the value is zero, there can be no noise. That's why the zero regions have no noise. If that doesn't make sense to you then you need to refresh your knowledge of what Poisson noise is. It's not simply additive noise you know.

[Anvinder Singh]

Oh, Thanks Image analyst. Just forgot it will have no poisson noise for '0' value. It seems to be good. One last thing, if i need the background to be white i am trying to change "grayimage = ones(256, 256);" and grayscale value but it doesnt seem to show up ?

[Image Analyst]

"White" is going to be the brightest value in your image, which is 500, not 1. Assign areas you want to be white to 500, not 1.