How to read the RGB value from a grayscale image (just like the data cursor does)

I have a DICOM image. I have read it and stored the value in variable 'I'. So, how to read the RGB value from a grayscale image (just like the data cursor does). Here is the image with the data cursor. Variable 'I' is consisted of the index values shown in the second line on the picture. I wonder whether we can read the RGB value by command just like the third value shown on the image. If yes, how to do it? Thanks a lot

Answer 1

[Walter Roberson]

cmap = colormap();

RGB = ind2rgb(YourImage, cmap);
RGB(290, 189)       %notice that X is columns and Y is rows

However, if you used imshow() to display the image, then chances are that imshow decided on the colormap for you instead of using the current colormap. If so, then instead of assigning colormap() to cmap, use

cmap = gray(256);

[Penny]

Hi, Walter, thank you. But the result value is 1, not the same to the data cursor value. Why does this happen?

[Walter Roberson]

For example,

Image = imread('cameraman.tif');
imshow(Image);

I dropped a data cursor on it, which said

[X,Y] [168 198]
Index: 60
[R,G,B]: [0.2353 0.2353 0.2353]

So if I now

cmap = gray(256)
cmap(60+1, :)

I get 0.235294117647059 0.235294117647059 0.235294117647059

so index 60 (uint8) does map to the same color as the data cursor shows.

Now,

RGB = ind2rgb(Image, cmap);

X = 168; Y = 198;
RGB(Y, X, :)

and the result is

ans(:,:,1) =
         0.235294117647059
ans(:,:,2) =
         0.235294117647059
ans(:,:,3) =
         0.235294117647059

so using ind2rgb with the appropriate colormap does get you the same results as the data cursor. Provided, that is, that you use Y for rows and X for columns, (Y,X,:) rather than the less confusing but incorrect (X,Y,:)

[Penny]

Hey Walter, thank you. Here is my code:

Image=dicomread('D:\CT\001\001.dcm');
figure;
imshow(Image,[]);
cmap=gray(256);
RGB=ind2rgb(Image,cmap);
X=189;Y=290;
RGB(Y,X,:)

and the result is:

    % code
ans(:,:,1)=1
ans(:,:,2)=1
ans(:,:,3)=1

and the result shown in Data cursor is:

[Adam]

If you are always using a greyscale colourmap then the answer is simply the normalised value at the defined point in your matrix, you don't even need the colourmap.

[Walter Roberson]

You can use mat2gray to normalize your array

[Penny]

thanks to Adam and Walter. It works by using the function mat2gray. But how to define the amin and amax value that correspoing to 0 and 1 in grayscale image? we have several nomalized value in 'Image' that corrsponding to 1. How to define the minimum one? thanks a lot to your help.

[Penny]

If I use the function mat2gray by the default setting,the code is

if true
Image=dicomread('D:\CT\001\001.dcm');
figure;
imshow(Image,[]);
I=mat2gray(Image);
figure;
imshow(I,[])
end

But the value has a little difference to the data cursor. Here is the picture:

Answer 2

[Chad Greene]

Alternatively, I wrote a function for this a while back. It's been a while since I wrote it, so no guarantees regarding how well it works. It's called colorpicker.

Answer 3

[Image Analyst]

The function you want is called impixelinfo():

hp = impixelinfo();
set(hp, 'Units', 'normalized');
set(hp, 'Position', [.5, .9, .3, .05]); % Or whatever size and location you want.

This will put up a status label where you can see the x,y and RGB values as you mouse around over the image.