Trying to save the red positions of an image

Hi,

I'm trying to save all the position's of an image with red color.

im = imread('image.png');
locations = find(im(:,:,1) == 36)

This code is giving me an empty array. Why this is happening? How can I fix it?

Answer 1

[Image Analyst]

Oliver, this works:

rgbImage = imread('image.png');
subplot(1, 2, 1);
imshow(rgbImage);
impixelinfo; % Let's you mouse around and see rgb values.
% Split image into components:
[r, g, b] = imsplit(rgbImage);
% Red spots are r=237, g=28, b=36.
% Get red mask
redMask = r > 200 & g < 50 & b < 50;
subplot(1, 2, 2);
imshow(redMask);
% Get rows and columns where the red pixels are
[redRows, redColumns] = find(redMask)

redRows and redColumns are paired row,column coordinates for each pixel that is red in the image, in column-major order. Not sure what you're going to do with that, but there you have it.

[Oliver Lestrange]

Hi there,

With your code I'm geting and error:

Undefined function or variable 'imsplit'

Maybe my MATLAB version doesn't have this function...

[Image Analyst]

You have an old version. Try this

r = rgbImage(:, :, 1);
g = rgbImage(:, :, 2);
b = rgbImage(:, :, 3);

Answer 2

[Sudheer Bhimireddy]

Does your image has that Red value? Double-check it.

To test your code, follow this example.

A       = imread('ngc6543a.jpg'); % Read MATLAB example figure
A_read  =  A(:,:,1);              % Read Red channel
A_ind   =  find(A_read == 36);    % Find non-zeros where the value is equal to 36
A_sz    = size(A_read); 
[A_row,A_col] = ind2sub(A_sz,A_ind); % Convert linear indices to matrix subscripts

If you change the above find statement to

A_ind   =  find(A_read == 3600);

you will get an empty A_ind array because there exists no such value inside A_read.

[Oliver Lestrange]

Hi Sudheer Bhimireddy,

Yes, it does.

If I print the im variable I get that value.

I've tried the example that you mentioned and works. I'm not seeing the difference between the codes. Just the .png vs .jpg but that shouldn't be a problem.

[Sudheer Bhimireddy]

can you attach the image file you are trying in this code?

[Oliver Lestrange]

This is just I test for what I want to do.

[Sudheer Bhimireddy]

With the image you attached I don't see any cells with value = 36.

Try this and see what you get, if you get 1 it means yes the value 36 is in the array , if you get 0, it means the im array doesnot have the value 36.

>> ismember(36,im)

[Oliver Lestrange]

I get one

[Sudheer Bhimireddy]

I am sorry, I should have been more clear. Ofcourse im will have 36 in its Blue channel. I meant to see

>> ismember (36,im(:,:,1))

the first index is the Red channel and will not have 36.

[Oliver Lestrange]

You right, doesn't have the 36. Why this is happening? If it's not 36, how can I know which number is? With the ouput of the last line it seems to be 237.

[Image Analyst]

Like I said in my answer, your values are r=237, g=28, b=36.

[Sudheer Bhimireddy]

To understand that, you need to understand the way image color data is stored in MATLAB.

For an RGB image, if you are storing the pixel values in a [m,n,3] array; where m and n gives the size of the image. If you are storing them in unit8 format, they can varry from [0 255] and if you store them in double format they varry from [0 1].

Now, lets say pixel at index (10,10) in your image is filled with red alone, then the corresponding cell values will be (in unit8 format)

(10,10,1) = 255;
(10,10,2) = 0;
(10,10,3) = 0;

Similarly if it is filled with blue alone, they will have values

(10,10,1) = 0;
(10,10,2) = 0;
(10,10,3) = 255;

So, the first channel of the image contains the amount of Red color on a scale of [0 255] at each pixel.

[Oliver Lestrange]

Thanks a lot guys!