remove pixels with certain rgb values

Hi, I want to remove pixels with R value >100 and G value>100, here is my code:

[height width rgb] = size (I); % I is RGB image
for coloum = 1:width
    for row =1:height

          if (I(row,coloum,1)> 100 && I(row,coloum,2)> 100)
              I(row,coloum,1)= 0;
              I(row,coloum,2)= 0;
              I(row,coloum,3)= 0;
          end

      end

The error is "Index exceeds matrix dimensions". Does anyone have an idea about this? Thanks in advance.

Answer 1

[Image Analyst]

Instead of all your code (nested for loops), try this vectorized approach:

mask = (I(:, :, 1) > 100) & (I(:, :, 2) > 100);
% Mask the image using bsxfun() function to multiply the mask by each channel individually.
maskedRgbImage = bsxfun(@times, I, cast(mask, 'like', I));

Also, I very much recommend you don't name your variable I because it looks too much like l (lower case L) and 1 (one).

[Tian Tian]

Thank you it works!

[Image Analyst]

You're welcome. Since we're done, then can you "Accept this answer" and vote for it?

[Perry Taneja]

What do you mean by removing? What value will say a pixel with value "101" acquire after you remove it?

[Image Analyst]

In my example, pixels with colors meeting the criteria (red > 100 and green > 200) will be "true" in the mask and therefore will be set to pure black (0,0,0).

A pixel with color of (101, 101, 101) will remain unchanged. Why? It's not in the mask because it's green value was only 101 (not greater than 200). Only the red channel met the criteria but the criteria required that both red and green meet the criteria to be considered as part of the mask.

[Jona Boender]

This worked great, thank you!

Answer 2

[Jos (10584)]

Is I really a 3D array or is the third dimension just 1?

Simply take a look at the value of rgb, after the size(I) command :)

[Tian Tian]

Thank you for comments. It's actually a 2D image, 2D array then. The 3rd dimension 1 2 3 is for rgb color.I am a little confused about the dimension and the rgb size, is rgb from 0-255 for each color channel? May you please explain my code problem? Thanks a lot.

[Jos (10584)]

There is a difference between indexed images (2D, like yours) and true colour images (3D, with RGB triplets in the 3rd dimension). This is explained here:

https://uk.mathworks.com/help/matlab/creating_plots/image-types.html

[Tian Tian]

Hi Jos, thanks for the information. I think mine is true color image, I said '2D' because of width and height of the image (physical dimension).My code was also based the link that you sent to me: pixel (2,3,1) where the last 1 is the color. Do you know where is the problem? Thank you in advance.

[Jos (10584)]

What does size(I) return?

What does a small portion of I, like I(1:4,1:4,:) contain?

[Tian Tian]

Also, I tried 'colormap', and got the error: 'Colormap must have 3 columns: [R,G,B].'. So it is not an indexed image.

[Tian Tian]

Thank you I imported a wrong image that's why. Thanks a lot for your help!!