How would you create a mask on an image?

I am doing a project and one of the tasks is to create a matrix with number of pixels x number of pixels x 3 as its size. In the previous task I created a matrix using the green container of the RGB image and I am suppose to duplicate this along the 3 dimensions. My question is what functions would I use in order to do this. I am not allowed to use any implicit functions only explicit.

[Image Analyst]

Define implicit and explicit. Which are cat(), repmat(), bsxfun(), and class()? Walter and I used those in our Answers below. Are you allowed to use them?

[Feynman]

Yes those I can use, I cannot use conditional statements or define a new function that I create. With the green container matrix that I made I was to replace any values of green that were smaller than the user was supposed to input a number 0 to 255 which I coded as

m1 = g>=n1;
will the code that you suggested be able to tell whether the green channel intensity was greater than the threshold?

[Image Analyst]

That will get you a logical mask, m1, from which you can use it as the mask in my code below to mask out pixels brighter than the user specified value. You can set the values in any of the color channels to anything you want, e.g. 0 or 255, or anything else.

redChannel(m1) = 0; % Mask inside to %255 or whatever.

or

redChannel(~m1) = 0; % Mask outside.

[Feynman]

I need to keep the values that are greater than the value the user entered. Will the code you suggested keep the values greater than the one the user inputs?

[Feynman]

I tried implementing the code you suggested and it keeps giving me an error of integers can only be combined with integers of the same class or scale doubles

[Image Analyst]

mark and redChannel have to be the same type of integer like it said. So you'd need to do

maskedRed = redChannel .* uint8(mask);

if redChannel is a uint8.

Answer 1

[Image Analyst]

Here's one method:

    % Multiply the mask by each color channel individually.
    maskedRed = redChannel .* mask;
    maskedGreen = greenChannel .* mask;
    maskedBlue = blueChannel .* mask;
    % Recombine separate masked color channels into a single, true color RGB image.
    maskedRgbImage = cat(3, maskedRed, maskedGreen, maskedBlue);

Here's another method, suggested by Sean D.

% Mask the image.
maskedRgbImage = bsxfun(@times, rgbImage, cast(mask,class(rgbImage)));

Answer 2

[Walter Roberson]

repmat() or cat(3,X,X,X)