Marking pixels above a certain value

Hello. I have a problem.Let's say we have an image called img1 and I want to do the following two things (without loops):

"1. Create logical image where every pixel is marked that has a green channel which is greater or equal 0.5. The result should be stored in mark_green."

"2. Set all pixels in the original image (the double image from step 2) to black where _mark_green is true. Store the result in image_masked. Use repmat to complete this task."

Any ideas?

Answer 1

[James Tursa]

Hint for the 1st part,

G = (insert code here to pick off the green plane)
mark_green = G >= 0.5; % the marked pixels

For the 2nd part, repmat ~mark_green so that it can be used for all of the color planes. You might want to .* the result with your image.

Give it a try, and then post your code if you have problems and need specific advice.

[Trn]

G = image(:,:, 2);
mark_green = G >= 0.5;

Should work right? I'm not sure how to use repmat for the second part.

[Christopher Glantschnig]

idxmat = repmat(image_mark_green,[1,1,3]);
image_masked = image;
image_masked(idxmat) = 0;

[Trn]

This works. Thanks! Could you shortly explain what it does exactly?

[rashi]

1,1,3 means three dimensional right. But from where do three dimensions come here..? Thanks.