how can i set an images pixels to black ?

hi guys , i m so new with Matlab and have a problem.

i have an image and i have to set all pixels to the 'black' which is greater then 0.5(or true it says) and without using loops. but repmat

can anyone help me ?

Answer 1

[li yan]

I(I > 0.5) = 0;

[Alprcnkt]

i m new.. can you explain little?

[Walter Roberson]

http://blogs.mathworks.com/steve/2008/01/28/logical-indexing/

[li yan]

Assuming that you have got a 2D-metrix I, and you want to set all pixels greater than 0.5 to 0. you can do like this

 row = 10; col = 10;
 I = 0.5 + .1 * randn(row, col);
 I(I > 0.5) = 0;

then you have set all pixels greater than 0.5 to 0 in I.

Answer 2

[Image Analyst]

You would not use repmat() - that's not appropriate. I don't even know how it could be used to do this. See if this explains it

% Display original image
subplot(2,2,1);
imshow(grayScaleImage, []);

% Get a "mask" of what pixels are brighter than 0.5
% Wherever the image is >0.5 mask will be true/1/white.
% Wherever the image is <= 0.5 the mask will be false/0/black.
mask = grayScaleImage > 0.5;
subplot(2,2,2);
imshow(mask);
title('Binary Image', 'FontSize', 20);

% Now everywhere that mask is true, set the original image to zero there.
grayScaleImage(mask) = 0;
% Display the result.
subplot(2,2,3);
imshow(grayScaleImage);
title('New Image', 'FontSize', 20);

[Alprcnkt]

thank you so much i also found something like that with help and it worked

green_c = lena_image(:,:,2); % to reaching green channels
black = green_c >= 0.5; % creating a matrix where green >= 0.5
black = repmat(black, [ 1 1 3]); %repeating the black matrix, 
lena_image(black)=0;
imshow(lena_image) % showing picture how it supposed to be

[Image Analyst]

You didn't say you were going to mask a color image according to if the green channel was greater than 0.5. To do that you'd normally do this:

mask = rgbImage(:,:,2) > 0.5; % Mask determined from green channel
% Mask the image using bsxfun() function
maskedRgbImage = bsxfun(@times, rgbImage, cast(mask, 'like', rgbImage));

And no repmat needed.