how can i set an images pixels to black ?

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Alprcnkt
Alprcnkt 2016-4-2
评论： li yan 2016-4-3
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 ?

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回答（2 个）

li yan
li yan 2016-4-2
I(I > 0.5) = 0;
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li yan
li yan 2016-4-3
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.

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Image Analyst
Image Analyst 2016-4-2
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);
  2 个评论
Alprcnkt
Alprcnkt 2016-4-2
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
Image Analyst 2016-4-2
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.

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