remove pixels with certain rgb values

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Tian Tian 2018-2-15

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https://ww2.mathworks.cn/matlabcentral/answers/382972-remove-pixels-with-certain-rgb-values

评论： Jona Boender 2020-4-28

采纳的回答： Image Analyst

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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.

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Image Analyst 2018-2-17

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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).

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Image Analyst 2018-7-9

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.