Finding x,y coordinates of pixel by RGB color value

Question

awezmm 2017-5-5

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https://ww2.mathworks.cn/matlabcentral/answers/338809-finding-x-y-coordinates-of-pixel-by-rgb-color-value

评论： Image Analyst 2018-1-23

Hi, Is there a reverse impixel function? I want to find the x and y coordinates of pixels in a color image by searching for them only with a RGB color value. I have tried converting into a binary image and searching for the '1' values by using

BW = im2bw(RGB, level)

and then [rows,cols] = find(BW==1);

but I am confused on how to set a level to implement an RGB value so that only my specific color pixels will show up as white in the binary image.

Is there a better way to find the coordinates of pixels only by using RGB value?

thanks!

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Answer 1

Image Analyst 2017-5-5

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Try this:

% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
% Find where each color plane is your exact desired color value.
mask = redChannel == desiredRedValue && ...
    greenChannel == desiredGreenValue & ...
    blueChannel == desiredBlueChannel;
[rows, columns] = find(mask);  % Note [rows, columns] = [y, x], NOT [x, y]

WHY do you want to do this. You probably don't need to and there is a better way if I just knew what you were going to do after this.

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awezmm 2017-5-6

Thank you for your response. I've only been doing image processing for about the past 40 Minutes so I too have just barely scratched the surface. I am trying to catch ahead though.

My previous inquiry was too confusing. I was looking for a false range mask for rows and a completely DIFFERENT false range mask for columns. For instance if i only wanted to search in X coordinates of 10-20 and Y coordinates of 90-100? Is there a way to syntax a DIFFERENT false ranges for columns and rows.

Eventually I will be finding the distance between coordinates. And, yes I have gone through your demos. However, I believe this is the best way to go forward because I figured out that the desiredColor value can be a range in each channel and thus I can be easily searching for many different RGB values at the same time.

Image Analyst 2017-5-6

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I don't know what a "false range" is.

If you want to know what coordinates have x between 10 and 20 and y between 90 and 100, then do

indexes = columns >= 10 & columns <= 20 & rows >= 90 & rows <= 100;
% Extract only those in the range
c2 = columns(indexes);
r2 = rows(indexes);

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Answer 2

Franklin Prashanth C 2018-1-22

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https://ww2.mathworks.cn/matlabcentral/answers/338809-finding-x-y-coordinates-of-pixel-by-rgb-color-value#answer_301258

编辑：Image Analyst 2018-1-23

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How can I get these xy coordinate values, so that I can use it.?

[rows, cols] = find(BW==1); or 
[rows, columns] = find(mask);

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Image Analyst 2018-1-23

It just depends on whether you want the columns called "col" or "columns" and whether your binary image is called BW or mask. The ==1 is not necessary and can get deleted.

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