MATLAB Answers

M@lik Ali

how to mark pixel as zero

Asked by M@lik Ali
on 23 Jul 2012

Hi all

I have a segmented image i want to process only the larger segment. so i want to make the remaining pixels as zero. Please help me how i can do it.


You don't need to make the remaining pixels zero in order to measure the largest "segment".

Thanks again. yes but actually i want to do some more processing on the largest segment like to extract features to display that image. that why i think i need it, what you suggest ..


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2 Answers

Answer by Image Analyst
on 23 Jul 2012
 Accepted answer

Use regionprops to get the areas then get them all in an array so you can determine the largest.

measurements = regionprops(labeledImage, 'Area');
allAreas = [measurements.Area];
[sortedAreas sortIndexes] = sort(allAreas);
% Then mask your image or create a new one from the largest blob.


Thanks Image Analyst, in this way i cant get the largest region, now i want to mark all pixels as zero except largest region. please help me how i can mark the pixel as zero.

You can get the largest region. It's sortIndexes(1). Just extract the label for the largest image with ismember(). Then turn it into a binary image and use that to "mark all pixels as zero except largest region" in your original image.

biggestLabel = ismember(labeledImage, sortIndexes(1));
binaryImage = biggestLabel > 0;
% If you want the original image masked:
maskedImage = grayImage;
maskedImage(~binaryImage) = 0;

If this code doesn't work for you, let me know tomorrow and maybe I can adapt my blobs demo (in my File Exchange) to find the largest blob. But like I said in the comment, you don't need to zero anything out to get the measurements of the largest blob, so I don't know why you think you do, unless you just want to display it for curiosity's sake.

Muhammad, you can color your labels like this, instead of all that code you gave in your comment below:

labeledImage = bwlabel(binaryImage, 8);     % Label each blob so we can make measurements of it
coloredLabels = label2rgb (labeledImage, 'hsv', 'k', 'shuffle'); % pseudo random color labels

Answer by jagadeeshwar
on 23 Jul 2012

hai imran just crop the image which segment u want it can be enlarge after crop operation


Not necessarily true. With a crop, you could have a portion of another blob included in the cropped image, so you could have more than one blob in the cropped image. Besides, he'd first need to know what the largest blob is before he could even crop it, but that's his main problem - finding the largest blob. He doesn't know which blob it is unless he sorts it like I showed him how. Plus there's no need to do any enlarging.

Ok thanks, my goal is to extract the features of the largest regions, that why i was thinking that once i have a complete image having some pixels zero values. then its easy to further process.

One more thing can i do like this

as i have a piece of code like this for c = 1 : size(ca, 2) for r = 1 : size(ca, 1) jj=ca{r,c}; indx=indx+1;

                 if  labels(indx)==1 
                    colorMap(r:c, 1) = 1;  
                    colorMap(r:c, 2) = 0;  
                    colorMap(r:c, 3) = 0;  
                elseif labels(indx)==2
                    colorMap(r:c, 1) = 1;  
                    colorMap(r:c, 2) = 1;  
                    colorMap(r:c,  3) =0;  
                    elseif labels(indx)==3 
                    colorMap(r:c, 1) = .5;  
                    colorMap(r:c, 2) = .5;  
                    colorMap(r:c,  3) = .5;  
                    elseif labels(indx)==4 
                    colorMap(r:c, 1) = 0;  
                    colorMap(r:c, 2) = 1;  
                    colorMap(r:c,  3) = 0;  
                    elseif labels(indx)==5 
                    colorMap(r:c, 1) = 1;  
                    colorMap(r:c, 2) = 0;  
                    colorMap(r:c,  3) = 1;  
                    elseif labels(indx)==6
                    colorMap(r:c, 1) = 127/255;  
                    colorMap(r:c, 2) =1;  
                    colorMap(r:c,  3) = 212/255;  
                    elseif labels(indx)==7 
%                     elseif data_idxs(indx)==7 
                    colorMap(r:c, 1) = 1;  
                    colorMap(r:c, 2) = 0;  
                    colorMap(r:c,  3) = 0;  
                    colorMap(r:c, 1) = 1;  
                    colorMap(r:c, 2) =0;  
                    colorMap(r:c,  3) = 1;  

using this code i color the different regions. now can i make the pixel values zero as in this case i put the different color for the pixel, so same way i want to put some pixel as zero means when

elseif labels(indx)==6 value (r:c)=0 hoe i can do like this.

i am able to do this my job will be easy.

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