# why does bwperim make the border WHITE?!!

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Steven on 25 Dec 2013
Edited: Steven on 1 Jan 2014
I want to use bwperim to calculate the perimeter of an object, but I got a strange thing!
I use bwperim in two different states: first to the original binary image and second to its inverse, i.e.,:
%%binaryImage
perim1 = bwperim (binaryImage);
perim2 = bwperim (~binaryImage);
The strange thing is that the first one (perim1) gives an image that is almost black (except the detected boundaries, of course), but:
Exactly, the borders of the whole image, I mean the FIRST row and column and the LAST row and column are white! I mean it has given the value of 1 to the pixels of the image border! Why?!!
For the second one (perim2), this is not the case and it is correct as the original image.
The binary image:
perim1 (with white border):
perim2:
Thanks so much!
Steven

Show 3 older comments
Walter Roberson on 25 Dec 2013
How did you create the binary image? What image did you start with and how did you process it?
Walter Roberson on 25 Dec 2013
but be careful about the possibility that some of your image touches the border.
Steven on 25 Dec 2013
for creating binary, I used:
binaryimage = im2bw(gray_scale_image,graythresh(gray_scale_image)

Image Analyst on 25 Dec 2013
I don't think perim1 should have the edges of the image as white. Can you post the binary image so I can check it out?

Show 1 older comment
Image Analyst on 25 Dec 2013
Well, the binary image is white at the edges. It assumes that that is the end (outer perimeter) of your object so it sets the edge pixels white. So it makes sense, don't you think? If you're going to assume that your object continues as white outside the boundaries of the image then you should erase the white lines after calling bwperim():
perim1(1,:) = false;
perim1(1,end) = false;
perim1(:,1) = false;
perim1(:,end) = false;
Steven on 25 Dec 2013
Thanks.
How about using imclearborder? It does the same right?
Thanks.
Steven
Walter Roberson on 25 Dec 2013
Erasing the borders yourself is safer if you might possibly have an object touching the edge. If you know you will not, then imclearborder() is more convenient.

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