why does bwperim make the border WHITE?!!

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Steven
Steven 2013-12-25
编辑： Steven 2014-1-1
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
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Walter Roberson
Walter Roberson 2013-12-25
http://www.mathworks.com/help/images/ref/imclearborder.html
but be careful about the possibility that some of your image touches the border.
Steven
Steven 2013-12-25
for creating binary, I used:
binaryimage = im2bw(gray_scale_image,graythresh(gray_scale_image)

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Image Analyst
Image Analyst 2013-12-25
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?
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Steven
Steven 2013-12-25
Thanks.
How about using imclearborder? It does the same right?
Thanks.
Steven
Walter Roberson
Walter Roberson 2013-12-25
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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