why does bwperim make the border WHITE?!!

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

[Walter Roberson]

Could you post your binaryImage and your perim1 and perim2 ?

[Walter Roberson]

I cannot tell for sure, but it looks to me as if the original might have a single-row outline of 0's all around it.

[Steven]

How can I avoid or correct it?

[Walter Roberson]

How did you create the binary image? What image did you start with and how did you process it?

[Walter Roberson]

http://www.mathworks.com/help/images/ref/imclearborder.html

but be careful about the possibility that some of your image touches the border.

[Steven]

for creating binary, I used:

binaryimage = im2bw(gray_scale_image,graythresh(gray_scale_image)

Answer 1

[Image Analyst]

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?

[Steven]

Thanks.

I posted them above.

Thanks

Steven

[Image Analyst]

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]

Thanks.

How about using imclearborder? It does the same right?

Thanks.

Steven

[Walter Roberson]

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.