calculating circumference of an open ended object in a skeletonized binary image

I have a question regarding the regionprops function. Can it be used to accurately measure the circumference of an open ended object? As an example to illustrate the problem I am facing , let us say I want to compare the length of an object of the shape 'C' and another object of the shape 'O' (similar diameters, thickness is negligible). when I do regionprops and get perimeters, I find that the perimeter of C is more than perimeter of O. I am interested in extracting only the outer circumference/perimeter of the objects .. in which case, the perimeter of 'C' should be half of the perimeter of 'O'. If regionprops does do the trick, I need advice on how to approach this problem. Thank you very much!

Answer 1

[Image Analyst]

Simply sum the pixels, if your image consists of only that object:

cPerimLength = sum(cImage(:));
oPerimLength = sum(oImage(:));

That may be good enough. If it's not, explain why.

[Dfranklab]

 My problem is slightly more confounding since I have 
multiple C's and O's in the same image and I would
 like to compare them.
About my images: the images are of small blood vessels
 taken using a fluorescent microscope. I have two stains 
that are imaged in different channels.One of the stain
 selectively marks a portion of the perimeter of some vessels
 like a 'C' and the other stains the full perimeter of the 
vessel. I need to compare the perimeter of these vessels. I have attached 
the zoomed images below.

[Image Analyst]

Well you could just get the areas of each blob

props = regionprops(binaryImage, 'Area');
allAreas = [props.Area];

Indexing is from top to bottom then left to right as you encounter blobs across the image.

[Image Analyst]

Use bwconvhull() to get the convex hull of each blob. Then pass that image in to regionprops() and ask for perimeter or area.