How to calculate diameter from side view?

Hi!

Imagine we have a side view of a circular object such as a coin. How can I accurately calculate the diameter of an object from side view image?

The simple thing is that the user determines the two sides (left and right) of the object as the diameter, but is there another way that MATLAB can detect it automatically and accurately?

Thanks!

Steven

[Walter Roberson]

Could you post a couple of sample images?

[Jan]

To calculate the diameter accurately, you need the parameter of the optical system and the distance also, when you want the real extent.

[Steven]

Thanks all. The sample is posted here.

Answer 1

[Image Analyst]

I don't know what you mean by side view. Do you mean like front side and back side (head side and tail side), or do you mean like an "edge-on" view, so that it looks like a rod or stick? Either way, you need to get a binary image of the object and then use regionprops to get the length or diameter of it. Post an image so we can see it. regionprops gives the answer in pixels. If you need real world units, like Jan brought up, then see my attached spatial calibration demo, below in blue text.

[Steven]

Thanks all. What I meant was something like this. Imagine this is a spreading case and I want to find the area of circle and so the diameter.

What I want is the length of red line (diameter). How can I get it? I mean even the pixel numbers?

One more question: I drew this line by plot and the user determines the two ends of the circle. If I find the pixel numbers between these two points (red line length) in grayscale image, is it right or I have to do so in binary image? [I got the pixel numbers by just decreasing x2 -x1]

Thanks

Steven

[Image Analyst]

You can have the user do it by calling imdistline() or improfile().

[Steven]

1. Is there a way that Matlab can find the length, since it is more accurate?

2. So this x2 -x1 in grayscale image is right? No need to use binary image? Right?

Thanks!

Steven

[Image Analyst]

It can if you can get the tilted disk by itself. For example you can ask regionprops to get the majoraxislength. Can you improve your image capture at all so it's not blurry and dark? Can you make it so that the disk is surrounded by a solid black border?

[Steven]

I am afraid I can't. Maybe I just crop it and adjust the contrast to do so.

[Image Analyst]

How can you crop if you don't know what size it is yet? Anyway, adjusting contrast by a linear stretch like imadjust() or non-linearly with histeq() won't help at all. You'd need more sophisticated things, like morphological filters like imopen().