Measure the distance of a curve within an image

I am trying to learn how to take measurements of different aspects in an image. With the image provided, how would I measure the length of the curved blue line and the diameter of the green circle if the hight of the red box is 3 meters. I will post my code if I can actually get something working. Thank you.

Answer 1

[Benjamin Thompson]

You could use imshow to load and display the image, then ginput get get a series of manually selected points on the line. If you have the image processing toolbox, imtool has a bit more image analysis capabilities, and colorThresholder can be used to create a blue color mask so you can get the list of pixels that are part of the blue line and then do further math on those pixels positions.

Answer 2

[Image Analyst]

I'd just get a mask of the blue line. Like

[r, g, b] = imsplit(rgbImage);
blueMask = r == 0 & g == 0 & b == 255;

or you can use the Color Thresholder app on the Apps tab of the tool ribbon.

If the line is known to be a single pixel wide then you can just count the number of non-zero pixels.

lineLength = nnz(blueMask);

If the line is thick, then you can get the skeleton image with bwskel first

blueMask = bwskel(blueMask);
lineLength = nnz(blueMask);