find the short length between two points

Dear all, I have video of growing plant root. I managed to make a binary image each frame. Now I would like to calculate the length of the root. I would be happy to use ginput(x) to define the source point that is not changing across the video, and to calculate the shortest distance from it to the root tip in each frame. HOW CAN I DO THAT? please.

In the image, you can the advance (left to right) form the original image, binary image, skeleton image, and skeleton image without branches. skeleton image without branches not good because of the data losses. skeleton image not perfect because of the no top defined source point.

please help, all I want is the total length of the root in the original image? Best, HM