In this tool you can measure distances in 2D photos.
First time you click you will mark the first point in the reference line. Second click ends the reference line. After the reference line is marked a popup will ask you for the length of the reference line. After the length is given pixels per millimeter (PPMM) vill be calculated as displayed.
When PPMM is available you can mark the starting and end points for the measurement lines and the length will be displayed.