When an image is loaded, three pairs of line segments are displayed against the background image in their initial position. Each "end point" can be moved by pick and drag. 

(The colored lines were retouched, because the original lines had been too thin to be clearly recognized in this shrinked image.)

We move the line segments to edges in the background image. 
  • green pair of lines are placed onto edges parallel to x-axis.
  • red   pair  of lines are placed onto edges parallel to y-axis.
  • blue  pair  of lines are placed onto edges parallel to z-axis.
In the process of moving line you can turn "Magnet Mode" on. In this mode Photo3D searches an edge in the background image near the line and  automatically fit the line to the edge.