C
Measurements within reference plane
Solve for homography H relating reference plane to image plane
•H maps reference plane (X,Y) coords to image plane (x,y) coords
•Fully determined from 4 known points on ground plane
–Option A:  physically measure 4 points on ground
–Option B:  find a square, guess the dimensions
–Option C:  Note  H = columns 1,2,4 projection matrix
»derive on board (this works assuming Z = 0)
•Given (x, y), can find (X,Y) by H-1 
reference plane
image plane