= Simple calibration =
Open again a test image in 'UVMAT_DEMO_FILES/EX01_pair'. We shall use the diameter of the half cylinder visible on the upper let of the image to set the calibration. Its physical diameter 
is 2cm. The corresponding diameter in pixels can be obtained with the ruler displayed by the menu bar '''Tools/ruler''' of uvmat.

First zoom on the cylinder to optimize the precison. Select zoom on, press the left mouse button and adjust the field with the directional key board arrows. Then unselect '''zoom on''' to allows for other mouse actions (otherwise zoom has priority). It is also useful to increase the contrast at the cylinder edge by setting '''MaxA''' to 100 in the frame '''Scalar''' (right side of uvmat).

Then select the menu bar the menu bar '''Tools/ruler''', press the left hand mouse button on the cylinder edge, draw a diameter keeping the mouse pressed, release it onthe opposite edge. The length in pixels, 140, is displayed.