The invention is a method for negotiating a parameter of an optical communication protocol between two devices. One device displays a first calibration image comprising a series of pixel patterns having a predetermined position and different sizes. The other device takes a first calibration picture of the first calibration image, identifies a set comprising at least one detectable pixel pattern in the first calibration picture and selects a setting data based on the set. Then it displays a second calibration image comprising this setting data which is read by the first device via another calibration picture. The first device retrieves the setting data from the calibration picture, selects a value based on the setting data and updates the parameter with the selected value.