A data processing unit configured to hardware calibrate a gaze tracking device with respect to a display unit , characterized in that the data processing unit comprises: a data output interface configured to produce control data (G-Ctrl) for presenting at least one graphical guide object (G1, G2, G3) on an active area of the display unit , the at least one graphical guide object (G1, G2, G3) designating at least one distinctive feature (121, 122; 123) of the gaze tracking device reflecting how the gaze tracking device may be positioned (x1, x2) on a first side of a frame of the display unit , and a user input interface configured to receive at least one user input movement command (MC) and a user input confirmation command (CC), and the data processing unit is configured to: produce the control data (G-Ctrl) such that at least one of the at least one graphical guide object (G1, G2, G3) moves on the active area in a first direction parallel to the first side of the frame in response to the at least one user input movement command (MC), assign a position value (Px) designating an actual position for the gaze tracking device on the first side of the frame based on a current position of the at least one graphical guide object (G1, G2, G3) on the active area , calculate an offset value (X-OFF) based on the assigned position value (Px) and a known measure of the active area , the offset value representing a distance in the first direction between a well-defined point of the gaze tracking device and a well-defined point of the first side , determine a width measure (X) and a height measure (Y) reflecting a physical extension of the active area ; and store, in response to a user input confirmation command (CC), the offset value (X-OFF), the width measure (X) and the height measure (Y) for use when determining a user's gaze point on the active area . a data output interface configured to produce control data (G-Ctrl) for presenting at least one graphical guide object (G1, G2, G3) on an active area of the display unit , the at least one graphical guide object (G1, G2, G3) designating at least one distinctive feature (121, 122; 123) of the gaze tracking device reflecting how the gaze tracking device may be positioned (x1, x2) on a first side of a frame of the display unit , and a user input interface configured to receive at least one user input movement command (MC) and a user input confirmation command (CC), and produce the control data (G-Ctrl) such that at least one of the at least one graphical guide object (G1, G2, G3) moves on the active area in a first direction parallel to the first side of the frame in response to the at least one user input movement command (MC), assign a position value (Px) designating an actual position for the gaze tracking device on the first side of the frame based on a current position of the at least one graphical guide object (G1, G2, G3) on the active area , calculate an offset value (X-OFF) based on the assigned position value (Px) and a known measure of the active area , the offset value representing a distance in the first direction between a well-defined point of the gaze tracking device and a well-defined point of the first side , determine a width measure (X) and a height measure (Y) reflecting a physical extension of the active area ; and store, in response to a user input confirmation command (CC), the offset value (X-OFF), the width measure (X) and the height measure (Y) for use when determining a user's gaze point on the active area . <EOS>