FAQ: I had the wrong PHYSICAL.INI / screen config settings, will my data be affected?

[Sam]

Configuring the EyeLink Host PC with a correct physical dimension for you Display PC setup are crucial for the Host PC to accurately calculate pixel-per-degree resolution, which is used to determine velocity and saccade amplitude. These settings are stored in the PHYSICAL.INI file (for EyeLink II / EyeLink 1000) or within the Screen Configuration settings (for EyeLink 1000 Plus / EyeLink Portable Duo). For more background information, please refer to our resource, What is visual angle and how does it apply to eye tracking?.

Important Note: If the physical settings do not match your experimental setup, do not be alarmed. The calibration routine ensures that the raw gaze data, which is in screen pixels, will still be accurate. However, the online event parser will not be accurate. To obtain correct velocity and saccade amplitude metrics, you can easily recalculate them using simple trigonometry with the correct measurements in post-processing.

To ensure that future gaze data is correct, it is essential to update these physical settings on the Host PC.

• Eyelink 1000 Plus / EyeLink Portable Duo: 
  
  From the Host Software, press CTRL+ALT+Q to exit to the File Manager. From the File Manager click the gear icon in the top left corner to access the Configurations Screen.
  
  
     
  
  From the Configurations Screen, select the Screen Settings Wizard icon to access the Screen Settings Wizard.
  
  
     
  
  Follow the on-screen directions to enter the values for Screen Size, Default Display Resolution, Eye-to-Screen Distance, and Camera-to-Screen Distance. Once complete, click the Save button and then click the EyeLink logo icon to restart the Host software.
  
  
• EyeLink 1000: 
  
  From the host software press CTL+ALT+Q to exit the Host software and enter the DOS environment. From the DOS prompt, execute the following command:
  
  
  Code:

  >> edit C:\ELCL\EXE\physical.ini
  
  This will open the PHYSICAL.INI file in a text editor. From here enter the correct parameters it asks for:
  
  
  Code:

  ## screen_phys_coords = <left>, <top>, <right>, <bottom>
;; Meaure the distance of the visible part of the display screen edge
;; relative to the center of the screen (measured in in millimeters).
;; <left>, <top>, <right>, <bottom>:
;; position of display area corners relative to display center
screen_phys_coords = -188.0, 146.0, 188.0, -146.0

## screen_pixel_coords = <left> <top> <right> <bottom>
;; Sets the gaze-position coordinate system, which is used for all
;; calibration target locations and drawing commands.  Usually set
;; to correspond to the pixel mapping of the participant display. 
;; Issue the calibration_type command after changing this to recompute
;; fixation target positions. 
;; You should also write a DISPLAY_COORDS message to the start of
;; the EDF file to record the display resolution.
;;            <left>: X coordinate of left of display area
;;            <top>: Y coordinate of top of display area
;;            <right>: X coordinate of right of display area
;;            <bottom>: Y coordinate of bottom of display area
screen_pixel_coords = 0.0, 0.0, 1024.0, 768.0

## screen_distance = <mm to center> | <mm to top> <mm to bottom>
;; Used for visual angle and velocity calculations. 
;; Providing <mm to top> <mm to bottom> parameters will give better
;; estimates than <mm to center>
;; <mm to center> = distance from display center to participant in millimeters.
;; <mm to top> = distance from display top to participant in millimeters.
;; <mm to bottom> = distance from display bottom to participant in millimeters.
screen_distance = 600 660
  
  If using the EyeLink 1000 in Remote mode, the camera-to-screen distance also needs to be edited. Add the following command to the FINAL.INI file, adjusting the dz parameter to accurately reflect the distance from the display monitor to the base of the camera lens in millimeters (the minus sign should be preserved).
  
  Code:

  ## remote_camera_position <rh> <rv> <dx> <dy> <dz>
;; <rh>: 10;    // rotation of camera from screen (clockwise from top)
;;              i.e. how much the right edge of the camera is closer than left edge of camera
;;              i.e. 10 assumes right edge is closer than left edge
;; <rv>: 17;  // tilt of camera from screen (top toward screen)
;; <dx>: -80;  // bottom-center of display in cam coords
;; <dy>:  60;
;; <dz>: -90;
remote_camera_position 10 17 -80 60 -90
  
  IMPORTANT NOTE: Please make sure that the values entered are in millimeters and that you enter the values as they are specified in the comments above. A common mistake is entering the values incorrectly or in the wrong units.
  
  
• EyeLink II:
  From the host software press CTL+ALT+Q to exit the Host software and enter the DOS environment. From the DOS prompt, execute the following command:
  
  
  Code:

  edit C:\EYELINK2\EXE\physical.ini
  
  Similar to the EyeLink 1000, You must modify the four commands below to specify the dimension and placement of your screen and markers relative to the subject.
  
  
  Code:

      ## marker_phys_coords = <x, y for top left> <x, y for bottom left>
    ##            <x, y for top right> <x, y for bottom right>
    ;; Physical position of LED markers in millimeters
    ;; The 4 LED markers are mounted at the corners of the display.  
    ;; These are their physical locations, measured with right and up
    ;; from screen center being positive, and left and down being negative.
    ;; The marker coordinate pairs are entered in this order:
    ;;                1    3
    ;;                2    4    
    ;; Physical coords: negative if to the left of the center of screen,
    ;;            negative if below the center of screen
marker_phys_coords = -220,145, -220,-145, 220,145, 220,-145


    ## screen_phys_coords = <left>, <top>, <right>, <bottom>
    ;; Meaure the distance of the visible part of the display screen edge
    ;; relative to the center of the screen (measured in in millimeters).
    ;; <left>, <top>, <right>, <bottom>:
    ;;    position of display area corners relative to display center
screen_phys_coords = -188.0, 146.0, 188.0, -146.0


    ## screen_pixel_coords = <left> <top> <right> <bottom>
    ;; Sets the gaze-position coordinate system, which is used for all
    ;; calibration target locations and drawing commands.  Usually set
    ;; to correspond to the pixel mapping of the subject display.  
    ;; Issue the calibration_type command after changing this to recompute
    ;; fixation target positions.  
    ;; You should also write a DISPLAY_COORDS message to the start of
    ;; the EDF file to record the display resolution.
    ;;    <left>: X coordinate of left of display area
    ;;    <top>: Y coordinate of top of display area
    ;;    <right>: X coordinate of right of display area
    ;;    <bottom>: Y coordinate of bottom of display area
screen_pixel_coords = 0.0, 0.0, 1024.0, 768.0

    
    ## simulation_screen_distance = <mm>
    ;; Used to create marker positions for simulated head tracking to
    ;; compute correct visual angles and velocities, in combination with
    ;; physical marker positions.  
    ;; <mm> = simulated distance from display to subject in millimeters.
    ;; Note that this only affects the simulation pixels-per-degree
    ;; measurements.
simulation_screen_distance = 600

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If you have data which was collected with the incorrect physical.ini measurements, you can reparse the gaze events post hoc using the Event Reparsing feature in Data Viewer.

If there is concern about the quality of the gaze data collected with the these invalid settings, please contact support@sr-research.com so we can discuss the possible implications. Please make sure to include a description of the experiment along with the original settings and the corrected settings in the PHYSICAL.INI.