Convert psychopy visual degrees to monitor pixels. Originally designed for iohub tobii eyetracker recordings.

eyetracker_visdeg_to_px.py

import math
from typing import Tuple

def vdeg_to_px(
    gaze_vdeg:Tuple[float, float], 
    screen_size_cm:Tuple[float, float], 
    screen_size_px:Tuple[int, int], 
    viewer_distance_cm:float,
) -> Tuple[float, float]:
    '''
    Convert from psychopy visdeg (0,0 at center, +y up, +x right)
    to normalized screen coordinates (0,0 at topleft, +y down, +x right)

    :param gaze_vdeg: (x,y) coordinates in visual degrees
    :param screen_size_cm: (width, height) of screen in cm
    :param screen_size_px: (width, height) of screen in px
    :param viewer_distance_cm: distance from screen to viewer in cm

    :return: (x,y) coordinates in px
    '''

    # to normalized screen coordinates
    norm_psychopy = [math.tan(math.radians(this_vdeg)) * viewer_distance_cm / this_scn_sz_cm 
                     for this_vdeg, this_scn_sz_cm 
                     in zip(gaze_vdeg, screen_size_cm)]

    # change from psychopy coordinate frame (0,0 at center, +y up, +x right) 
    #        to normalized monitor coordinate frame (0,0 at topleft, +y down, +x right)
    norm_monitor = [norm_psychopy[0] + 0.5, norm_psychopy[1] * -1 + 0.5]

    # to pixels:   norm x/y * screen resolution in pixels
    px = [this_norm_monitor * this_scn_sz_px 
          for this_norm_monitor, this_scn_sz_px 
          in zip(norm_monitor, screen_size_px)]

    return px

##################################################################
# test the function
vdeg_to_px_args = {
    'screen_size_cm' : (34.5, 19.5),
    'screen_size_px' : (1920, 1080),
    'viewer_distance_cm' : 65,
}

coords_to_test_visdeg = [
    (0, 0),    # center of the screen
    (0, 8.5),  # near top, horizontally centered
    (14, 0),   # near left, vertically centered
    (0, -8.5), # near bottom, horizontally centered
    (-14, 0),  # near right, vertically centered
]

for coord in coords_to_test_visdeg:
    print(f"{str(coord):<12}=>  {vdeg_to_px(coord, **vdeg_to_px_args)}")
    
# (0, 0)      =>  [960.0, 540.0]
# (0, 8.5)    =>  [960.0, 1.976395143139884]
# (14, 0)     =>  [1861.9169494153318, 540.0]
# (0, -8.5)   =>  [960.0, 1078.0236048568602]
# (-14, 0)    =>  [58.08305058466807, 540.0]


##################################################################
# example with pandas
import pandas as pd

df = pd.DataFrame(coords_to_test_visdeg, columns=['gaze_x_deg', 'gaze_y_deg'])
print('Pre-conversion:')
print(df)

# convert to pixels
df[['gaze_x_px', 'gaze_y_px']]  = df.apply(lambda row: vdeg_to_px((row['gaze_x_deg'], row['gaze_y_deg']), **vdeg_to_px_args), axis=1).apply(pd.Series)
print('\n\nPost-conversion:')
print(df)

# Pre-conversion:
#    gaze_x_deg  gaze_y_deg
# 0           0         0.0
# 1           0         8.5
# 2          14         0.0
# 3           0        -8.5
# 4         -14         0.0


# Post-conversion:
#    gaze_x_deg  gaze_y_deg    gaze_x_px    gaze_y_px
# 0           0         0.0   960.000000   540.000000
# 1           0         8.5   960.000000     1.976395
# 2          14         0.0  1861.916949   540.000000
# 3           0        -8.5   960.000000  1078.023605
# 4         -14         0.0    58.083051   540.000000