Convert a line thickness from degrees of visual angle to pixels

The defaults (30 inch viewing distance, 96 dpi) match Szafir (2018)'s own assumed viewing conditions. Override both to match an actual lab setup (measured pixel pitch and viewing distance), rather than trusting a reported dpi, which is frequently wrong for actual displays.

Usage

visual_angle_to_px(
  deg,
  viewing_distance_in = 30,
  dpi = 96,
  convention = .C4A$angle_convention
)

Arguments

deg

line thickness in degrees of visual angle

viewing_distance_in

viewing distance, in inches

dpi

display resolution, in dots (pixels) per inch

convention

"szafir" or "standard" – see Details. Defaults to the angle_convention package option (see c4a_options()).

Value

line thickness in pixels

Details

The standard chord formula for the linear size subtending a visual angle deg at distance d is 2 * d * tan(deg/2) – this is what convention = "standard" computes. Szafir (2018)'s own in-text px-equivalents (e.g. "0.25 deg (6px)", "2.0 deg (50px)" for points; "0.05 deg (1px)", "0.35 deg (9px)" for lines) are consistently exactly half of that formula's result at their own stated 30in/96dpi assumptions – across both experiments, at four independently checked (angle, px) pairs, which is why this is treated as their own conversion convention rather than a one-off rounding difference. convention = "szafir" (the default, and the package default via c4a_options("angle_convention")) reproduces that, so preview sizes in palette_prob_bg() / c4a_gui() visually match the paper's own figures; convention = "standard" gives the textbook-correct value, which is what you want once calibrating against a real measured display.

See also

px_to_visual_angle(), line_discrim_prob(), c4a_options()