Functions to convert screen sizes

In visual experiments, a very common conversion is done between stimuli sizes. There are three ways to measure the stimuli size: pixels, millimeters, and visual angle degrees. While we need to set the pixels of any stimuli on screen when programming the experiment, we need to report the visual angle degrees, which depend also on the screen distance.

Convert pixels into visual angle degrees

The function px2deg2() converts pixels into visual angle degrees. It can take a vector of the width and height (in that order) of the stimulus of interest, or only take a single value (not assuming either width or height).

Usage example:

stim_size_px = [100, 133]; 		% width: 100px, height: 133px
screen_size_px = [800, 600]; 	% width: 800px, height: 600px
screen_distance_mm = [635]; 	% eye to center of screen distance: 635 mm.
screen_size_mm = [320, 240]; 	% width: 320mm, height: 240mm.
stim_center_px = [400, 300]; 	% The center of the stimuli matches the center of the screen: 800/2, 600/2px

[stim_size_deg, stim_size_mm] =...
		px2deg2(stim_size_px, screen_size_px, screen_distance_mm, screen_size_mm, stim_center_px)

stim_size_deg is a 1-by-2 vector with the width and height of the stimulus in visual angle degrees.

stim_size_mm is a 1-by-2 vector with the width and height of the stimulus in millimeters.

Convert visual angle degrees into pixels

The function deg2px2() converts visual angle degrees into pixels. It can take a vector of the intended width and height (in that order) of the stimulus of interest, or only take a single value (not assuming either width or height).

Usage example:

stim_size_deg = [3.6080, 4.7974]; 		% width: 3.608°, height: 4.7974°
screen_size_px = [800, 600]; 	% width: 800px, height: 600px
screen_distance_mm = [635]; 	% eye to center of screen distance: 635 mm.
screen_size_mm = [320, 240]; 	% width: 320mm, height: 240mm.
stim_center_deg = [0, 0]; 		% The center of the stimuli matches the center of the screen: degrees of eccentricity

[stim_size_px, stim_size_mm, stim_center_px] =...
		px2deg2(stim_size_deg, screen_size_px, screen_distance_mm, screen_size_mm, stim_center_deg)

stim_size_px is a 1-by-2 vector with the width and height of the stimulus in screen pixels.

stim_size_mm is a 1-by-2 vector with the width and height of the stimulus in millimeters.

stim_center_px is a 1-by-2 vector with the screen position of the stimulus center, in screen pixels.

Correction of visual angle degrees

This function corrects for the position of the stimulus, if the stimulus is not centered. I took the principles from here: https://www.sr-research.com/eye-tracking-blog/background/visual-angle/