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This code has been written to make it easier to report visual angle in
papers, and to ensure a standardized calculation is used.

This code can take variable input (e.g. horizontal, vertical, or diagonal
screen measurements in any units), and will calculate any number of 
degrees of visual angle in pixels, as well as degrees of visual angle 
spanned by the computer screen or an image. 

The code is provided on: http://saliency.mit.edu/

Two versions of the code are available for convenience: 
(1) computeDegVisAngle takes in a variable-length input argument list
(2) computeDegVisAngle_gui is a simple user interface

Usage and input parameters are specified in the respective functions.

Example usage of (1):
>> computeDegVisAngle('diagonal','19.5 inch','distance','24 inch','hres',1280,'vres',1024)

Example usage of (2):
>> computeDegVisAngle_gui

NOTE: if you provide the screen diagonal and resolution, then the screen
height and width will be estimated under the assumption that the aspect
ratio of the screen dimensions matches that of the set resolution. If
these estimates look wrong, either:
(1) you measured the monitor dimensions, instead of the screen dimensions
(2) the screen resolution aspect ratio does not match the screen
(3) explicitely enter the screen height and width instead of the diagonal

Common mistake: remember that the number of pixels per unit size of screen 
or image is a linear function, but number of pixels per degree of visual 
angle is NOT.

Remember: when writing a paper, include enough information for visual 
angle to be calculated - providing screen resolution without screen 
dimensions or vice versa is insufficient! Also, remember to measure the
screen itself, rather than the whole monitor.

Zoya Bylinskii, Aug 2014

Resources used: http://osdoc.cogsci.nl/miscellaneous/visual-angle/, 
Thank you to Lavanya Sharan and Melissa Vo for helpful discussion.