>Psychtoolbox>PsychCal

[dpi, SP] = MeasureDpi([theScreen=max][, useCreditCard=0]);

Helps the user to accurately measure the screen’s dots per inch by use of a
reference object of known size.

INPUT:

‘theScreen’ Screen parameter, if the screen is not provided, the
screen with maximum index (typically an external display) will be used.

‘useCreditCard’ Credit Card parameter, it allows you to use a credit card as
an object of known size if set to 1. Otherwise it is recommended to have a
ruler or some other object of known size at hand, and the script will ask for
the size of that object.

OUTPUT:

‘dpi’ Measured dots per inch.
‘SP’ Screen properties reported by the display driver (e.g., based on EDID).
It will provide you with a structure containing the physical size of the
screen (width and height) in mm, the screen resolution in pixels, the
dot pitch and dpi automatically.

Note: ‘SP’ relies on information from Screen(‘DisplaySize’). Read the notes
in ‘Screen DisplaySize?’ on potential limitations in reliability or accuracy
of this driver reported info.

Denis Pelli

History

5/28/96 dgp Updated to use new GetMouse.
3/20/97 dgp Updated.
4/2/97 dgp FlushEvents.
4/26/97 dhb Got rid of call to disp(7), which was writing ‘7’ to the
command window and not producing a beep.
8/16/97 dgp Changed “text” to “theText” to avoid conflict with TEXT function.
4/06/02 awi Check all elements of the new multi-element button vector
returned by GetMouse on Windows.
Replaced Chicago font with Arial because it’s available on
both Mac and Windows
11/6/06 dgp Updated from PTB-2 to PTB-3.
01/31/16 mk Fix some wrong assumptions for PTB-3. Get rid of GetChar.
8/13/19 mgg Modify to use it with a credit card (common object of a
known size).
9/10/19 mgg Optional output: Withdraw the presets from the screen
inc. the screen size, resolution, dot pitch and distance independent dpi.