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function [dpi, SP]=MeasureDpi(theScreen, CC)
% [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.
st = dbstack; funname = [st.name]; % This provides the name of the calling function, so when throwing an error, the user knows from where.
if nargin>2 || nargout>2
error([funname,':>> Error wrong call to function, use it as: dpi=MeasureDpi(Screen)', newline, 'or [dpi, SP]=MeasureDpi([Screen], [CC])']);
end
if nargin==0 || isempty(theScreen)
theScreen=max(Screen('Screens')); % Use the second screen, most of the cases.
disp([funname,':>> No screen value has been provided so I assume ',...
'that either you would like to use the secondary screen, or the main one if you only have one...']);
end
AssertOpenGL;
[unitInches, units, unit] = EnglishOrSI();
try
if nargin>1 && isequal(CC,1) % Using a credit card
objectInches= 8.56*(1/2.54); % ID-1 format, ISO/IEC 7810 credit card (85.60 × 53.98 mm).
else
disp('Please find an object of known width to hold against the display.');
disp('E.g. a ruler or an 8.5-inch page.');
objectInches=input(sprintf('How wide is your object, in %s? ',units))*unitInches;
disp('A small correction will be made for your viewing distance and the thickness of');
disp('the screen''s clear front plate, which separates your object from the screen''s');
disp('light emitting surface.');
end
isLCD = input('Is your display a flat panel, instead of a CRT? [y/n]: ', 's');
if isLCD == 'y'
thicknessInches=0.1;
fprintf('I assume that the display is a flat panel display, \n');
fprintf('with a thin (%.1f inch) clear front plate.\n',thicknessInches);
else
thicknessInches=0.25;
fprintf('I assume that the display is a CRT, \n');
fprintf('with a thick (%.1f inch) clear front plate.\n',thicknessInches);
end
distanceInches=input(sprintf('What is your viewing distance, roughly, in %s? ',units))*unitInches;
[window,screenRect]=Screen('OpenWindow',theScreen);
white=WhiteIndex(window);
black=BlackIndex(window);
% This might work if you have a modern monitor with reasonable updated drivers.
[SP.ScreenWidthOS, SP.ScreenHeightOS] = Screen('DisplaySize', theScreen);
[SP.Xpixels, SP.Ypixels] = Screen('WindowSize', window);
SP.DotPitchX= (SP.ScreenWidthOS/SP.Xpixels);
SP.DotPitchY=(SP.ScreenHeightOS/SP.Ypixels);
SP.DotPitchDiag= (SP.DotPitchX + SP.DotPitchY) /2;
SP.dpi=floor((sqrt((SP.Xpixels)^2+(SP.Ypixels)^2))/(sqrt((SP.ScreenWidthOS)^2+(SP.ScreenHeightOS)^2)/25.4));
% Instructions
if nargin>1 && isequal(CC,1) % Using a credit card
s=sprintf('Hold your credit card against the display.');
else
s=sprintf('Hold your %.1f-%s-wide object against the display.',objectInches/unitInches,unit);
end
theText={s,'Press, drag, and release the mouse to draw a bar'...
,'that matches the width of your object. Use one eye.'};
Screen('TextFont',window,'Arial');
s=24;
Screen('TextSize',window,s);
textLeading=s+8;
textRect=Screen('TextBounds',window,theText{1});
textRect(4)=length(theText)*textLeading;
textRect=CenterRect(textRect,screenRect);
textRect=AlignRect(textRect,screenRect,RectTop);
textRect(RectRight)=screenRect(RectRight);
dragText=theText;
dragTextRect=textRect;
% Animate
% Track horizontal mouse position to draw a bar of variable width.
for i=1:length(dragText)
Screen('DrawText',window,dragText{i},dragTextRect(RectLeft),dragTextRect(RectTop)+textLeading*i,black);
end
barRect=CenterRect(SetRect(0,0,RectWidth(screenRect),20),screenRect);
fullBarRect=barRect;
top=RectTop;
bottom=RectBottom;
left=RectLeft;
right=RectRight;
Screen('FillRect',window,white,barRect);
Screen('FrameRect',window,black,barRect);
Screen('Flip',window);
oldButton=0;
while 1
[x,~,button]=GetMouse(window);
if any(button)
if ~oldButton
origin=x;
barRect(left)=origin;
barRect(right)=origin;
else
if x<origin
barRect(left)=x;
barRect(right)=origin;
else
barRect(left)=origin;
barRect(right)=x;
end
end
if ~IsEmptyRect(barRect)
Screen('FillRect',window,black,barRect);
end
backgroundRect=barRect;
backgroundRect(left)=screenRect(left);
backgroundRect(right)=barRect(left);
if ~IsEmptyRect(backgroundRect)
Screen('FillRect',window,white,backgroundRect);
end
backgroundRect(left)=barRect(right);
backgroundRect(right)=screenRect(right);
if ~IsEmptyRect(backgroundRect)
Screen('FillRect',window,white,backgroundRect);
end
for i=1:length(dragText)
Screen('DrawText',window,dragText{i},dragTextRect(RectLeft),dragTextRect(RectTop)+textLeading*i,black);
end
Screen('FrameRect',window,black,fullBarRect);
Screen('Flip',window);
if ~IsEmptyRect(barRect)
Screen('FillRect',window,black,barRect);
end
else
if oldButton
objectPix=RectWidth(barRect);
dpi=objectPix/objectInches;
dpi=dpi*distanceInches/(distanceInches+thicknessInches);
clear theText
if unitInches==1
theText{1}=sprintf('%.0f dots per inch.',dpi);
else
theText{1}=sprintf('%.0f dots per inch. (%.0f dots/cm.)',dpi,dpi/2.54);
end
theText{2}='Click once to do it again; twice if you''re done.';
textRect=Screen('TextBounds',window,theText{2});
textRect(4)=length(theText)*textLeading;
textRect=CenterRect(textRect,screenRect);
textRect=AlignRect(textRect,screenRect,RectTop);
Screen('FillRect',window,white,InsetRect(dragTextRect,0,round(-0.3*textLeading)));
for i=1:length(theText)
Screen('DrawText',window,theText{i},textRect(RectLeft),textRect(RectTop)+textLeading*i,black);
end
Screen('Flip',window);
i=GetClicks;
if ~IsEmptyRect(barRect)
Screen('FillRect',window,white,barRect);
end
Screen('FillRect',window,white,InsetRect(textRect,0,round(-0.3*RectHeight(textRect))));
if i>1
break
end
for i=1:length(dragText)
Screen('DrawText',window,dragText{i},dragTextRect(RectLeft),dragTextRect(RectTop)+textLeading*i,black);
end
Screen('FrameRect',window,black,fullBarRect);
Screen('Flip',window);
end
end
oldButton=any(button);
end
Screen('Close',window);
catch
sca;
psychrethrow(psychlasterror);
end
function [unitInches, units, unit] = EnglishOrSI()
% Ask user whether to use the International System of Units or the English
% / Ameerican System(inches).
inches=input('Do you prefer inches (1) or cm (0)? ');
if inches
unitInches=1;
units='inches'; % e.g. distance in inches
unit='inch'; % e.g. 5 inch object
else
unitInches=1/2.54;
units='cm';
unit='cm';
end
end
end
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