```% Clear the workspace and the screen
sca;
close all;
clear;

% Here we call some default settings for setting up Psychtoolbox
PsychDefaultSetup(2);

% Get the screen numbers. This gives us a number for each of the screens
% attached to our computer. For example, when I call this I get the vector
% [0 1]. The first number is the native display for my laptop and the
% second referes to my secondary external monitor. By native display I mean
% the display the is physically part of my laptop. With a non-laptop
% computer look at your screen preferences to see which is the primary
% monitor.
screens = Screen('Screens');

% To draw we select the maximum of these numbers. So in a situation where we
% have two screens attached to our monitor we will draw to the external
% screen. If I were to select the minimum of these numbers then I would be
% displaying on the physical screen of my laptop.
screenNumber = max(screens);

% Define black and white (white will be 1 and black 0). This is because
% luminace values are genrally defined between 0 and 1.
white = WhiteIndex(screenNumber);
black = BlackIndex(screenNumber);

% Do a simply calculation to calculate the luminance value for grey. This
% will be half the luminace value for white
grey = white / 2;

% Open an on screen window and color it grey. This function returns a
% number that identifies the window we have opened "window" and a vector
% "windowRect".
% "windowRect" is a vector of numbers: the first is the X coordinate
% representing the far left of our screen, the second the Y coordinate
% representing the top of our screen,
% the third the X coordinate representing
% the far right of our screen and finally the Y coordinate representing the
% bottom of our screen.
[window, windowRect] = PsychImaging('OpenWindow', screenNumber, grey);

% Now that we have a window open we can query some of its properties. So,
% lets to the following:

% This function call will give use the same information as contained in
% "windowRect"
rect = Screen('Rect', window);

% Get the size of the on screen window in pixels, these are the last two
% numbers in "windowRect" and "rect"
[screenXpixels, screenYpixels] = Screen('WindowSize', window);

% Get the centre coordinate of the window in pixels.
% xCenter = screenXpixels / 2
% yCenter = screenYpixels / 2
[xCenter, yCenter] = RectCenter(windowRect);

% Query the inter-frame-interval. This refers to the minimum possible time
% between drawing to the screen
ifi = Screen('GetFlipInterval', window);

% We can also determine the refresh rate of our screen. The
% relationship between the two is: ifi = 1 / hertz
hertz = FrameRate(window);

% We can also query the "nominal" refresh rate of our screen. This is
% the refresh rate as reported by the video card. This is rounded to the
% nearest integer. In reality there can be small differences between
% "hertz" and "nominalHertz"
% This is nothing to worry about. See Screen FrameRate? and Screen
nominalHertz = Screen('NominalFrameRate', window);

% Here we get the pixel size. This is not the physical size of the pixels
% but the color depth of the pixel in bits
pixelSize = Screen('PixelSize', window);

% Queries the display size in mm as reported by the operating system. Note
% that there are some complexities here. See Screen DisplaySize? for
% information. So always measure your screen size directly.
[width, height] = Screen('DisplaySize', screenNumber);

% Get the maximum coded luminance level (this should be 1)
maxLum = Screen('ColorRange', window);

% Wait for a keyboard button press to exit
KbStrokeWait;

% Clear the screen. "sca" is short hand for "Screen CloseAll". This clears
% all features related to PTB. Note: we leave the variables in the
% workspace so you can have a look at them.
sca;
```