# Single Dot

> Source: [singleDot](https://peterscarfe.com/singleDot.html)

This very simple demo draws a single dot on the screen in a randomly determined position. Each time you run the code the dot will appear at a different position as we randomly seed the number generator in the code.

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

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

% Seed the random number generator. Here we use the an older way to be
% compatible with older systems.
rng('shuffle')

% Get the screen numbers. This gives us a number for each of the screens
% attached to our computer. For help see: Screen Screens?
screens = Screen('Screens');

% 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. When only one screen is attached to the monitor we will draw to
% this. For help see: help max
screenNumber = max(screens);

% Define black and white (white will be 1 and black 0). This is because we
% are defining luminace values between 0 and 1 through the use of the PTB
% default setting call above.
% For help see: help WhiteIndex and help BlackIndex
white = WhiteIndex(screenNumber);
black = BlackIndex(screenNumber);

% Open an on screen window and color it black.
% For help see: Screen OpenWindow?
[window, windowRect] = PsychImaging('OpenWindow', screenNumber, black);

% Get the size of the on screen window in pixels.
% For help see: Screen WindowSize?
[screenXpixels, screenYpixels] = Screen('WindowSize', window);

% Get the centre coordinate of the window in pixels
% For help see: help RectCenter
[xCenter, yCenter] = RectCenter(windowRect);

% Enable alpha blending for anti-aliasing
% For help see: Screen BlendFunction?
% Also see: Chapter 6 of the OpenGL programming guide
Screen('BlendFunction', window, GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

% Set the color of our dot to full red. Color is defined by red green
% and blue components (RGB). So we have three numbers which
% define our RGB values. The maximum number for each is 1 and the minimum
% 0. So, "full red" is [1 0 0]. "Full green" [0 1 0] and "full blue" [0 0
% 1]. Play around with these numbers and see the result.
dotColor = [1 0 0];

% Determine a random X and Y position for our dot. NOTE: As dot position is
% randomised each time you run the script the output picture will show the
% dot in a different position. Similarly, when you run the script the
% position of the dot will be randomised each time. NOTE also, that if the
% dot is drawn at the edge of the screen some of it might not be visible.
dotXpos = rand * screenXpixels;
dotYpos = rand * screenYpixels;

% Dot size in pixels
dotSizePix = 20;

% Draw the dot to the screen. For information on the command used in
% this line type "Screen DrawDots?" at the command line (without the
% brackets) and press enter. Here we used good antialiasing to get nice
% smooth edges
Screen('DrawDots', window, [dotXpos dotYpos], dotSizePix, dotColor, [], 2);

% Flip to the screen. This command basically draws all of our previous
% commands onto the screen. See later demos in the animation section on more
% timing details. And how to demos in this section on how to draw multiple
% rects at once.
% For help see: Screen Flip?
Screen('Flip', window);

% Now we have drawn to the screen we wait for a keyboard button press (any
% key) to terminate the demo. For help see: help KbStrokeWait
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 if you want.
% For help see: help sca
sca;

```
