% 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. 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. screenNumber = max(screens); % Define black and white (white will be 1 and black 0). This is because % in general luminace values are defined between 0 and 1 with 255 steps in % between. All values in Psychtoolbox are 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 values for white grey = white / 2; % Open an on screen window using PsychImaging and color it grey. [window, windowRect] = PsychImaging('OpenWindow', screenNumber, grey); % Measure the vertical refresh rate of the monitor ifi = Screen('GetFlipInterval', window); % Retreive the maximum priority number topPriorityLevel = MaxPriority(window); % Length of time and number of frames we will use for each drawing test numSecs = 1; numFrames = round(numSecs / ifi); % Numer of frames to wait when specifying good timing. Note: the use of % wait frames is to show a generalisable coding. For example, by using % waitframes = 2 one would flip on every other frame. See the PTB % documentation for details. In what follows we flip every frame. waitframes = 1; %-------------------------------------------------------------------------- % NOTE: The aim in the following is to demonstrate how one might setup code % to present a stimulus that changes on each frame. One would not for % instance present a uniform screen of a fixed colour using this approach. % The only reason I do this here to to make the code as simple as possible, % and to avoid a screen which flickers a different colour every, say, % 1/60th of a second. Virtually all the remianing demos show a stimulus % which changes on each frame, so I want to show an approach which will % generalise to the rest of the demos. Therefore, one would clearly not % write a script in this form for an experiment; it is to demonstrate % principles. % % Specifically, % % vbl + (waitframes - 0.5) * ifi % % is the same as % % vbl + 0.5 * ifi % % As here waitframes is set to 1 (i.e. (1 - 0.5) == 0.5) % % For discussion see PTB forum thread 20178 for discussion. % %-------------------------------------------------------------------------- %------------ % EXAMPLE #1 %------------ % First we will demonstrate a poor way in which to get good timing of % visually presented stimuli. We generally use this way of presenting in % the demos in order to allow the demos to run on potentially defective % hardware. In this way of presenting we leave much to chance as regards % when our stimuli get to the screen, so it is not reccomended that you use % this approach. for frame = 1:numFrames % Color the screen grey Screen('FillRect', window, [0.5 0.5 0.5]); % Flip to the screen Screen('Flip', window); end %------------ % EXAMPLE #2 %------------ % Here we now specify a time at which PTB should be ready to draw to the % screen by. In this example we use half a inter-frame interval. This % specification allows us to get an accurate idea of whether PTB is making % the stimulus timings we want. vbl = Screen('Flip', window); for frame = 1:numFrames % Color the screen red Screen('FillRect', window, [0.5 0 0]); % Flip to the screen vbl = Screen('Flip', window, vbl + (waitframes - 0.5) * ifi); end %------------ % EXAMPLE #3 %------------ % Here we do exactly the same as the second example, but we additionally % first set the PTB prority level to maximum. This means PTB will take % processing priority over other system and applicaiton processes. I % normally do this before and after stimulus presentation, however, on % modern multi-core processors keeping Priority on is unlikely to overload % system resources. Plus, on Linux this operation can take much much longer % then on Windows and OSX (up to minutes in some use cases). So it is now % suggested that you set Priority once at the start of a script after % setting up your onscreen window. % See PTB forum thread 20178 (and those linked to it) for discussion. Priority(topPriorityLevel); vbl = Screen('Flip', window); for frame = 1:numFrames % Color the screen purple Screen('FillRect', window, [0.5 0 0.5]); % Flip to the screen vbl = Screen('Flip', window, vbl + (waitframes - 0.5) * ifi); end Priority(0); %------------ % EXAMPLE #4 %------------ % Finally we do the same as the last example except now we additionally % tell PTB that no more drawing commands will be given between coloring the % screen and the flip command. This, can help acheive good timing when one % is needing to do additional non-PTB processing between setting up drawing % and flipping to the screen. Thus, you would only use this technique if % you were doing this. So, if you are not, go with example #3 Priority(topPriorityLevel); vbl = Screen('Flip', window); for frame = 1:numFrames % Color the screen blue Screen('FillRect', window, [0 0 0.5]); % Tell PTB no more drawing commands will be issued until the next flip Screen('DrawingFinished', window); % One would do some additional stuff here to make the use of % "DrawingFinished" meaningful / useful % Flip to the screen vbl = Screen('Flip', window, vbl + (waitframes - 0.5) * ifi); end Priority(0); % Clear the screen. sca;