That is, the top right of the window has coordinates (1,1), the bottom left is (-1,-1). In normalised (‘norm’) units the window ranges in both x and y from -1 to +1. Requires :No monitor information Normalised units ¶ Spatial frequency: cycles per stimulus (so will scale with the size of the stimulus). (These can be determined at any point by the Window.size attribute.) Obviously it has the disadvantage that the location of the right and left edges of the screen have to be determined from a knowledge of the screen dimensions. This type of unit can be useful in that it scales with window size, unlike Degrees of visual angle or Centimeters on screen, but stimuli remain square, unlike Normalised units units. For a standard widescreen (16:10 aspect ratio) the bottom left of the screen is (-0.8,-0.5) and top-right is (+0.8,+0.5). As a result, the dimensions of a screen with standard 4:3 aspect ratio will range (-0.6667,-0.5) in the bottom left to (+0.6667,+0.5) in the top right. With ‘height’ units everything is specified relative to the height of the window (note the window, not the screen). If you want to store the window size of your participants device in an online study, you can add a code component and use thisExp.addData('windowSize', win.size). If using height units on a tablet/touchscreen device,currently 1 unit height corresponds to the height of the screen when the device is held in landscape.ĭegrees of visual angle are not currently supported for online use, but you can estimate pixels per cm using a screen scaling method (this demo was shared by Wakefield Morys Carter 2021) and then use pixel units to present stimuli in cm see Li et al (2020) for more details. The easiest solution here is to use Height units, this means that the size of stimuli will be scaled relative to the height of that participants screen - which usually means it is possible to run studies even on smartphones! Because of this it makes it difficult to use units like deg or cm - because we need to know both the participants viewing distance and the number of pixels that make up a cm on that participants screen. Furthermore it is going to be more difficult for you to control factors like viewing distance. It is likely that your experiment will be run on a wide variety of devices all with differing screen resolutions. If you are running a study online the easiest units to use will be those that require no monitor info. For running an experiment it’s usually best to use something like ‘cm’ or ‘deg’ so that the stimulus is a fixed size irrespective of the monitor/window.įor all units, the centre of the screen is represented by coordinates (0,0), negative values mean down/left, positive values mean up/right.įor help understanding spatial units visually, try the builder demo “spatialUnits” under “Understanding PsychoPy” (version 2021.2). For conducting demos, the two normalised units (‘norm’ and ‘height’) are often handy because the stimulus scales naturally with the window size. Your choice of unit depends on the circumstances. In PsychoPy ®, after providing information about your monitor, via the Monitor Center, you can simply specify your stimulus in the unit of your choice and allow PsychoPy ® to calculate the appropriate pixel size for you. In most other systems you provide the stimuli at a fixed size and location in pixels, or percentage of the screen, and then have to calculate how many cm or degrees of visual angle that was. One of the key advantages of PsychoPy ® over many other experiment-building software packages is that stimuli can be described in a wide variety of real-world, device-independent units.
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