Display ID numbers

Setting up your displays and using the setup protocol to label them appropriately for BITTSy is covered earlier in this manual, but this section explains the assigning of display ID numbers and side labels in more depth, and what kinds of changes to your system can affect your DISPLAYS starting definition.

What are display ID numbers?

When you have multiple displays connected (and configured to extend rather than duplicate each other), your operating system will assign them each an ID number. In Windows 10, you can see what the system is assigning to which screen by going into the Display Settings (right click on the desktop and select this option), then clicking Identify, as shown on the image below.

This will cause number labels to appear temporarily in the corner of each of your connected displays. If you would like to change which number is associated with which monitor, you can click and drag the numbered boxes that appear above the Identify button. However, be careful to verify that your changes "stick" whenever your computer is restarted, or your displays are turned off and back on.

What determines how display IDs are assigned by Windows?

Except for when you, the user, specify an order (by clicking and dragging numbers in the Display Settings as mentioned above), Windows will assign display IDs solely by the port where the monitor is plugged into your computer. Windows doesn't "know" that you've arranged your monitors on one side or another of your desk or room - it only knows that there's a cable coming out of that particular display output jack, and a display plugged in on the other end.

This has several important implications for using displays for experiments in BITTSy.

  1. Because the operating system doesn't tell BITTSy anything about where monitors are located, we have to do that within the protocol file (that is, we need a DISPLAYS ARE starting definition!)

  2. BITTSy relies on how the operating system numbers monitors, and if the operating system switches the display IDs for any reason, BITTSy will need to be given an updated DISPLAYS definition to preserve the matching of physical devices to their locations.

How does BITTSy use display ID numbers?

Whenever you validate a protocol, BITTSy will fetch the display ID numbers from your operating system, and will match them, in order, to the side names that appear in that protocol's DISPLAYS starting definition.

For example, let's say the experimenter's monitor (where the BITTSy window will be, and no stimuli) had ID 1, a right-side monitor has ID 2, and a left-side monitor has ID 3. BITTSy will take the lowest ID that is not the experimenter's monitor, and assign it the first side name in the DISPLAYS ARE definition. This means that our right-side monitor (#2) will be chosen first, then the left-side one (#3), so the DISPLAYS ARE statement should label those sides in that order.


The goal of the setup protocol is to help you identify the display IDs of your monitors, and label them with the appropriate side names so that BITTSy will display images/videos to the intended location whenever you use an action statement. But you can also use Windows to show the display IDs, as described on this page; your DISPLAYS ARE statement should simply list the side names, in order, that correspond to those ID numbers.

When and why can display ID numbers change?

The operating system's display ID assignment depends on where the monitor is plugged into your computer. Therefore, anytime that you unplug and replug a display cable, you should run the setup protocol to verify that your system is still assigning the same IDs to the same devices, and the DISPLAYS starting definition is still the correct order to match where your displays are positioned in your room.

If you experience monitors swapping IDs for any reason other than unplugging and replugging into your computer, let us know! Some users have experienced this issue in Windows 10, and others not at all. There may be a numbering that is more stable on your system than the default, which you can check by reordering your monitors in Display Settings (or swapping where monitors are plugged in) and checking that the display IDs assigned stay consistent across restarting your computer. Some TVs connected via HDMI appear to take priority over other connected monitors; you can try manually setting them to be a lower ID number. If your computer has an upgraded graphics card, it may matter if you have some displays connected directly to the motherboard/on-board graphics card and others connected to ports that are located on your installed graphics card. It also may matter whether your displays are all powered on before turning on your computer, or whether you're turning them on after the computer has started up.

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