Introduction To Video In QLab

QLab 5’s video system, built on Apple’s Metal framework, gives you a powerful, flexible system for cueing, routing, and displaying still and moving images. The input side gives you precise positioning and timing of both prerecorded and live imagery, while the output side gives you sophisticated pixel-perfect control over how your video is displayed on monitors, televisions, projectors, LED walls, Syphon servers, and NDI feeds.

A note on style

This manual follows the nomenclature used in professional theater in the US, where “projection” is taken to mean any form of video or film used in a live theatrical performance. Whether the imagery is moving or still, digital or analogue, displayed via a projector, a TV, an LED wall, or a projection-enabled intelligent light, it’s all grouped together under the name “projection design.” In QLab, Video, Camera, and Text cues are the types of cues that deal with projection. “Screen” means any physical device that displays the contents of these cues. The words “screen” and “display” are used more or less interchangeably.

Sources Of Video

Video cues play back prerecorded video or still images stored on your Mac.

Camera cues display live video from webcams, Blackmagic Design video capture devices, Syphon servers, and NDI streams.

Text cues render styled text as still images using fonts installed on your Mac.

Output Of Video

QLab’s video output patches are called stages. Cues are assigned to stages, and then stages are rendered onto one or more actual video output devices, with a few intermediary steps in between. You may not need to use every feature that’s available in every one of these steps, but taken together they afford the maximum possible flexibility.

The signal path for video goes like this:

  1. Cues receive a source of video and display it on a stage.
  2. Stages are divided into regions, which cover some or all of the stage.
  3. Each region is assigned to exactly one output route.
  4. Each output route represents exactly one output device.
  5. Output devices are connections to physical video displays like monitors and projectors, Syphon outputs, and NDI outputs.

QLab can use any or all of the following as output devices:

  • Any display that appears in the Displays section of System Preferences.
  • Outputs of Blackmagic Design devices.
  • Any number of Syphon outputs1.
  • Any number of NDI outputs1.

While it’s possible that USB DisplayLink displays will also work with QLab, we do not recommend nor do we support using them because of their many unpredictable variables. Likewise, AirPlay is not supported in QLab due to its variable latency.

Most Macs these days use USB-C connections for video, a thorough discussion of which can be found in the USB-C tutorial in this manual.

Sizes and Shapes

Many media servers have ties to the cinema or broadcast worlds, in which the size and shape of a video signal conforms to an exact standard. You may be familiar with terms like “standard definition”, “1080p”, or “4K”; these are all terms for video standards that have specific resolutions, aspect ratios, frame rates, and other attributes.

QLab is completely agnostic when it comes to these standards. Video cues can play any compatible media file onto any stage, without the need to preemptively match resolution or frame rate.

When you create or edit a stage, you can set its width and height to suit your exact needs2. If the stage you create is larger than the size of the imagery you project onto it, the surrounding area will be filled with black pixels. If the stage is smaller than the imagery, the imagery will simply extend off the “canvas” of the stage. Individual cues can be scaled up or down to fit on any surface.

Frame rate is another question that you pretty much don’t need to worry about in QLab. Cues’ sources can have any frame rate; they do not need to match the frame rate of the output device or devices in use. You can even display multiple cues on the same stage at the same time with different frame rates. While it’s impossible to guarantee that there will never be visible artifacts if you have extreme mismatches between frame rates, generally speaking the Mac does a very good job of invisibly handling the necessary computation.

Take Your Time

As has been said in other parts of this manual, the secret to success with projection design is time. Give yourself time to experiment, time to troubleshoot, and time to learn the powers and limitations of your Mac.

  1. Limited by your computer’s resources.
  2. Up to a maximum of 16384 × 16384 pixels.

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