Scrutinizing Video Footage — Surveillance AB Repeat

Hank Liu
Hank Liu
Dec 2, 2014 · 3 min read

I’m sure you’ll recall a scene in CSI(and many investigation movies) where the agent replays a video footage over and over again to look for evidence. This is called the AB repeat function. In surveillance interface, this function comes in handy for it allows users conveniently look again for clues and find details on things machine cannot discern.

VLC Player AB repeat

AB repeat does not only exist in surveillance products. Take the VLC player for example, this function is designed as a button. When clicked once, it sets the A point on the timeline, and the button changes state to indicate that A point is set. Clicking it again will set the B point. It will now repeat playing from A point to B point. To cancel AB repeat, click on the button again and the button will change to normal state.

While this design is enough for consumer products, it is not suitable for surveillance products. First, it can’t alter AB points when set, this is inconvenient because user seldom get the right length the first time and may need to adjust the points many times afterwards when using surveillance products. Second, the sequence is fixed, users need to assign the A point first then the B point, but in many times when watching a footage it is the opposite, you’ll know when the event ends first and then go back to look for an beginning point. Third, it can only be canceled but not disabled, users cannot view footage beyond the AB point while the function is on, users need to cancel the function first to view and set up the function again afterwards.

KM Player AB repeat

KM player AB repeat improves the design by separating the function into three buttons, it can now alter AB points separately, disable/enable the function, and doesn’t restrict the setting sequence.

So is this the right design fit for surveillance products? It depends on the characteristic of the interface. Most users of surveillance products spend their time interacting with the timeline, where they can browse for events and indicate event types. AB repeat function design on player products lack reference with the timeline. Let’s look at products of other domains and see how they tackle this design.

Adobe Flash AB Repeat

Adobe Flash is a software for animation. It’s AB repeat is designed with one button, while activated, the repeat range will appear on the timeline with editable starting/ending point. This design relates to the timeline while encompassing a good usability. It can be a good reference for surveillance products.

GeoVision VMS 9.0 AB repeat

This is what we come up in the end. This is GeoVision VMS 9.0 software. There are two ways to activate the AB repeat function. User can simply drag the cursor to form a desire range, or they can use the AB repeat buttons above the timeline.

This is our process in designing the AB repeat function, so where to go
from here? What do you think?

Originally published at on November 27, 2014.

    Hank Liu

    Written by

    Hank Liu

    Design Mgr at LexisNexis, from Taiwan lives in Shanghai, have 2 cats, learning Processing and Blender, watch movies, enjoy travel, amateur writer, love design