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Realtime Editing for Psychology Experiment

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Luke Holme
Realtime Editing for Psychology Experiment
on Jun 23, 2015 at 3:01:49 pm

Hi guys,

I'm currently helping to run a psychology experiment in which participants watch a video on a screen whilst having their eye movements tracked.

On the experimenter desk, we have one monitor which displays a duplicate of what the participants can see (i.e. the video), and another, where their eye movements are displayed in real time. This is shown as a blank box in the corner of the screen, with a dot which moves around showing their fixation. Both of these are connected to their respective PCs using a VGA cable, as is the larger screen which participants watch.

Basically, we would like to add a third screen which combines these two views. In other words, I would like to - in real time - overlay the box from the eye tracking screen onto the video feed. I imagine it would be necessary to select the background colour of the box to be filtered out, select the box as the area of the screen I would like overlaid, and then stretch it so that it covers the entire area of the video.

Does anyone have any idea how this might be achieved? In a previous but similar setup this was done using a BlackMagic Media Express, as well as two "scan converters", a TVone Cs-320 Connect, by Corio; and a Scan Converter AVT-3170, by AVTool. However, I am unsure if these are still needed, since monitor resolutions have advanced so much in the meantime. I contacted blackmagic support, who pointed us towards an ATEM switcher (, which they said could "overlay any input over another with the ability to move it, resize it, key it and put borders on it". This sounds promising, but will we need any of the "scan converter" things as well? What do they even do? We are wary of spending so much money for something which might not work :)

I'm sorry for such basic questions, but media is really not my strong suit. Any information you can offer which might help us would be more than welcome. We're not committed to BlackMagic hardware either - any hardware which can achieve this would be a potential alternative.

Thanks very much for your time!

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Rob Duncan
Re: Realtime Editing for Psychology Experiment
on Jun 23, 2015 at 8:52:07 pm

This is not really a Blackmagic-specific question, but perhaps I can help you in the right direction.

It sounds as if you are attempting to chroma key the eye pattern on top of the material that the subject is viewing, which likely involves a few different parts:

  1. the chroma key (i.e. removing a specific colour from the eye scan)

  2. scaling the eye pattern to match the boundaries of the subject material

  3. possible horizontal mirroring of the eye pattern (if the subject is looking at the left side of the screen, will your scanner readout also show that to the left?

The scalers you mention both scale the image down to composite video (yellow RCA jack). The ATEM switcher is really designed for HD-SDI and HDMI sources, not SD analog sources that these scalers output.

If the previous image quality was not an issue, I would suggest that an SD composite switcher with Luma or Chroma key capability like the Roland V-4 or Panasonic MX50 might be found cheap on EBay and suit your needs. At least one of those scalers has some zoom and positioning controls to align the pattern.

Alternately, your VGA computer outputs could be scaled up to HDMI quality and put into a ATEM (maybe they already have HDMI on the computer?) but I don't know that you would be able to scale a keyed source. Perhaps an ATEM expert could chime in!

Best of luck.


Freaky People Productions,
Live Event Technical Production
Toronto, CANADA

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