Detect when image changes

Suppose I have an AJA video input that is showing a static image, i.e. the SDI source is paused. Is there some way for Touch Designer to detect when that image changes, so I can take some action in Python? Unfortunately this particular SDI source does not include timecode.

you could use a Cache TOP and have it output a previous frame (set active to on and set index to a negative value). Then feed that and the live video both into a Diff TOP. Use an Analyze TOP (set to perhaps Average or Sum?) to detect if any change is showing in the Diff TOP. Then you could use something like a TOP To CHOP followed by Logic CHOP to define within what range (there will always be some noise between frames) your trigger will fire.

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