VIP Scaler II Megacore: Garbage at bottom of image

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Also, an underflow makes sense if the scaler can't keep up with the CVO, but I definitely don't understand the overflow condition. The backpressure between the VIP cores should prevent that (I think).

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Overflow is from the clocked video input. It can't stop accepting data from the pins, and backpressure only controls Scaler reading from the FIFO.

In other words: the scaler isn't accepting data fast enough for the CVI, and isn't producing data fast enough for the CVO.