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My unterstanding from the VIP Suite User Guide is the following:
As soon as a new input frame starts, the scaler starts upscaling the video data delivered by the CVI core. When the fifo at the CVO core has reached the set threshold, output of the active image data starts. This provides some synchronization to lock both framerates, but it violates the set timing values for the output if the framerates do not match otherwise.
Is this correct or did I miss an important point?
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Your understanding is correct. CVO underflow is indicating data is leaving the FIFO faster than it arrives. The first pixel leaves the CVO when the threshold is hit, and then over the course of your first ~1020 lines it is draining down to zero. You should be able to observe this fact with signaltap or simulation. It's been a while, but I believe the root cause will be a clock/line timing mismatch between the input and the output.
I believe you should have been able to change the FIFO size and threshold and seen corresponding effect on location/size of your garbage. Try doubling/halving the threshold and see if it changes. (if it doesn't, the behavior described above is not why you're getting underflow and you'll have to figure out what is the reason).
I'm also not sure why nearest neighbor worked fine and 8 vertical tap filters didn't. The difference between the two should have simply been additional (8) lines of latency between the first pixel coming into the scaler and the first output pixel being emitted.
I agree with the suggestion to rig up a simple simulation to sanity check your clocks/timings. (as you can expect, the simulation execution itself will take some time..)
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