Capture frame from Avalon-ST video using NIOS II Processor

You can do something like use the Frame Buffer IP to get the frame data written to memory, and the Frame Reader to inject the results.

You can also probably accomplish what you would like to do using the SGDMA.

I'm not aware of an example that walks you through either of these approaches, however.

Probably the simplest thing to get started with would be just inserting the Frame Buffer in your existing video pipeline and start peeking at the data from NIOS.