Alex let's talk about this. Good point above! Some HD pics are without a line even with constant panning movement and a vertical bar as a subject. Sometimes it's near the top or bottom. The key is that there is only 0-1 line max. So it is possible to save it fast enough. It's just a matter of timing. Some experimentation is needed. Instead of varying the timing delay, we can vary which part of the image buffer to detect change in. For example, if you look at the last line as a trigger, the camera DMA may catch up to the saving process. Assuming it's faster. So we might try using the first line as the trigger. If the saving/reading process is slower it wont catch the DMA cycle while it's writing. It'll stay behind it. Understand? Good theory?
Even if this works for HD, or any specific buffer size and rate, a different strategy may be needed for the LV buffer. There may be a delay between scans or a different frame rate. Certainly there is less memory to fill or save. Maybe the opposite process is faster in this case. Maybe the writing is faster but there's a pause between frames. Maybe you are correct, if the attempted buffer size is too big there will be 2 or more lines. Then we cannot save fast enough to stay in sync. Only in this case would it be impossible. 5x5 for example.
I volunteer to do a simple experiment when it's time. This has to be done for each mode and memory buffer. They might not share the same result. Pan to the left and see which half is shifted left, the top or the bottom. This will prove which of the 2 processes, reading or writing, is faster. One of you has probably already done this experiment? I can't remember which way I was panning for a particular picture.
It would be much simpler if you could just look at the pointers to get the timing of the buffers. Unfortunately this only tells you when each buffer is ready, not when it's started. Or maybe it tells you when it's started. Maybe we'll get lucky. But it cannot do both, and no way will it tell us when it's 1/2 written. Which may be needed. I'll explain those details later.
Some of these ideas may help Hires 5x5 stay in sync across buffers? Baby steps.