I've just uploaded a patch to disable negative borders. It also removes some warnings.
This happens for example in 124 beers later demo, just before the first rotozoom effect.
Left and right borders are starting with a size of -48. Then, the coder increase them by one until they reach +5.
Why do they use nagative borders size ?
I think this generates something (maybe a hard zoom ?)
If this is the case, how can I code this ?
OK, I've tested that, it's exactly what I suspected. Take a look into my Videl file -- what do you see (what are you supposed to see, to be precise) is decreasing distance between "gfx display area" and "visible screen area" (my first ASCII picture in the document), i.e. some kind of "opening". So, both left and right border sizes are decreasing till they reach normal screen size area.
What does it mean for you: my wild guess is you forget to adjust HDB value (skipping from 1st half line to 2nd or vice versa). If not, then your / my ;-) formulas are wrong, there's nothing like negative border size.
Btw, you should set border color to content of $ffff9800.w -- this white color while watching the demo isn't anything pleasant :)