English Amiga Board - View Single Post - Blitter poly-line draw & filling

Leffmann · 29 March 2014, 22:56

Ok, I misunderstood your post at first, but I see now what you mean with the "EOR trick"

The problem is that you're rounding the memory address of the first pixel to an even address. The bchg instruction (and bset, bclr and btst) with memory destination operate on a single byte, so half of the time you're drawing 8 pixels to the left, because the memory address has been rounded. You can just pass the address as it is to the Blitter, it doesn't have to be even.

The better solution is the one you've already found: pointing D to a couple of scratch bytes.

29 March 2014, 22:56	#4
Leffmann Join Date: Jul 2008 Location: Sweden Posts: 2,269	Ok, I misunderstood your post at first, but I see now what you mean with the "EOR trick" The problem is that you're rounding the memory address of the first pixel to an even address. The bchg instruction (and bset, bclr and btst) with memory destination operate on a single byte, so half of the time you're drawing 8 pixels to the left, because the memory address has been rounded. You can just pass the address as it is to the Blitter, it doesn't have to be even. The better solution is the one you've already found: pointing D to a couple of scratch bytes.