Interlaced display in BLITZ mode.

[Anakirob]

Does anyone know any way of getting the display library to create an interlaced screen?

I can't find any reference to it in the documentation, but surely it must be possible.

[Coagulus]

I've a vague memory of using setint (5?) to offset the y position of the screen with interlace flag set but I may have dreamed this!

Edit no it wasn't this I think...

[Daedalus]

I've never tried it myself, but it probably needs to be done by setting up BPLCON0 manually. According to the docs, bit 2 needs to be set to enable an interlaced display.

[earok]

Did anyone have any luck with this?

With display library it appears that BPLCON0 is set in the copper list, so presumably if you can find the copper list in memory (the slice mode commands for this don't appear to work?) you can set bit 2 on the word that gets pushed into BPLCON0 by the copper?

[idrougge]

This sounds like a job for CustomString.

[earok]

Looking at it further, it seems like there's much more required than simply setting BPLCON0 bit 2, since interlace mode requires toggling between two copper lists every frame.. hmm, probably why Display Library doesn't support it to begin with.

I haven't yet attempted to implement this but my working theory on how to handle this is:

- Create two separate Display Library copper lists
- Use CustomString as suggested above to set the BPLCON0 bit (though the problem with this is of course you'd need to know what the original BPLCON0 value was. I guess you could just use WinUAE's debugger to see what the value was and hardcode it rather than programatically just try to find it in memory)
- Use DisplayBitmap to point the two copper lists to two separate bitmaps, one with the odd lines and one with the even lines
- Set a Vertical Blank interrupt, and do the appropriate CreateDisplay based on the result of testing bit 7 of $DFF004

[earok]

SUPER NASTY but it seems to work. This assumes that copperlist 0 is set up with your even number lines and copperlist 1 is set up with your odd number lines (may be vice versa).

SetInt 5

	VP = Peek.w($DFF004)

	X=1-X
	if X=0
		CreateDisplay 0
		VP = VP BITSET 15				
	else
		CreateDisplay 1
		VP = VP BITCLR 15						
	endif
	
	Poke.w $DFF02A,VP

End SetInt

[idrougge]

I didn't know the 68000 bit operations were implemented as operators in Blitz.

[Cylon]

How would one do this in ASM? You could stick that ASM routine in the SetInt5 block in Blitz and you're done, aren't you?

[earok]

@idrougee, me neither until yesterday. Up until I spotted those in the manual I was doing everything with bitwise operations.

@cyclon there's an ASM tutorial at https://github.com/alpine9000/amiga_...mode/README.md but I failed to adapt it. In essence you're supposed to set bit 2 of bplcon0 and check bit 15 of vposr to see which copper list to set. But bit 15 never seemed to get set so I forced it.