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Understanding clean X/Y split
http://forums.nesdev.com/viewtopic.php?f=2&t=16280
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Author:  GradualGames [ Sun Jul 30, 2017 1:42 pm ]
Post subject:  Understanding clean X/Y split

I've been studying this wiki article:

Split X Y Scroll

In order to get a split screen sufficient for scrolling a large boss on the screen. That was fairly easy to get working.

I decided to proceed to trying to create a generic system where I could specify as many split points as I wanted, so I could use this system both for scrolling a large boss around or for, say, parallax scrolling.

Once I got that working, I couldn't seem to get fine horizontal scrolling to work. I noticed the Split X Scroll section of the wiki is probably aimed towards this particular use case. I also noticed Doug's tutorial on splits use this latter technique.

So my question is, why can't I use the Split X Y Scroll to successfully arbitrarily set the fine X and Y scroll at the split point? I.e., as I increment X, the ppu seems to "pop" over a few pixels further rather than smoothly scrolling.

I'm fine with using the two techniques separately, just curious why Split X/Y can't be used in both cases (apparently).

*edit* I did find a couple of threads on this issue, and the only hint I could find towards my question was that I probably have the timing off; and that it's a bit trickier to get the fine timing right for clean X/Y scroll. I did try fiddling with the timing (the empty loop that pushes the code into the next hblank) but never got the "pop" to go away.

Author:  glutock [ Sun Jul 30, 2017 1:53 pm ]
Post subject:  Re: Understanding clean X/Y split

I don't know if it'll help, but here's a good talk about the 2005/2006 split : viewtopic.php?p=78593#p78593

Author:  GradualGames [ Sun Jul 30, 2017 1:59 pm ]
Post subject:  Re: Understanding clean X/Y split

glutock wrote:
I don't know if it'll help, but here's a good talk about the 2005/2006 split : viewtopic.php?p=78593#p78593

Yeah that's one of the ones I was looking at. I actually just figured it out, I missed an lsr when computing the formula listed in the wiki for the last write. Sweet, generic system working now.

Author:  dougeff [ Sun Jul 30, 2017 4:16 pm ]
Post subject:  Re: Understanding clean X/Y split

I'm not clear on exactly what you are trying. Can you post code of every $2000,$2005,$2006 write?

When I am confused, I try to find a game that does what I want, and set breakpoints for scroll changes.

Author:  rainwarrior [ Sun Jul 30, 2017 4:34 pm ]
Post subject:  Re: Understanding clean X/Y split

I think the result was just that the full X/Y scroll does work, but GradualGames had a bug that made it seem like it didn't.

Author:  tokumaru [ Sun Jul 30, 2017 4:41 pm ]
Post subject:  Re: Understanding clean X/Y split

Good luck finding a game from back in the day that uses the $2006/5/5/6 trick. AFAIK, that was discovered by reverse engineering the hardware, not by looking at existing software. Old games did mess with $2005/6 mid-frame in various ways, but I don't think any of them managed to set arbitrary X/Y positions anywhere on the screen like we can now.

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