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Old 05 September 2019, 01:37   #16
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Join Date: Jul 2015
Location: The Netherlands
Posts: 1,379
Originally Posted by Shatterhand View Post
Yes Also doubles the amount of blocks you have to blit on screen.
Well, a smart person only blits about a block per frame for the optimal scroller. So that would make it two per frame. Not a big deal really

The link Retro1234 shared has an example that shows exactly how to do that. I took a quickly glance at it and I was only able to understand it because of the post you made earlier about how the Copper works
Yup, it did. And happy to be of service.

What's really funny is that's not exactly "garbage", it looks like it's just the bitmap but with its colors garbled. I really don't think this is a safe thing to do, but I would like some confirmation (And of course, understanding what the hell is happening is always interesting too)
I just gave you confirmation it's not safe. Would you like me to do so again?

What is happening is that you're moving the bitplane pointers up or down through memory. This has two consequences. The first is that (part of) the display shows up distorted. The second is that part of the display now shows data from outside of the display boundaries as you allocated them.

If you write in those regions by say, using the Block command, then you're writing in unallocated memory. Anything can be there, including code & data belonging to your program or any other program that is running.
Originally Posted by Retro1234 View Post
Yeah I would say its not that dangerous as doing very similar horizontally I.e the corkscrew is considered good practice.
Only if you have allocated the required extra space for it, otherwise it actually is dangerous.

For reference, horizontal corkscrew scrolling requires you to allocate one extra line for the display for each screen you scroll horizontally. Vertical scrolling requires you to allocate one extra line for the display for each line you scroll (unless you wrap the display with the Copper, in which case you don't need to allocate extra for vertical scrolling).

Last edited by roondar; 05 September 2019 at 01:43.
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