Tutorial about zooming with BPLCON1 (102) : Looking for examples

[Yragael]

Hi guys,


I'm finishing a little tutorial about horizontal hardware zooming using the famous BPLCON1 trick.



I am looking for examples of this zoom in famous demos. I was a bit suprised to notice that it does not seem to have been used in Elysium by Sanity. I found it is used in Jesterday by Sanity for the rolling menu.



If you know any demo that uses this trick, I may include some screenshots in my tutorial to point the reader to them.

[DanScott]

Rink-A-Dink Redux
Desert Dream

and if you want to see a strange variation on this, disassemble my copper-list from the Star Wars scroller in "The Fall"

[Yragael]

Thanks. Great demos. I will definitely pick the Star Wars scroller. I had a look at the Copper list for the Star Wars scroller (nice!). The Copper updates BPLCON1, as expected, and the low address pointers for bitplanes 1 and 2. It seems that this is a way to jump by 128 pixels every 64 pixels in the middle of a line because the text is drawn in a much wider bitplane than the screen ?

[Yragael]

Well the tutorial is published here, width downloadable code :



http://www.stashofcode.fr/zoom-hardw...on1-sur-amiga/


In french but should translate well with Google. Will translate it one day...

[ross]

Another great tutorial


The ultimate usage for the (modulo/$102) effect is sure in the rotozoomer/wobbler of Brian the Lion:
[ Show youtube player ]
(an awesome piece of code by Kreator/Anarchy)

Well, is not only $102 but also copper at best, blitter and sorcery

[Yragael]

Yes I noticed it. Awesome.

Although I just checked the Copper list with the MK III, and it does not appear that this very talented guy did use the BPLCON1 trick (which makes the FX even more impressive!). The structure of the Copper list of the menu, with "Brian" rotating in the background, is, per line, just some MOVE along with NOPs :

MOVE ?,BPLCON1
MOVE ?,BPL1MOD
MOVE ?,BPL2MOD

Most of the good zooms do not use the BPLCON1 trick for horizontal stretching, maybe because of its limitations, as explained in my tutorial... But all of them do use the BPLxMOD trick for vertical stretching.

Originally Posted by ross

Another great tutorial


The ultimate usage for the (modulo/$102) effect is sure in the rotozoomer/wobbler of Brian the Lion:
[ Show youtube player ]
(an awesome piece of code by Kreator/Anarchy)

Well, is not only $102 but also copper at best, blitter and sorcery

[nogginthenog]

Originally Posted by Yragael

Well the tutorial is published here, width downloadable code :



http://www.stashofcode.fr/zoom-hardw...on1-sur-amiga/


In french but should translate well with Google. Will translate it one day...

Nice tutorial! Perfectly readable in English with Google translate.

[LeCaravage]

Bravo.

[Yragael]

Some figures in the tutorial got fixed this morning: the scenario for the hardware horizontal zoom, and the matching "magic table". Not easy to translate this assembly code in Excel figures

[DanScott]

Originally Posted by Yragael

Thanks. Great demos. I will definitely pick the Star Wars scroller. I had a look at the Copper list for the Star Wars scroller (nice!). The Copper updates BPLCON1, as expected, and the low address pointers for bitplanes 1 and 2. It seems that this is a way to jump by 128 pixels every 64 pixels in the middle of a line because the text is drawn in a much wider bitplane than the screen ?

Exactly

But there are some DMA glitches... so I used dual playfield, and hid the glitches with sprites (with strange priority values i seem to remember)

[Yragael]

Well done. Hiding the problem if it cannot be solved easily, or cannot be solved at all, is definitely part of the art of demomaking.

Originally Posted by DanScott

Exactly

But there are some DMA glitches... so I used dual playfield, and hid the glitches with sprites (with strange priority values i seem to remember)

[sandruzzo]

Since blitter has a barrel schift, is it possible to do the same thing with it?

[Toffee]

I'm assuming scroll register zoom doesn't work with AGA fmode register set to 3 as you can only shrink at every 64 pixels?

[DanScott]

Originally Posted by sandruzzo

Since blitter has a barrel schift, is it possible to do the same thing with it?

Theoretically.. although you would have to work on word wide strips, and probably multiple passes to mask and merge adjacent edges.

This $102 register trick is just making use of a hardware "feature" and is effectively free (less the copper instructions to move to the register)

[Yragael]

Originally Posted by Toffee

I'm assuming scroll register zoom doesn't work with AGA fmode register set to 3 as you can only shrink at every 64 pixels?

Yes. This won't work because of this reason. You would only be able to shrink the image every 64 pixels. I did not try.

[Yragael]

Originally Posted by DanScott

Theoretically.. although you would have to work on word wide strips, and probably multiple passes to mask and merge adjacent edges.

This $102 register trick is just making use of a hardware "feature" and is effectively free (less the copper instructions to move to the register)

zoom3.s and zoom4.s contain the code to shrink horizontally the picture using the Blitter. Such code is required to shrink the picture horizontally when the possibilities of the hardware horizontal zoom are exhausted (ie: when BPLCON1 has reached 15). This kind of software horizontal zoom is done in real-time, but it should be run before the zoom to precompute intermediate pictures.

[Toffee]

It's a shame AGA doesn't help this trick. Would be cool doing a full screen 8 bitplane zoomer.

If you increase scroll register between reads does it give you blank data?

I still find it incredible how someone stumbled upon this. I'm sure Chaos said he got the trick from another Sanity coder before Jesterday.