Need explanation of a demo effect - screen zoom/rotate
 

Hi all -

There were a few demos back in the day that used this effect. It appeared to be random, and a 50% dither was employed. The screen would rotate and gradually expand, creating swirls of colour as it did.

I kind of got the idea that it was rotating the screen and pasting it back in a feedback loop, but I'm not so sure.

Anyone know how it was done? I can't recall any of the actual demos that did this, but one that sticks in my mind used pink and a kind of light green colour for the effect.

Thanks,

D.

Are we talking the blitter tornado effect as seen in Stardust and the following Prestige cracktro?

http://www.youtube.com/watch?v=7S238EpfqbA

I have wondered for years how this was done. Amazing effect. Used to stare at that Prestige cracktro for ages...

Never made one but I know roughly how it's done:

Push random pixels into the centre of the display (watch the effect and look closely at the centre of the screen, you'll see it)

Blitter copy the bitmap data around and out from the centre to the edges.

The random pixels coupled with the way the blitter copies are done gives the "fractal" looking spirals.

That's the one!

Indeed. I experimented with a simple zoom instead of the blitter copy, and it didn't look good. I'd love to know how the copy is actually performed.

D.

Ah, but to know that is to take away the magic. :)

Experiment with various ways of copying and see what happens. My guess would be that was exactly what the original coder of the effect did.

Original is in the 40kb intro Bananamen by Stellar if I remember rightly.

That's the first good looking feedback effect of this kind coded by Dweezil and most known. The original effect was coded by Tizzy and appeared in Shining's 40k intro released at TP'92.

OK, nice one Stinger :)

I'll have to check that one. :)

Bumping a very old thread

I liked this effect. There was an intro/cracktro (crystal/melon/lemon.?) with this effect (and the music by Radix Paul The Penguin)
Unlike the others it appeared to be more than 1bpl (i.e. it was in colour) but I can't find it :(

The Bananamen one is multicolor, but haven't looked closely enough to determine if they just repurpose an older frame for the other bitplane..

This?

Yes like that. I still can't find the exact intro I was looking for.

Yeah, I too was convinced that there was an intro with that effect but I couldn't find it..
What you see in the animated gif is my version.

2+ bpls is simple: interleaved bitmap, blit height x2+, random pixels in the middle have random colors.
Full Moon (aga) by VD/Fairlight, which kind of matches that gif's palette, has 4 bpls but it's prerendered animation.

Full Moon, right!

The thing about my colors version is that I pretty much randomly picked a couple, liked them and left it here :)

But.. what do you mean by "prerendered animation"?

I had noticed it repeats on a very short loop unlike others I've seen. But the "animation" is quite long 1s / 25 frames?

Gah.. just noticed that from the youtube video.. probably because they needed frame time for the other overlay effect?
(what you see from my gif is a grab from winuae, for a 2 bitplanes version and it can work on base A500)

EDIT: I will use it shortly in a little intro; it's nothing new, but nice to look at :D

If you look in memory you'll see a whole lot of prerendered screens with that fx (it generates the animation at the start of the end-part and then plays it in a loop). "Only" the zoom scroller is realtime. I don't have the exact numbers, but let say fullscreen is 10kb *4bpl *25 = 1mb. No problem for A1200.
You can also add a watch at $dff058 in winuae and you'll see it only does 1 word wide blits (other than blit clear), tornado is typically 3 words (2 and +1 for shift) wide.

Yeah, it's in my top5 fx. One of my retro friends never gets enough of it, his #1 fav intro is Dark Room :).

Oh man it's been a minute. I managed to get this coded up - here's my BASIC interpreter running the effect in 800x480, 256 colours:

https://www.youtube.com/watch?v=D6XIkSDwgZc