Cracktro effect

[absence]

Does anyone know what kind of effect is used in this cracktro (screenshot attached)? It zooms continously/infinitely into that cloud of colours while rotating. It runs smoothly on a regular A500, so it's probably nothing too computationally expensive.

[Galahad/FLT]

I think its a fractal zoomer, and I think Bloodsuckers were first with it as well.

[WayneK]

I think it's more commonly known as a "Blitter Tornado" (!), and it's discussed in a lot of detail over at the ADA forums:-

http://ada.untergrund.net/forum/inde...&page=0#msg534

[StingRay]

It's a blitter feedback effect and as far as I remember Tizzy/Shining made the first one in his Party'92 40k intro. Most known is the one by Dweezil though.

[Leffmann]

Basically you use the blitter to modify one image frame into the next by skewing the image both horizontally and vertically to achieve rotation, typically in steps of 16 or 32 pixels. F.ex for a 256x256 pixel frame you would treat the frame as 16 columns and 16 rows, and add further displacement outwards to achieve the zooming effect. The latter is trickier to get working and I was only ever able to get rotation going when I tried this.

The jitter of pixels that give it the "fractal" look is simply aliasing that propagates from modifying one frame into the next in crude steps.

If anyone has some commented source code for a working example I'd love to look at it.

[Asman]

Hi

There you can find resourced version of this effect ( thx StingRay for correction ) ( Look into Prestige cracktros ) http://cyberpingui.free.fr/rsources.htm.

Regards

[StingRay]

He asked for commented sources (i.e. comments for the effect code) though, the one on cyberpingui's site is a resourced version.

[Asman]

You're right. Thx for correction.

[absence]

Originally Posted by Leffmann

If anyone has some commented source code for a working example I'd love to look at it.

Originally Posted by StingRay

He asked for commented sources (i.e. comments for the effect code) though, the one on cyberpingui's site is a resourced version.

Seems xscreensaver has a similar effect in the kumppa module. While not commented, the code is C, so it's probably a little easier to figure out than 68k assembler with the Amiga blitter.

[KevG]

Hi folks.

Here is the basic source code for same effect. It was written by Paul Rene over at the Blitz Basic web site. The code is for Blitz Plus so just download the PC demo first then load the code in below... Looks really nice and shouldn't be too difficult to convert to the Amiga.

See ya, Kev G.

;
; Kumppa v1.0
;
; Ported by Paul Rene Jørgensen, paulrene@pilen33.org
;
Global screenWidth, screenHeight
Global middleX, middleY
Global stateX, stateY
Global rotSizeX, rotSizeY
Global rx, ry
Dim xRotations(1)
Dim xRotTable(1)
Dim yRotations(1)
Dim yRotTable(1)
Dim chks(1)
Dim rotateX(1)
Dim rotatey(1)

screenWidth = 800
screenHeight = 600

middleX = screenWidth/2
middleY = screenHeight/2

Graphics screenWidth,screenHeight,32,2


initializeRotations(0.05,0.05)

While Not KeyHit(1)
SetBuffer BackBuffer()

r=Sin(a+0)*255
g=Sin(a+90)*255
b=Sin(a+180)*255
Color r,g,b
If(Rand(0,10)<2) Then Color 0,0,0
a=a+4 : If a>=360 Then a=a-360
If Not KeyDown(15)
Line middleX,middleY,middleX+Cos(a)*4,middleY+Sin(a)*4
EndIf

While KeyDown(57) : Delay 25 : Wend

rotate()
CopyRect 0,0,screenWidth,screenHeight,0,0,FrontBuffer(),BackBuffer()
;VWait
Flip

Wend


Function rotate()
rx=xRotTable(stateX+1)-xRotTable(stateX)
ry=yRotTable(stateY+1)-yRotTable(stateY)
For x=0 To rx
If x<>0 Then rotateX(x)=middleX-1-xRotations(xRotTable(stateX+1)-x) Else rotateX(x)=0
Next
For x=0 To rx
If x=rx Then rotateX(x+rx+1)=screenWidth-1 Else rotateX(x+rx+1)=middleX+xRotations(xRotTable(stateX)+x)
Next
For y=0 To ry
If y<>0 Then rotateY(y)=middleY-1-yRotations(yRotTable(stateY+1)-y) Else rotateY(y)=0
Next
For y=0 To ry
If y=ry Then rotateY(y+ry+1)=screenHeight-1 Else rotateY(y+ry+1)=middleY+yRotations(yRotTable(stateY)+y)
Next

If rx>ry Then x=rx Else x=ry

For dy=0 To ((x+1)*2)-1
For dx=0 To ((x+1)*2)-1
If rx>ry Then y=ry-rx Else y=0
If ((dy+y)>=0 And dy<((ry+1)*2) And dx<((rx+1)*2))
If ((dy+y+dx)<=(ry+rx) And (dy+y-dx)<=(ry-rx))
palaRotate((rx*2)+1-dx,dy+y)
palaRotate(dx,(ry*2)+1-dy-y)
EndIf
EndIf
If ry>rx Then y=rx-ry Else y=0
If ((dy+y)>=0 And dx<((ry+1)*2) And dy<((rx+1)*2))
If ((dy+y+dx)<=(ry+rx) And (dx-dy-y)>=(ry-rx))
palaRotate(dy+y,dx)
palaRotate((rx*2)+1-dy-y,(ry*2)+1-dx)
EndIf
EndIf
Next
Next
stateX=stateX+1
If stateX=rotSizeX Then stateX=0
stateY=stateY+1
If stateY=rotSizeY Then stateY=0
End Function

Function palaRotate(x,y)
SetBuffer FrontBuffer()
ax=rotateX(x)
ay=rotateY(y)
bx=rotateX(x+1)+2
by=rotateY(y+1)+2
cx=rotateX(x)-(y-ry)+x-rx
cy=rotateY(y)+(x-rx)+y-ry
If cx<0
ax=ax-cx
cx=0
EndIf
If cy<0
ay=ay-cy
cy=0
EndIf
If (cx+bx-ax)>screenWidth Then bx=ax-cx+screenWidth
If (cy+by-ay)>screenHeight Then by=ay-cy+screenHeight
If (ax<bx And ay<by)
CopyRect ax,ay,bx-ax,by-ay,cx,cy,BackBuffer(),FrontBuffer()
EndIf
End Function


Function initializeRotations(xSpeed#, ySpeed#)
a=0:b=0:c=0:f=0:g=0:j=0:k=0:l=0:maxi=0
m#=0.0m#=0.0k#=0.0:d#=0.0:ix#=0.0:iy#=0.0


rotSizeX = (2.0/xSpeed#)+1
ix = (Float middleX+1.0) / Float rotSizeX

rotSizeY = (2.0/ySpeed#)+1
iy = (Float middleY+1.0) / Float rotSizeY

Dim xRotations(middleX+2)
Dim xRotTable(rotSizeX+1)
Dim yRotations(middleY+2)
Dim yRotTable(rotSizeY+1)

m = middleX
If(middleY>middleX) Then m = middleY
Dim chks(m)

maxi = 0
c = 0
d = 0
g = 0
For a=0 To (middleX-1)
chks(a) = True
Next

For a=0 To (rotSizeX-1)
xRotTable(a)=c
f=Int (d+ix#)-g
g=g+f
If g>middleX
f=f-(g-middleX)
g=middleX
EndIf
For b=0 To (f-1)
m=0
For j=0 To (middleX-1)
If chks(j)=True
om=0
ok=1
l=0
While ((j+l)<middleX And (om+12*ok)>m)
If (j-l)>=0
If chks(j-l)
om=om+ok
EndIf
ElseIf chks(l-j)
om=om+ok
EndIf
If chks(j+l) Then om=om+ok
ok=ok/1.5
l=l+1
Wend
If om>=m
k=j
m=om
EndIf
EndIf
Next
chks(k)=False
l=c
While (l>=xRotTable(a))
If l<>xRotTable(a) Then xRotations(l)=xRotations(l-1)
If (k>xRotations(l) Or l=xRotTable(a))
xRotations(l)=k
c=c+1
l=xRotTable(a)
EndIf
l=l-1
Wend
Next
d=d+ix
If maxi<(c-xRotTable(a)) Then maxi=c-xRotTable(a)
Next
xRotTable(a)=c
Dim rotateX((maxi+2)*2)

maxi=0
c=0
d=0
g=0
For a=0 To (middleY-1)
chks(a) = True
Next

For a=0 To (rotSizeY-1)
yRotTable(a)=c
f=(d+iy)-g
g=g+f
If g>middleY
f=f-(g-middleY)
g=middleY
EndIf
For b=0 To (f-1)
m=0
For j=0 To (middleY-1)
If chks(j) Then
om=0
ok=1
l=0
While ((j+l)<middleY And (om+12*ok)>m)
If (j-l)>=0
If chks(j-l)
om=om+ok
EndIf
ElseIf chks(l-j)
om=om+ok
EndIf
If chks(j+l) Then om=om+ok
ok=ok/1.5
l=l+1
Wend
If om>=m
k=j
m=om
EndIf
EndIf
Next
chks(k)=False
l=c
While (l>=yRotTable(a))
If l<>yRotTable(a) Then yRotations(l)=yRotations(l-1)
If (k>yRotations(l) Or l=yRotTable(a))
yRotations(l)=k
c=c+1
l=yRotTable(a)
EndIf
l=l-1
Wend
Next
d=d+iy
If maxi<(c-yRotTable(a)) Then maxi=c-yRotTable(a)
Next
yRotTable(a)=c
Dim rotateY((maxi+2)*2)

Dim chks(1) ; Free mem
End Function

[Photon]

It's quite uninteresting to convert a derivative back to the original platform. But it was a while since I saw While/Wend, thanks for that

"Blitter feedback"? Then a right to left scroller is blitter feedback. That doesn't mean it's a nice idea though

[StingRay]

Originally Posted by Photon

"Blitter feedback"? Then a right to left scroller is blitter feedback.

Errrm? Care to eloborate how a scroller is a feedback effect? Because I don't get it at all...

[hitchhikr]

If i remember well this was basically a grid of squares and each of them was scrolled individually by the blitter (in x/y) and copied into the back buffer, then at regular interval a pattern was drawn into that buffer. The whole process was randomly moved in x/y to give that fractals look and moved the opposite way with the copper to stabilize it on the screen.

Nothing fancy but it was a rather creative piece of code back then.

[Photon]

No, YOU said feedback You explain what you mean with Blitter Feedback!

I still say this is what effects is all about. New, maybe simple, maybe complex ideas... but it takes a coder with imagination to invent them, and a strict coder to make them perfect.

Here's the one by Dweezil: http://bitworld.bitfellas.org/demo.php?id=9813

The Shining one was good, but they ruined it a little by making it 1bpl, so it looked like a fast RotPlot with 5 shadow bitplanes...