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Hi again,

I abandoned trying to get DE working for the 4D ring I was using for the "real" 3D Mandelbrot and decided to try rendering it adding it as an option in the beta Julibrot formula that I posted which uses solid based on iteration density rather than solid based on iteration - it's not as optimum as using DE but considerably more optimum than plain iteration and produces results very similar to using DE.
So I did a test render of the same minibrot I did previously but this time allowing the rest of the Mandelbrot to appear in the background.
Here's the result:



http://www.fractalforums.com/gallery/?sa=view;id=695

Hi all, it's not simple, but I think solving this will give the derivative in general cases (where a solution is possible):

For deriving the derivative of 4 functions of 4 reals, we have:
 
 x1 = a(x,y,z,w)
 y1 = b(x,y,z,w)
 z1 = c(x,y,z,w)
 z2 = d(x,y,z,w)
 
We want the derivative of the system where:
 
 N.p = 0
 
Which is given by the limit(v->0) of:
 
( |          i                       j                      k                      l               | ).(x1,y1,z1,w1) = 0
  | a(x+v,y,z,w)-x1  b(x+v,y,z,w)-y1  c(x+v,y,z,w)-z1  b(x+v,y,z,w)-w1  |
  | a(x,y+v,z,w)-x1  b(x,y+v,z,w)-y1  c(x,y+v,z,w)-z1  b(x,y+v,z,w)-w1  |
  | a(x,y,z+v,w)-x1  b(x,y,z+v,w)-y1  c(x,y,z+v,w)-z1  b(x,y,z+v,w)-w1  |
  | a(x,y,z,w+v)-x1  b(x,y,z,w+v)-y1  c(x,y,z,w+v)-z1  b(x,y,z,w+v)-w1  |

All values treated as reals - obviously treating all values as complex would maybe provide a solution in cases where using reals does not.

Duh - OK this is just WIP - I guess my recollection of getting a normal was a little hazy, but maybe just omitting the bottom row of the matrix.....or make it 5*5 with the last column as ident ?

Aaaargh !! I really should think things through more before posting stuff:

I tend to rethink stuff from first principles - I don't have any high levels maths books and working it out myself is usually quicker than ordering books from the library
In this case I was considering the fact that for distance estimation (to be used in ray-tracing) we need the derivative of a formula and thinking about it I suddenly thought that that is very closely related to the normal to the surface - hence my calculation above.
In fact it turns out that if you calculate the 4*4 matrix above (below the i,j,k,l) to the limit of v then you get the transpose of the Jacobian so I guess I'm not completely off target

hmm while i haven't completely understood the method you're proposing, i think it's not possible to infer global characteristics from local sampling: basically the normal can vary chaotically at any point inside the field of consideration, so we should not expect there to be a "straight line" to the point of nearest intersection, at any level of iteration. moreover i've found that with fractals there is never really a "surface" at all, making normals extremely troublesome to even estimate reasonably

Ok, did a higher res. still shot of the "true 3D" minibrot:



or:

http://makinmagic.deviantart.com/art/quot-Real-3D-quot-Minibrot-126708978

The minibrot is at approx. -1.76 on the real axis and is viewed from 30 degrees above and 60 degrees around (camera target point set slightly nearer the main cardioid).
Bailout was x^2+y^2+z^2+w^2>4 and the maximum iteration count allowed was 60, though solid was based on iteration density so rarely if ever would 60 iterations actually be used.
Rendered to disk in Ultra Fractal at a resolution of 3840*2880, time 1hour 51mins on a (heat impaired) 3GHz P4HT effectively in single-threaded mode due to the render being performed to a screen buffer in the global section of a UF formula.

Hope you guys like it

Just for comparison with the "True 3D" Minibrot:

http://makinmagic.deviantart.com/art/Hypercomplex-Minibrot-126754381

http://makinmagic.deviantart.com/art/Quaternionic-Minibrot-126754860

Hi, well if you look at the math for the "real 3D" "roundy version:

* | r   i   j   k
-------------
r | r   i    j   k
i | i  -r   k   j
j | j   k  -r  i
k | k  j   i  -r

Which gives z^2+c:

              xi = mx
              yyy = my
              zzz = mz
              mx = mx*mx - my*my - mz*mz - mw*mw + x
              my = 2.0*(my*xi + mz*mw) + y
              mz = 2.0*(mz*xi + yyy*mw) + zz
              mw = 2.0*(mw*xi + yyy*zzz) + w

Assuming the start value is always (0,0,0,0) then:

if we start with a 3D cut such that w=0 and from that a 2D cut such that zz is 0 then the iterate is reduced to:

              xi = mx
              mx = mx*mx - my*my + x
              my = 2.0*my*xi + y

if w=0 and y is 0 then it's reduced to:

              xi = mx
              mx = mx*mx - mz*mz + x
              mz = 2.0*mz*xi + zz

And if we take a 3D cut where y=0 (or zz=0) and a 2D cut where zz=0 (or y=0):

              xi = mx
              mx = mx*mx - mw*mw + x
              mw = 2.0*mw*xi + w

However, for example in the first two cases (i.e. a 3D cut where w=0) then although the 2D X-Y slice and 2D X-Z slice both are the 2D Mandelbrot Set any other rotation of a plane including the X axis around the X axis will result in other images.

For the "squarry" version the iteration depth was only 10 The above holds for the 2D slices in that case also (as the only difference is that in the full iteration it uses "mw = 2.0*(mw*xi - yyy*zzz) + w" instead of "mw = 2.0*(mw*xi + yyy*zzz) + w").

I hope that makes things clearer

Here's the minibrot for the "squarry" version:

http://makinmagic.deviantart.com/art/quot-Real-3D-quot-Minibrot-Squarry-126885372