• March 18, 2018, 10:22:49 AM

### Author Topic:  M-set "auto-zoom" method  (Read 90 times)

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#### gerrit

• 3b
• Posts: 711
##### M-set "auto-zoom" method
« on: February 20, 2018, 05:08:21 AM »
I came up with a method to zoom into an initial image of interest to arbitrary depth (depending on patience) to create an even more interesting image.
Basic idea is to take a location c and do a Newton iteration on finding the zero of the Greens function.
The Greens function is $$G(c)=e^{-\log(2) m(c)}$$ where $$m(c)$$ is the smoothed iteration count:
$$m(c) = n_{escape}-\log(\log(|z|)/\log(2)$$, and z is the value after escape. It is zero on the boundary and increases away from it, and has the properties of a potential field around a charge object (the M-set).
The usual distance estimate is obtained by solving $$G(c+\Delta c)=0$$ by linearizing (using derivative of G).
As G is real and c is complex there are infinitely many solutions and the distance estimate is obtained by selecting the one with smallest $$d=|\Delta c |$$.
Newtons method is then obtained by stepping c by $$\Delta c$$, where $$\Delta c = -d \frac{z/|z|}{z'/|z'|}$$, which is the distance including direction. ($$d = |z|\log(|z|)/|z'|$$.)

It does not find anything interesting but I like it as a method to "complexify" an image. Note the convergence is not quadratic because G has an essential singularity (meaning all the derivatives are zero), not sure if there is a fix for that. Typically you get closer to the boundary (zoom factor) by a factor 10-100 every iteration.

Considerable speedup is obtained by a partial "line search", meaning instead of shifting by $$\Delta c$$ you shift by $$\alpha \Delta c$$ and you search for the best $$\alpha$$. Theoretically $$1/2\leq \alpha\leq 2$$.
Optimal $$\alpha$$ seems constant in practice but varying by location. So I run Newton+line search for a few iterations, the skip the line search and just use the best $$\alpha$$.

I'll post some images in my image thread.

#### claude

• Fractal Frogurt
• Posts: 426
##### Re: M-set "auto-zoom" method
« Reply #1 on: February 20, 2018, 05:37:53 AM »
$$\Delta c = -d \frac{z/|z|}{z'/|z'|}$$, which is the distance including direction. ($$d = |z|\log(|z|)/|z'|$$.)
why do it in two steps with absolute values that cancel out?  I've played with directional de before, without knowing the theory behind it, just $$z \log(|z|) / z'$$... an early example https://math.stackexchange.com/questions/1153052/interior-distance-estimate-for-julia-sets-getting-rid-of-spots

#### gerrit

• 3b
• Posts: 711
##### Re: M-set "auto-zoom" method
« Reply #2 on: February 20, 2018, 05:43:29 AM »
why do it in two steps with absolute values that cancel out?  I've played with directional de before, without knowing the theory behind it, just $$z \log(|z|) / z'$$... an early example https://math.stackexchange.com/questions/1153052/interior-distance-estimate-for-julia-sets-getting-rid-of-spots
For clearer exposition.

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