END OF AN ERA, FRACTALFORUMS.COM IS CONTINUED ON FRACTALFORUMS.ORG

OK, I volunteer ...  

(By the way, this is a nice parameter / formula combo)

For a quick check, I first reduced the size of the render by a factor of 4 (to 600 x 600) while decreasing the DE stop by the same factor (to 7.5).

After rendering, I adjusted some of the parameters in the 'Calculation' tab, enabled the 'Recalculate a selection' function in the 'Post processing' window, drew a rectangle around one of the more obvious noise artefacts, and re-rendered the selection by pressing the 'calculate' button (in the 'Recalculate a selection' part, the one beside the 'Enable' checkbox).

Basically, I activated 'normals on DE' and adjusted the 'Stepwidth limiter' to 0.03, but the most helpful thing seems to be to reduce the value of the 'Raystep multiplier' to 0.001, which seems to eliminate most of the artefacts (a slight noise on the edges of the ribbon structures can still be seen), but unfortunately also increases the render time to ~16 hours on my machine (for the render with the original size of 2400 x 2400 with DE stop reset to 30) ...

As you can see, one artefact in the center of the gnarl structure persists, however. Decreasing the values of the Raystep multiplier and Stepwidth limiter by several orders of magnitude did not help in this matter. However (and strange enough), you can get rid of the artefact by setting the 'Raystep divisor' in the 'Recalculate a Selection' section to 10 (with Stepwidth limiter and Raystep multiplier at 0.03 and 0.001, respectively), and then do the recalculation of the selection, which should take about 10 to 15 min.

Finally, before exporting the image, you have to recalculate the ambient shadows once again (this will be done for the whole render, but takes only some additional minutes); the recalculated section will blend in with the rest of the image. In effect, you can use this procedure as a kind of cosmetic surgery to selectively remove some artefacts from any image without having to re-render the whole thing using extremely slow calculation parameters.  

I do not really understand why increasing the 'Raystep divisor' by a factor of 10 does the trick, while reducing the 'Raystep multiplier' by the same factor does not achieve this: according to the tooltip, the Raystep multiplier is divided by the Raystep divisor.  

If you are interested, I may send you the final 2400 x 2400 image via private mail (I don't know if it is possible within this forum to somehow exchange email addresses ...)

Thanks Cytotox! I'll keep the Ray step divisor trick in mind. I do like to work with combo's with "difficult" DE's now and then, so I'll have a try next time.

After doing the homework indicated by cytotox, I uploaded the resulting 2400x2400 version in dA  http://mario837.deviantart.com/art/Glitched-Gnarly-Fixed-217136542

The only addition I made is increasing the 'Stepcount for binary search' to the maximum value of 31.

As you can see, a big difference!

The params, before the 'Recalculate Selection' step, are below:

Mandelbulb3Dv17{
P.....K0...M7...61...2A....EvC/5c8v3.zWn5CipTH/khI09ZcDQG.g7Aa5KaOdtzEiWrEA07S/k
................................yJp/JtKqP/2........Y./..................y.2.....
................/Mk58....6Uxb...J0....E4.....M1XswUFy0iD/..........c./...w1....w
//kPGAcC/.../2.........2.1........zD........I.A........4./E......y1...sD...../..
.wnX0LDDj5fLtH1Yvvn0opYuteKnyA2Zw.pCc8jj242nhB7JlvH5XhbcA4awyWn4k.C.vLjD30k9LeoH
TvvIMHxg7yvyye2GJE0N62jj......ojP7............kD.2....sD.6kz....................
.............................UTRZE.YqJ0/cQL72.wRZE.KsJ0/kXL72UMSZE..............
................ezzzzzzzz1.U..6.f....66...k8....53...E3...U8....A.....Xt...1GHx8
...U.0aZczDM6/nz....zX6dE166lc..zrhe.w1osMgdlHzjRy8pJs/mpz1...........W.8.UQgk0.
aFBqwnQahzn6mIqp.ta..1..........U6U0.659g.U9M4n5/A7mz0CwQOr9PExj...........60c..
F3JI.EgI7TMmL4.EU1Dbqxm4Iz9...........WA8..k.1A.8LsffaWeYzv8CXieHq7../..........
/6U0.wzzz1...................................UTVR.kzz16.aB/k.1A..1Ak.In4.1Ak..Ak
.1kdX.2U.0.y3q/.rumsha..snwF.gaCPOPi.gdht0UXAVTVR..y3q/.H1qaqa9.POPi.AOPDxoH.Qxp
L1kzz/6U./.k.1A....y3q/.zz/k.1A.yz1yAT2.xzpaqa9.................................
E....E...../....2....A....EEh3aSdtqNU6oPs/.IjR5.......................k/9.......
..........kkpX0LD8QzzUf53iSIsuvDg53iSIsulz1.....................................
................................................................................
.....................c.....3....3....wpNi3aQg71F.A4R..pPr/......................
.....................g53iSIsu/zDOaNaNaNatyXaNaNaNaNiz........E/E..........2.....
................................................................................
................................8....E/...E.....TB3QcJaQZNoPgFKOiRKA............
...........0............................Ez1.....................................
................................................................................
..........................................E/....I....Y....kOjB4OTBKRWJ4....IjR5.
.............................MU/4..................wz.........zD........kz1.....
..............................................................................yD
........Uz9.............................................}