Continuous Potential

Note: This option can only be used with 256 color modes.

Fractint's images are usually calculated by the "level set" method, producing bands of color corresponding to regions where the calculation gives the same value. When "3D" transformed (see "3D" Images ), most images other than plasma clouds are like terraced landscapes: most of the surface is either horizontal or vertical.

To get the best results with the "illuminated" 3D fill options 5 and 6, there is an alternative approach that yields continuous changes in colors.

Continuous potential is approximated by calculating

           potential =  log(modulus)/2^iterations

where "modulus" is the orbit value (magnitude of the complex number) when the modulus bailout was exceeded, at the "iterations" iteration. Clear as mud, right?

Fortunately, you don't have to understand all the details. However, there ARE a few points to understand. First, Fractint's criterion for halting a fractal calculation, the "modulus bailout value", is generally set to 4. Continuous potential is inaccurate at such a low value.

The bad news is that the integer math which makes the "mandel" and "julia" types so fast imposes a hard-wired maximum value of 127. You can still make interesting images from those types, though, so don't avoid them. You will see "ridges" in the "hillsides." Some folks like the effect.

The good news is that the other fractal types, particularly the (generally slower) floating point algorithms, have no such limitation. The even better news is that there is a floating-point algorithm for the "mandel" and "julia" types. To force the use of a floating-point algorithm, use Fractint with the "FLOAT=YES" command-line toggle. Only a few fractal types like plasma clouds, the Barnsley IFS type, and "test" are unaffected by this toggle.

The parameters for continuous potential are:

      potential=maxcolor[/slope[/modulus[/16bit]]]

"Maxcolor"

is the color corresponding to zero potential, which plots as the TOP of the mountain. Generally this should be set to one less than the number of colors, i.e. usually 255. Remember that the last few colors of the default IBM VGA palette are BLACK, so you won't see what you are really getting unless you change to a different palette.

"Slope"

affects how rapidly the colors change -- the slope of the "mountains" created in 3D. If this is too low, the palette will not cover all the potential values and large areas will be black. If it is too high, the range of colors in the picture will be much less than those available. There is no easy way to predict in advance what this value should be.

"Modulus"

is the bailout value used to determine when an orbit has "escaped". Larger values give more accurate and smoother potential. A value of 500 gives excellent results. As noted, this value must be < 128 for the integer fractal types (if you select a higher number, they will use 127).

"16bit":

If you transform a continuous potential image to 3D, the illumination modes 5 and 6 will work fine, but the colors will look a bit granular. This is because even with 256 colors, the continuous potential is being truncated to integers. The "16bit" option can be used to add an extra 8 bits of goodness to each stored pixel, for a much smoother result when transforming to 3D.

Fractint's visible behavior is unchanged when 16bit is enabled, except that solid guessing and boundary tracing are not used. But when you save an image generated with 16bit continuous potential:

• The saved file is a fair bit larger.
• Fractint names the file with a .POT extension instead of .GIF, if you didn't specify an extension in "savename".
• The image can be used as input to a subsequent [3] command to get the promised smoother effect.
• If you happen to view the saved image with a GIF viewer other than Fractint, you'll find that it is twice as wide as it is supposed to be. (Guess where the extra goodness was stored!) Though these files are structurally legal GIF files the double-width business made us think they should perhaps not be called GIF - hence the .POT filename extension.

A 16bit (.POT) file can be converted to an ordinary 8 bit GIF by [R]estoring it, changing "16bit" to "no" on the [Y] options screen, and [S]aving.

You might find with 16bit continuous potential that there's a long delay at the start of an image, and disk activity during calculation. Fractint uses its disk-video cache area to store the extra 8 bits per pixel - if there isn't sufficient memory available, the cache will page to disk.

The following commands can be used to recreate the image that Mark Peterson first prototyped for us, and named "MtMand":

          TYPE=mandel
          CORNERS=-0.19920/-0.11/1.0/1.06707
          INSIDE=255
          MAXITER=255
          POTENTIAL=255/2000/1000/16bit
          PASSES=1
          FLOAT=yes

Note that prior to version 15.0, Fractint:

• Produced "16 bit TGA potfiles" This format is no longer generated, but you can still (for a release or two) use [R] and [3] with those files.
• Assumed "inside=maxit" for continuous potential. It now uses the current "inside=" value - to recreate prior results you must be explicit about this parameter.

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