Logarithmic Palettes and Color Ranges

By default, Fractint maps iterations to colors 1:1. I.e. if the calculation for a fractal "escapes" (exceeds the bailout value) after N iterations, the pixel is colored as color number N. If N is greater than the number of colors available, it wraps around. So, if you are using a 16-color video mode, and you are using the default maximum iteration count of 150, your image will run through the 16-color palette 150/16 = 9.375 times.

When you use Logarithmic palettes, the entire range of iteration values is compressed to map to one span of the color range. This results in spectacularly different images if you are using a high iteration limit and are zooming in on an area near a "lakelet".

When using a compressed palette in a 256 color mode, we suggest changing your colors from the usual defaults. The last few colors in the default IBM VGA color map are black. This results in points nearest the "lake" smearing into a single dark band, with little contrast from the blue (by default) lake.

Fractint has a number of types of compressed palette, selected by the "Log Palette" line on the [X] screen, or by the "logmap=" command line parameter:

logmap=1: for standard logarithmic palette.

logmap=-1: "old" logarithmic palette. This variant was the only one used before Fractint 14.0. It differs from logmap=1 in that some colors are not used - logmap=1 "spreads" low color numbers which are unused by logmap=-1's pure logarithmic mapping so that all colors are assigned.

logmap=N ( > 1): Same as logmap=1, but starting from iteration count N. Pixels with iteration counts less than N are mapped to color 1. This is useful when zooming in an area near the lake where no points in the image have low iteration counts - it makes use of the low colors which would otherwise be unused.

logmap=-N (< -1): Similar to logmap=N, but uses a square root distribution of the colors instead of a logarithmic one.

logmap=2 or -2: Auto calculates the logmap value for maximum effect.

Another way to change the 1:1 mapping of iteration counts to colors is to use the "RANGES=" parameter. It has the format:


Iteration counts up to and including the first value are mapped to color number 0, up to and including the second value to color number 1, and so on. The values must be in ascending order.

A negative value can be specified for "striping". The negative value specifies a stripe width, the value following it specifies the limit of the striped range. Two alternating colors are used within the striped range.

Example: RANGES=0/10/30/-5/65/79/32000

  This example maps iteration counts to colors as follows:

      color    iterations
        0      unused (formula always iterates at least once)
        1       1 to 10
        2      11 to 30
        3      31 to 35, 41 to 45, 51 to 55, and 61 to 65
        4      36 to 40, 46 to 50, and 56 to 60
        5      66 to 79
        6      80 and greater

Note that the maximum value in a RANGES parameter is 32767 and the maximum value for the number of iterations is also 32767 when using RANGES.

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Noel Giffin,