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Re: decorative random colors algorithm

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Am Sonntag, 1. Juli 2012 14:15:11 UTC+2 schrieb JohnF:
> I'm writing a toy program to generate some decorative string art
> (a better waste of time than playing computer games, anyway:).
> Sample output is at http://www.forkosh.com/decorative.ps
> 
> Every line (in that sample) starts with constant r,g,b=95,0,95
> out of 255 (or .37 0 .37 if you view that ps file) at one end point,
> and varies linearly to randomly selected r,g,b values at the other end.
> (There's a thread in comp.programming.postscript about this,
> where Gernot Hoffmann provided the ps code that does this.)
> 
> Originally, I just used the C function drand48() to select three
> random numbers for r,g,b values. But that looked pretty "washed out",
> probably because the uniform distribution is likely to give similar
> enough values so the composite color is pretty often grayish.
> (I didn't "do the math" to figure that out; I just looked at it.)
> 
> After failing to google much of anything about this kind of topic,
> or even figuring out what search terms might be relevant, I played
> around a bit, and settled on the following C code snippet...
> 
>    /* --- constant rgb0 initial color supplied by user --- */
>    red0   = ((double)fgred0)/255.;    /* in the example case */
>    green0 = ((double)fggreen0)/255.;  /* fgr,g,b0 are 95,0,95 */
>    blue0  = ((double)fgblue0)/255.;   /* as stated above */
>    /* --- randomize rgb1 end color --- */
>    red1   = pow(drand48(),1.0);       /* exponent 1.0 does nothing */
>    green1 = pow(drand48(),1.0);       /* now, may be changed later */
>    blue1  = pow(drand48(),1.0);
>    /* --- enhance largest, diminish other two --- */
>    if ( red1 >= green1 && red1 >= blue1 ) { /* red is largest */
>      red1 = pow(red1,0.333);
>      green1 = pow(green1,2.0); blue1 = pow(blue1,2.0); }
>    else if ( green1 >= blue1 ) {            /* green is largest */
>      green1 = pow(green1,0.333);
>      red1 = pow(red1,2.0); blue1 = pow(blue1,2.0); }
>    else {                                   /* blue is largest */
>      blue1 = pow(blue1,0.333);
>      red1 = pow(red1,2.0); green1 = pow(green1,2.0); }
>    /* --- emit postscript instruction --- */
>    fprintf(fp,"%.2f %.2f %.2f %.2f %.2f %.2f setrgb01\n",
>    red0,green0,blue0,red1,green1,blue1);
> 
> ...which sets the initial constant color, chooses final random
> colors with drand48(), and then:
>   (a) replaces the largest of the three random r,g,b numbers
>       with its cube root (makes it even larger, closer to 1),
>   (b) and squares the other two (makes them smaller, closer to 0).
> However, it still looks a bit "washed out" to my eye. But pushing the
> exponents (1/3 and 2) too much just "degenerates" the palette to rgb.
> So maybe my whole idea is just no good. Is there some standard,
> or not-so-standard, way to choose random rgb numbers that both
> (a) remain pretty much random, and that also (b) make the resulting
> image more "vibrant" to the eye (whatever that quantitatively means;
> I can tell what it subjectively means just by looking)?
> -- 
> John Forkosh

Hello John, 

you'll get better control over your colors if you use 
basically the HLS-model (and convert to RGB). 
For instance, you may map your pseudo-random-numbers to 
a limited Hue-range, Lightness-range and Saturation-range,
for instance S=0.5...1.0 in order to avoid littele satu-
rated or washed-out colors.
(1)
http://www.fho-emden.de/~hoffmann/hlscone03052001.pdf
The doc contains procedures for the conversions HLS<-->RGB
in Pascal and PostScript.

Pseudo-Random-Number-Generators in PostScript can deliver 
different number sequences, depending on the implementation. 
(2)
http://www.fho-emden.de/~hoffmann/ranpoint14032004.pdf
This doc contains a directly programmed PostScript generator
including spectra y distributions. There is as well some 
advice, how to convert an Equal distribution into a Gaussian.
[Dealing with real number sequences, the distributions are 
never truly Equal or Gaussian.]    

Hope this helps, especially (1). 

Best regards --Gernot Hoffmann

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Thread

decorative random colors algorithm JohnF <john@please.see.sig.for.email.com> - 2012-07-01 12:15 +0000
  Re: decorative random colors algorithm gernot.hoffmann@hs-emden-leer.de - 2012-07-01 07:59 -0700
    Re: decorative random colors algorithm JohnF <john@please.see.sig.for.email.com> - 2012-07-02 07:58 +0000
      Re: decorative random colors algorithm JohnF <john@please.see.sig.for.email.com> - 2012-07-02 12:29 +0000
  Re: decorative random colors algorithm Nobody <nobody@nowhere.com> - 2012-07-01 16:25 +0100
    Re: decorative random colors algorithm JohnF <john@please.see.sig.for.email.com> - 2012-07-02 08:28 +0000
  Re: decorative random colors algorithm gernot.hoffmann@hs-emden-leer.de - 2012-07-03 01:17 -0700
    Re: decorative random colors algorithm JohnF <john@please.see.sig.for.email.com> - 2012-07-03 10:27 +0000
  Re: decorative random colors algorithm gernot.hoffmann@hs-emden-leer.de - 2012-07-03 21:50 -0700
    Re: decorative random colors algorithm JohnF <john@please.see.sig.for.email.com> - 2012-07-04 05:15 +0000
      Re: decorative random colors algorithm JohnF <john@please.see.sig.for.email.com> - 2012-07-04 08:01 +0000
  Re: decorative random colors algorithm gernot.hoffmann@hs-emden-leer.de - 2012-07-04 02:20 -0700
    Re: decorative random colors algorithm JohnF <john@please.see.sig.for.email.com> - 2012-07-04 09:51 +0000
      Re: decorative random colors algorithm JohnF <john@please.see.sig.for.email.com> - 2012-07-04 10:59 +0000
        Re: decorative random colors algorithm Wally W. <ww84wa@aim.com> - 2012-07-04 09:03 -0400
          Re: decorative random colors algorithm JohnF <john@please.see.sig.for.email.com> - 2012-07-04 13:46 +0000
            Re: decorative random colors algorithm Wally W. <ww84wa@aim.com> - 2012-07-04 12:39 -0400
  Re: decorative random colors algorithm gernot.hoffmann@hs-emden-leer.de - 2012-07-04 07:10 -0700
    Re: decorative random colors algorithm JohnF <john@please.see.sig.for.email.com> - 2012-07-04 15:05 +0000
  Re: decorative random colors algorithm gernot.hoffmann@hs-emden-leer.de - 2012-07-04 09:12 -0700
    Re: decorative random colors algorithm JohnF <john@please.see.sig.for.email.com> - 2012-07-05 03:18 +0000
      Re: decorative random colors algorithm JohnF <john@please.see.sig.for.email.com> - 2012-07-05 05:54 +0000
        Re: decorative random colors algorithm John Forkosh <forkosh@panix.com> - 2012-07-05 12:23 +0000
  Re: decorative random colors algorithm "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2012-07-05 07:48 +0200
    Re: decorative random colors algorithm JohnF <john@please.see.sig.for.email.com> - 2012-07-05 06:14 +0000
    Re: decorative random colors algorithm Wally W. <ww84wa@aim.com> - 2012-07-05 07:30 -0400

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