poisson.psvg

<!-- poisson.psvg           -->
<!-- poisson disk sampling  -->
<!-- based on paper https://www.cs.ubc.ca/~rbridson/docs/bridson-siggraph07-poissondisk.pdf -->
<!-- and this impl. https://editor.p5js.org/codingtrain/sketches/4N78DFCXN                  -->
<psvg width="300" height="300" background="black">

  <def-dist2 x0="" y0="" x1="" y1="">
    <var dx="{x0-x1}"/>
    <var dy="{y0-y1}"/>
    <return value="{dx*dx+dy*dy}"/>
  </def-dist2>

  <def-poissondisk r="4" k="30">
    <var active=""/>
    
    <var w="{r/SQRT(2)}"/>
    <var r2="{r*r}"/>
    <var cols="{FLOOR(WIDTH/w)}"/>
    <var rows="{FLOOR(HEIGHT/w)}"/>
    
    <var grid="{FILL(-1,cols*rows)}"/>
    <var samples="{WIDTH/2} {HEIGHT/2}"/>

    <for i="0" true="{i<COUNT(samples)}" step="2">
      <asgn active="{CAT(active,NTH(samples,i),NTH(samples,i+1))}"/>
      <var col="{FLOOR(NTH(samples,i)/w)}"/>
      <var row="{FLOOR(NTH(samples,i+1)/w)}"/>
      <asgn grid="{UPDATE(grid,row*cols+col,i/2)}"/>
    </for>

    <while true="{COUNT(active)}">
      <var ridx="{FLOOR(RANDOM()*COUNT(active)/2)}"/>
      <var pos="{TAKE(DROP(active,ridx*2),2)}"/>
      <var found="0"/>
      <for n="0" true="{n<k}" step="1">
        <var sr="{RANDOM()*r+r}"/>
        <var sa="{RANDOM()*PI*2}"/>
        <var sx="{NTH(pos,0)+sr*COS(sa)}"/>
        <var sy="{NTH(pos,1)+sr*SIN(sa)}"/>
        <var col="{FLOOR(sx/w)}"/>
        <var row="{FLOOR(sy/w)}"/>
        <if true="{col>0 &amp;&amp; row>0 &amp;&amp;
                   col<cols-1 &amp;&amp; row<rows-1}">
          <var ok="1"/>
          <for i="-1" true="{i<=1}" step="1">
            <for j="-1" true="{j<=1}" step="1">
              <var idx="{(row+i)*cols+col+j}"/>
              <var nbr="{NTH(grid,idx)}"/>
              <if true="nbr!=-1">
                <var d="{dist2(sx,sy,NTH(samples,nbr*2),NTH(samples,nbr*2+1))}"/>
                <if true="{d<r2}">
                  <asgn ok="0"/>
                </if>
              </if>
            </for>
          </for>
          <if true="{ok}">
            <asgn found="1"/>
            <asgn grid="{UPDATE(grid,row*cols+col,COUNT(samples)/2)}"/>
            <var sample="{sx} {sy}"/>
            <asgn active="{CAT(active,sample)}"/>
            <asgn samples="{CAT(samples,sample)}"/>
          </if>
        </if>

      </for>
      <if false="{found}">
        <asgn active="{CAT(TAKE(active,ridx*2),DROP(active,ridx*2+2))}"/>
      </if>
    </while>
    
    <for i="0" true="{i<COUNT(samples)}" step="2">
      <fill color="hsl({i/5},80%,80%)"/>
      <circle cx="{NTH(samples,i)}" cy="{NTH(samples,i+1)}" r="1.2"/>
    </for>
  </def-poissondisk>

  <poissondisk/>

</psvg>