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/usr/share/xscreensaver/config/tessellimage.xml is in xscreensaver-data 5.36-1ubuntu1.

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The actual contents of the file can be viewed below.

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<?xml version="1.0" encoding="ISO-8859-1"?>

<screensaver name="tessellimage" _label="Tessellimage">

  <command arg="-root"/>

  <video href="https://www.youtube.com/watch?v=JNgybysnYU8"/>

  <hgroup>
   <vgroup>
    <number id="delay" type="slider" arg="-delay %"
            _label="Frame rate" _low-label="Low" _high-label="High"
            low="0" high="100000" default="30000"
            convert="invert"/>

    <number id="duration2" type="slider" arg="-duration2 %"
            _label="Speed" _low-label="0.1 second" _high-label="4 seconds"
            low="0.1" high="4.0" default="0.4"/>

    <number id="duration" type="slider" arg="-duration %"
            _label="Duration" _low-label="10 seconds" _high-label="10 minutes"
            low="10" high="600" default="120"/>

    <number id="depth" type="slider" arg="-max-depth %"
            _label="Complexity" _low-label="Shallow" _high-label="Deep"
            low="1000" high="100000" default="30000"/>

   </vgroup>

   <vgroup>
    <xscreensaver-image />

    <boolean id="fill"    _label="Fill screen" arg-unset="-no-fill-screen"/>
    <boolean id="outline" _label="Outline triangles" arg-unset="-no-outline"/>
    <boolean id="showfps" _label="Show frame rate" arg-set="-fps"/>

    <xscreensaver-updater />
   </vgroup>
  </hgroup>

  <_description>
Converts an image to triangles using Delaunay tessellation, and
animates the result at various depths.

More triangles are allocated to visually complex parts of the image.
This is accomplished by first computing the first derivative of the
image: the distance between each pixel and its neighbors (which is
essentially edge detection or embossing).  Then the Delaunay control
points are chosen by selecting those pixels whose distance value is
above a certain threshold: those are the pixels that have the largest
change in color/brightness.

https://en.wikipedia.org/wiki/Delaunay_triangulation

Written by Jamie Zawinski; 2014.
  </_description>
</screensaver>