Here is a Tcl/Tk adaptation of a graphics demo that was made in the early 90s for the Acorn Archimedes, a British home computer line which represents some of the first computers to include an ARM processor.

The algorithm for that demo was posted in the August '92 issue of Acorn User magazine, and that was used as the reference for this Tcl/Tk implementation.

The demo is a 2D simulation the surface of a body of water. Click inside the window to watch water-like ripples appear and move around.

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#!/usr/bin/tclsh

# 2D water surface demo/toy
# Tcl/Tk version by dusthillresident
#  dusthillresident@gmail.com
#  https://mastodon.me.uk/@dusthillperson
# based on an algorithm posted in the August '92 issue of Acorn User magazine
# for more information, please refer to this link:
# https://archive.org/details/AcornUser1992Magazine/AcornUser9208/page/n67/mode/2up

package require Tk

tk appname "AcornUser August 92 water demo, original by Michel Grimminck"

set wh 63 ;# width and height of the water surface
set sc 8  ;# scale size used for drawing
set xs 1
set ys 1

# initialise arrays
for {set x 0} {$x<$wh} {incr x} {
 lappend empty 0
}

for {set y 0} {$y<$wh} {incr y} {
 lappend empty2 $empty
}

set height $empty2
set velocity $empty2
unset empty empty2

array set cols {
 -4 \#406000
 -3 \#406a20
 -2 \#407040
 -1 \#407a60
  0 \#407a80
  1 \#4080A0
  2 \#408aC0
  3 \#4090E0
  4 \#409aFF
}

proc process_frame {} {
 set friction 0.02
 set tension 0.5
 global wh height velocity
 for {set x 1} {$x<$wh-1} {incr x} {
  for {set y 1} {$y<$wh-1} {incr y} {
   lset velocity $x $y [expr {(1-$friction)*([lindex $velocity $x $y]-$tension*(4*[lindex $height $x $y]-[lindex $height [expr {$x-1}] $y]-[lindex $height [expr {$x+1}] $y]-[lindex $height $x [expr {$y-1}]]-[lindex $height $x [expr {$y+1}]]))}]
  }
 }
 for {set x 1} {$x<$wh-1} {incr x} {
  for {set y 1} {$y<$wh-1} {incr y} {
   lset height $x $y [expr {[lindex $height $x $y]+[lindex $velocity $x $y]}]
  }
 }
}

image create photo img
image create photo displ
pack [label .squares -image displ] -fill both -expand 1
img put black -to 0 0 $wh $wh

set redraw 0

proc display_frame {} {
 global wh height cols redraw
 for {set x 1} {$x<$wh-1} {incr x} {
  for {set y 1} {$y<$wh-1} {incr y} { 
   set colour [expr {int([lindex $height $x $y]*4)}]
   if {$colour<-4} {set colour -4}
   if {$colour>4}  {set colour 4}
   set colour $cols($colour)
   img put $colour -to $x $y
  }
 }
 if {$redraw} { image delete displ; image create photo displ }
 displ copy img -zoom $::xs $::ys
}

set mouse_pressed 0

proc clickaction {x y} {
 global height xs ys
 set xx [expr {int($x/$xs)}]
 set xx [expr { !($xx<1||$xx>=$::wh-1) ? $xx : [return] }]
 set yy [expr {int($y/$ys)}]
 set yy [expr { !($yy<1||$yy>=$::wh-1) ? $yy : [return] }]
 lset height $xx $yy 10
}

bind .squares <ButtonPress> {
 set mouse_pressed 1
 clickaction %x %y
}

bind .squares <ButtonRelease> {
 set mouse_pressed 0
}

bind .squares <Motion> {
 if $mouse_pressed {
  clickaction %x %y
 }
}

bind . <Configure> {
 set redraw 1
 set ::xs [expr {[winfo width .squares]/$::wh}]
 set ::ys [expr {[winfo height .squares]/$::wh}]
 set ::xs [expr {$::xs<1 ? 1 : $::xs}]
 set ::ys [expr {$::ys<1 ? 1 : $::ys}]
}

proc updater {} {
 process_frame
 display_frame
 after 16 updater
}
updater

after 32 {wm geometry . 256x256}
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