<dt><code><b>tsb::canvas</b> ?-width w -height h?</code></dt>
 <dd>Creates a <code>canvas</code> emulation implementing enough
     methods for Plotchart, returns a widget command.</dd><br>
 <dt><code><b>tsb::canvascmd</b> cmd ...</code></dt>
 <dd>Implementation of the canvas widget command, supports
     <code>create</code>, <code>delete</code> and other methods plus
     method <code>svg</code> which renders the canvas as SVG after
     the invoking input field. The <code>svg</code> method takes
     optionally four numeric arguments which describe the SVG view box
     and resemble the scroll region of a real Tk canvas widget.
     The very first call of the <code>svg</code> method remembers
     the context w.r.t. the output field in the page. Subsequent
     calls of <code>svg</code> replace the content of that output
     field (useful for animations).</dd><br>
 <dt><code><b>tsb::clear</b></code></dt>
 <dd>Clears the entire page.</dd><br>
 <dt><code><b>tsb::eval</b> id</code></dt>
 <dd>Re-evaluates field with number <code>id</code>.</dd><br>
 <dt><code><b>tsb::load</b> ?filename?</code></dt>
 <dd>Load page from given <code>filename</code>, if file name omitted,
     present file selection.</dd><br>

1 {h2 "Geneva Drive"} 2 {#HTML
<p>How a film is moved through a movie projector? The film must
advance frame by frame with each frame pausing in front of the
lens for 1/24 of a second.<p>
 
<p>This intermittent motion is achieved using a Geneva Drive.</p>
 
<p>The name derives from the devices earliest application in mechanical
watches, Geneva being an important center of watchmaking. Other
application include pen change mechanism in plotters, automated
sampling devices, and so on.</p>

<p>This is an adapted version of Keith Vetter's "Geneva Drive" demo
from the Tcl'ers Wiki.<p>} 3 {array set S {
    title "Geneva Drive" w 500 h 500 lw 3
    animate 1 aid "" delay 50 angle 0 angle2 45
}
 
# Gear centers and radii
set V(gear1,o) {0 57}
set V(gear1,r0) 2
set V(gear1,r1) 10
set V(gear1,r2) 90
set V(gear1,r3) 161
set V(gear1,clr,r1) \#ccce34
set V(gear1,clr,r2) \#ccce34
set V(gear1,clr,r3) \#9c9a04
 
# Driving peg
set V(gear1,o2) {-114 0}
set V(gear1,p) 0              ;# Used for computing angles
set V(gear1,r10) 2
set V(gear1,r11) 10
set V(gear1,r12) 83
set V(gear1,clr,p) black
set V(gear1,clr,r0) black
set V(gear1,clr,r10) black
set V(gear1,clr,r11) red
set V(gear1,clr,r12) \#ccce34
 
set V(gear2,o) {0 -103}
set V(gear2,r0) 2
set V(gear2,r1) 10
set V(gear2,clr) \#64ce9c
set V(gear2,clr,r0) black
set V(gear2,clr,r1) \#64ce9c} 4 proc\ DoDisplay\ \{\}\ \{\n\ \ \ global\ S\ C\n\ \ \ set\ C\ \[tsb::canvas\ .c\ -width\ \$S(w)\ -height\ \$S(h)\]\n\ \ \ #\ Set\ the\ SVG\ viewBox\n\ \ \ set\ S(vbox)\ \[list\ \\\n\ \ \ \ \ \ \ \[expr\ \{0\ -\ \$S(w)\ /\ 2\}\]\ \[expr\ \{0\ -\ \$S(h)\ /\ 2\}\]\ \\\n\ \ \ \ \ \ \ \$S(w)\ \$S(h)\ \\\n\ \ \ \]\n\} 5 proc\ Gear1\ \{\}\ \{\n\ \ \ global\ V\ S\ C\n\ \ \ foreach\ \{x0\ y0\}\ \$V(gear1,o)\ break\n\ \ \ #\ Big\ disk\ and\ middle\ pin\n\ \ \ foreach\ who\ \{r3\ r1\ r0\}\ \{\n\ \ \ \ \ \ \ set\ xy\ \[MakeBox\ \$x0\ \$y0\ \$V(gear1,\$who)\]\n\ \ \ \ \ \ \ \$C\ create\ oval\ \{*\}\$xy\ -tags\ gear1,\$who\ -fill\ \$V(gear1,clr,\$who)\ \\\n\ \ \ \ \ \ \ \ \ \ \ -width\ \$S(lw)\n\ \ \ \}\n\ \ \ #\ Outer\ peg\n\ \ \ foreach\ \{x1\ y1\}\ \$V(gear1,o2)\ break\n\ \ \ set\ x1\ \[expr\ \{\$x0\ +\ \$x1\}\]\n\ \ \ set\ y1\ \[expr\ \{\$y0\ +\ \$y1\}\]\n\ \ \ foreach\ who\ \{p\ r11\ r10\}\ \{\n\ \ \ \ \ \ \ set\ xy\ \[MakeBox\ \$x1\ \$y1\ \$V(gear1,\$who)\]\n\ \ \ \ \ \ \ set\ xy2\ \[eval\ RegularPolygon2\ \$xy\ -start\ 0\ -extent\ 360\]\n\ \ \ \ \ \ \ \$C\ create\ polygon\ \{*\}\$xy2\ -tags\ \[list\ gear1\ gear1,\$who\]\ \\\n\ \ \ \ \ \ \ \ \ \ \ -fill\ \$V(gear1,clr,\$who)\ -width\ \$S(lw)\ -outline\ black\n\ \ \ \ \}\n\ \ \ \ #\ Rotating\ inner\ disk:\ concatentation\ of\ two\ arcs\n\ \ \ \ set\ xy2\ \[eval\ RegularPolygon2\ \[MakeBox\ \$x0\ \$y0\ \$V(gear1,r2)\]\ \\\n\ \ \ \ \ \ \ \ -start\ 135\ -extent\ 270\]\n\ \ \ \ set\ xy3\ \[eval\ RegularPolygon2\ \[MakeBox\ \$x1\ \$y1\ \$V(gear1,r12)\]\ \\\n\ \ \ \ \ \ \ \ -start\ -47\ -extent\ -94\]\n\ \ \ \ set\ xy\ \[concat\ \$xy2\ \$xy3\]\n\ \ \ \ \$C\ create\ polygon\ \{*\}\$xy\ -tags\ gear1\ \\\n\ \ \ \ \ \ \ \ -fill\ \$V(gear1,clr,r2)\ -outline\ black\ -width\ \$S(lw)\n\ \ \ \ \$C\ raise\ gear1,r1\n\ \ \ \ \$C\ raise\ gear1,r0\n\} 6 proc\ Gear2\ \{\}\ \{\n\ \ \ \ global\ V\ S\ C\n\ \ \ \ foreach\ \{x0\ y0\}\ \$V(gear1,o)\ break\n\ \ \ \ foreach\ \{Gx\ Gy\}\ \$V(gear2,o)\ break\n\ \ \ \ set\ xy\ \[MakeBox\ \$x0\ \$y0\ \$V(gear1,r2)\]\n\ \ \ \ set\ xy0\ \[eval\ RegularPolygon2\ \$xy\ -start\ 135\ -extent\ 90\]\n\ \ \ \ #\ Create\ one\ cusp\ and\ inlet\n\ \ \ \ foreach\ \{x1\ y1\}\ \[lrange\ \$xy0\ end-1\ end\]\ break\n\ \ \ \ set\ dx\ \[expr\ \{\$x1-\$x0\}\]\n\ \ \ \ set\ dy\ \[expr\ \{\$y1-\$y0\}\]\n\ \ \ \ set\ dist\ \[expr\ \{hypot(\$dx,\$dy)\}\]\n\ \ \ \ set\ nx\ \[expr\ \{\$dy\}\]\n\ \ \ \ set\ ny\ \[expr\ \{-\$dx\}\]\n\ \ \ \ set\ x2\ \[expr\ \{\$x1\ +\ \$dx\ *\ 10\ /\ \$dist\}\]\n\ \ \ \ set\ y2\ \[expr\ \{\$y1\ +\ \$dy\ *\ 10\ /\ \$dist\}\]\n\ \ \ \ set\ x3\ \[expr\ \{\$x2\ +\ \$nx\ *\ 80\ /\ \$dist\}\]\n\ \ \ \ set\ y3\ \[expr\ \{\$y2\ +\ \$ny\ *\ 80\ /\ \$dist\}\]\n\ \ \ \ set\ x4\ \[expr\ \{\$x3\ +\ \$dx\ *\ 26\ /\ \$dist\}\]\n\ \ \ \ set\ y4\ \[expr\ \{\$y3\ +\ \$dy\ *\ 26\ /\ \$dist\}\]\n\ \ \ \ set\ x5\ \[expr\ \{\$x4\ -\ \$nx\ *\ 80\ /\ \$dist\}\]\n\ \ \ \ set\ y5\ \[expr\ \{\$y4\ -\ \$ny\ *\ 80\ /\ \$dist\}\]\n\ \ \ \ set\ x6\ \[expr\ \{\$x5\ +\ \$dx\ *\ 10\ /\ \$dist\}\]\n\ \ \ \ set\ y6\ \[expr\ \{\$y5\ +\ \$dy\ *\ 10\ /\ \$dist\}\]\n\ \ \ \ lappend\ xy0\ \$x1\ \$y1\ \$x2\ \$y2\ \$x3\ \$y3\ \$x4\ \$y4\ \$x5\ \$y5\ \$x6\ \$y6\n\ \ \ \ #\ Rotate\ 3\ times\ and\ join\ all\ the\ points\n\ \ \ \ set\ xy1\ \[RotateCoords\ \$xy0\ \$Gx\ \$Gy\ -90\]\n\ \ \ \ set\ xy2\ \[RotateCoords\ \$xy0\ \$Gx\ \$Gy\ -180\]\n\ \ \ \ set\ xy3\ \[RotateCoords\ \$xy0\ \$Gx\ \$Gy\ -270\]\n\ \ \ \ set\ xy\ \[concat\ \$xy0\ \$xy1\ \$xy2\ \$xy3\]\n\ \ \ \ \$C\ create\ polygon\ \{*\}\$xy\ -tags\ gear2\ -fill\ \$V(gear2,clr)\ \\\n\ \ \ \ \ \ \ \ -outline\ black\ -width\ 4\n\ \ \ \ foreach\ who\ \{r1\ r0\}\ \{\n\ \ \ \ \ \ \ \ set\ xy\ \[MakeBox\ \$Gx\ \$Gy\ \$V(gear2,\$who)\]\n\ \ \ \ \ \ \ \ \$C\ create\ oval\ \{*\}\$xy\ -tags\ gear2,\$who\ \\\n\ \ \ \ \ \ \ \ \ \ \ \ -fill\ \$V(gear2,clr,\$who)\ -width\ \$S(lw)\n\ \ \ \ \}\n\} 7 {proc MakeBox {x y r} {
    return [list [expr {$x-$r}] [expr {$y-$r}] [expr {$x+$r}] [expr {$y+$r}]]
}} 8 {# from https://wiki.tcl-lang.org/Regular%20Polygons%202
proc RegularPolygon2 {x0 y0 x1 y1 args} {
    array set V {-sides 0 -start 90 -extent 360} ;# Default values
    foreach {a value} $args {
        if {! [info exists V($a)]} {error "unknown option $a"}
        if {$value == {}} {error "value of \"$a\" missing"}
        set V($a) $value
    }
    if {$V(-extent) == 0} {return {}}
    set xm [expr {($x0+$x1)/2.}]
    set ym [expr {($y0+$y1)/2.}]
    set rx [expr {$xm-$x0}]
    set ry [expr {$ym-$y0}]
    set n $V(-sides)
    if {$n == 0} {                              ;# 0 sides => circle
        set n [expr {round(($rx+$ry)*0.5)}]
        if {$n <= 2} {set n 4}
    }
    set dir [expr {$V(-extent) < 0 ? -1 : 1}]   ;# Extent can be negative
    if {abs($V(-extent)) > 360} {
        set V(-extent) [expr {$dir * (abs($V(-extent)) % 360)}]
    }
    set step [expr {$dir * 360.0 / $n}]
    set numsteps [expr {1 + double($V(-extent)) / $step}]
    set xy {}
    set DEG2RAD [expr {acos(-1)*2/360}]
    for {set i 0} {$i < int($numsteps)} {incr i} {
        set rad [expr {($V(-start) - $i * $step) * $DEG2RAD}]
        set x [expr {$rx*cos($rad)}]
        set y [expr {$ry*sin($rad)}]
        lappend xy [expr {$xm + $x}] [expr {$ym - $y}]
    }
    # Figure out where last segment should end
    if {$numsteps != int($numsteps)} {
        # Vecter V1 is last drawn vertext (x,y) from above
        # Vector V2 is the edge of the polygon
        set rad2 [expr {($V(-start) - int($numsteps) * $step) * $DEG2RAD}]
        set x2 [expr {$rx*cos($rad2) - $x}]
        set y2 [expr {$ry*sin($rad2) - $y}]
        # Vector V3 is unit vector in direction we end at
        set rad3 [expr {($V(-start) - $V(-extent)) * $DEG2RAD}]
        set x3 [expr {cos($rad3)}]
        set y3 [expr {sin($rad3)}]
        # Find where V3 crosses V1+V2 => find j s.t.  V1 + kV2 = jV3
        set j [expr {($x*$y2 - $x2*$y) / ($x3*$y2 - $x2*$y3)}]
        lappend xy [expr {$xm + $j * $x3}] [expr {$ym - $j * $y3}]
    }
    return $xy
}} 9 {# From https://wiki.tcl-lang.org/CanvasRotation
proc RotateItem {w tagOrId Ox Oy angle} {
    set angle [expr {$angle * atan(1) * 4 / 180.0}] ;# Radians
    foreach id [$w find withtag $tagOrId] {
        # Do each component separately
        set xy {}
        foreach {x y} [$w coords $id] {
            # rotates vector (Ox,Oy)->(x,y) by angle clockwise
            set x [expr {$x - $Ox}]             ;# Shift to origin
            set y [expr {$y - $Oy}]
            set xx [expr {$x * cos($angle) - $y * sin($angle)}] ;# Rotate
            set yy [expr {$x * sin($angle) + $y * cos($angle)}]
            set xx [expr {$xx + $Ox}]           ;# Shift back
            set yy [expr {$yy + $Oy}]
            lappend xy $xx $yy
        }
        $w coords $id {*}$xy
    }
}} 10 {proc RotateCoords {xy Ox Oy angle} {
    set angle [expr {$angle * atan(1) * 4 / 180.0}] ;# Radians
    set xy2 {}
    foreach {x y} $xy {
        # rotates vector (Ox,Oy)->(x,y) by angle clockwise
        set x [expr {$x - $Ox}]             ;# Shift to origin
        set y [expr {$y - $Oy}]
        set xx [expr {$x * cos($angle) - $y * sin($angle)}] ;# Rotate
        set yy [expr {$x * sin($angle) + $y * cos($angle)}]
        set xx [expr {$xx + $Ox}]           ;# Shift back
        set yy [expr {$yy + $Oy}]
        lappend xy2 $xx $yy
    }
    return $xy2
}} 11 {proc StepIt {dir} {
    global S V C
    foreach {x0 y0} $V(gear1,o) break
    RotateItem $C gear1 $x0 $y0 $dir
    set S(angle) [expr {($S(angle) + $dir) % 360}]
    if {$S(angle) == 45} {
        set S(angle2) 45
    } elseif {$S(angle) > 45 && $S(angle) <= 135} {
        foreach {Gx Gy} $V(gear2,o) break
        foreach {x1 y1} [$C coords gear1,p] break
        set dx [expr {$x1 - $Gx}] ; set dy [expr {$y1 - $Gy}]
        set degree [expr {
            round((acos($dy /hypot($dx,$dy))) * 180 / acos(-1))
        }]
        set S(degree) $degree
        set da [expr {-abs($degree - abs($S(angle2)))}]
        if {$da != 0} {
            RotateItem $C gear2 $Gx $Gy $da
            incr S(angle2) $da
        }
    }
}} 12 {proc Animate {} {
    after cancel $::S(aid)
    StepIt 5
    if {$::S(animate)} {
        set ::S(aid) [after $::S(delay) Animate]
    }
    lassign $::S(vbox) vx vy vw vh
    # Retro feeling: jitter on the viewBox
    set vx [expr {$vx + 1 * (rand() - 0.5)}]
    set vy [expr {$vy + 1 * (rand() - 0.5)}]
    set vw [expr {$vw + 1 * (rand() - 0.5)}]
    set vh [expr {$vh + 1 * (rand() - 0.5)}]
    $::C svg $vx $vy $vw $vw
}} 13 {DoDisplay
Gear1
Gear2
Animate} 14 {tsb::save geneva.tsb}

# Taygete Scrap Book - a poor man's clone of Jupyter Notebook
#
# chw June 2019
#
####################################################################

# Load webview and other stuff.

package require twv
package require Markdown

# On MacOSX we need Tk early, otherwise crashes occur.

	# Should be the ping, Vasily.
	# But can be the reload, too.
	catch {uplevel \#0 $str}
	return
    }
    set ::H($id) $str
    if {$str eq ""} {
	$::W call Wclear $id 0
	return
    }
    set newfield 0
    set ::ID $id
    if {[string first "#HTML" $str] == 0} {
	set n [string first "\n" $str]
	if {$n > 4} {

	set str [Markdown::convert $str]
	$::W call Wraw $id $str 1
	$::W call Inhide $id 1
	set newfield 1
    } elseif {[catch {uplevel \#0 $str} ret opts]} {
	if {[dict get $opts -code] == 4} {
	    # continue
	    $::W call Wclear $id 0
	} else {
	    $::W call Werror $id $::errorInfo
	}
    } else {
	if {$ret ne ""} {
	    $::W call Wresult $id $ret
	} else {
	    $::W call Wclear $id 0
	}
	set newfield 1
    }
    if {$newfield} {
	incr id
	if {![info exists ::H($id)]} {
	    set ::H($id) ""

    if {$selname} {
	set cmd [dict get [info frame -1] cmd]
	set newcmd $cmd
	lappend newcmd $name
	::tsb::change_field $cmd $newcmd
    }
    if {[info exists ::ID]} {
	$::W call Wclear $::ID 0
    }
    set t $title
    if {$file ne ""} {
	append t " - [file tail $file]"
    }
    set t [htmlquote $t]
    set f [open $name w]

		    }
		} elseif {[llength $coords]} {
		    dict set C($c,$id) -coords $coords
		} else {
		    set ret [dict get $C($c,$id) -coords]
		}
	    }
	    fi* {
		# find ?withtag?
		set args [lassign $args tags]
		if {$tags eq "withtag"} {
		    lassign $args tags
		}
		set ret [tagfind $c $tags]
	    }
	    sv* {
		# svg
		set vx ""
		set vy ""
		set vw ""
		set vh ""
		lassign $args vx vy vw vh
		package require can2svg
		set xml "<?xml version='1.0'?>\n"
		append xml "<svg width='$C($c,width)' height='$C($c,height)'"
		if {$vx ne "" && $vy ne "" && $vw ne "" && $vh ne ""} {
		    append xml " viewBox='$vx $vy $vw $vh'"
		}
		append xml " version='1.1'"
		append xml " xmlns='http://www.w3.org/2000/svg'"
		append xml " xmlns:xlink='http://www.w3.org/1999/xlink'>\n"
		foreach item $C($c,dlist) {
		    set type [dict get $C($c,$item) -type]
		    switch -- $type {
			image - window {

				lappend cmd $key [dict get $C($c,$item) $key]
			    }
			}
		    }
		    append xml "  " [can2svg::can2svg $cmd {*}$args] "\n"
		}
		append xml "</svg>\n"
		# If (output) field set, use it; otherwise use
		# the current one from ::ID and remember it.
		if {[info exists C($c,field)]} {
		    $::W call Wclear $C($c,field) 1
		    set fid $C($c,field)
		} elseif {[info exists ::ID]} {
		    set fid $::ID
		    set C($c,field) $fid
		}
		if {[info exists fid]} {
		    $::W call Wraw $fid $xml
		}
	    }
	}
	return $ret
    }

    proc canvas {args} {
	variable C

	};

	/* The 2nd sin: this drives the Tcl event loop. */
	var Gtimer = window.setInterval(function() {
	    window.external.invoke("0 ::tsb::ping");
	}, 20);

	var Wclear = function(id, later) {
	    if (later) {
		needsClear[id] = true;
		return;
	    }
	    if (!needsClear[id]) {
		return;
	    }
	    var output =
		document.getElementById('out' + id + '-pre').firstChild;
	    if (output.innerHTML.length > 0) {
		output.style.color = 'inherit';
		output.innerHTML = '';
	    }
	    output = document.getElementById('out' + id + '-raw');
	    if (output.innerHTML.length > 0) {
		output.innerHTML = '';
	    }
	    needsClear[id] = null;
	};

	var Wresult = function(id, str) {
	    var output =
		document.getElementById('out' + id + '-pre').firstChild;
	    str = str.replace(/&/g, '&');
	    str = str.replace(/</g, '&lt;');

	    }
	};

	var Winput = function(id, str) {
	    var input = document.getElementById('code' + id);
	    input.value = str;
	    var lines = str.split(/\r?\n|\r/);
	    var nlines = (lines.length < 50) ? lines.length : 50;
	    input.rows = nlines;
	};

	var Feval = function(id) {
	    var input = document.getElementById('code' + id);
	    needsClear[id] = true;
	    RunTcl("" + id + " " + input.value);

		    RunTcl("" + id + " " + input.value);
		    event.preventDefault();
		    return false;
		}
	    });
	    input.addEventListener('input', function(event) {
		var lines = input.value.split(/\r?\n|\r/);
		var nlines = (lines.length < 50) ? lines.length : 50;
		input.rows = nlines;
	    });
	    input.focus();
	    return input;
	};

	var InitField = function(id, str) {
	    var input = Field(id);
	    input.value = str;
	    var lines = str.split(/\r?\n|\r/);
	    var nlines = (lines.length < 50) ? lines.length : 50;
	    input.rows = nlines;
	};

	var GotoTop = function() {
	    window.location.href = "#top";
	};