From Newsgroup: comp.lang.tcl

When you run the following code, a canvas comes up and you can
click-hold your mouse and drag it around to create bezier curves.

The line is jagged and it has a double curve. I wonder if it is possible
to make it smooth and reduce it to a plain s shape. Any ideas?


package req Tk


canvas .c -width 500 -height 500
pack .c -fill both -expand 1

bind .c <Button-1> [list mark %x %y]
bind .c <B1-Motion> [list draw %x %y]

proc mark {x y} {
set ::id 0
set ::ax $x
set ::ay $y
}

proc draw {x y} {
.c delete $::id

set xdiff [expr {($x - $::ax) / 2.0}]
set tx2 [expr {$::ax + 2 * $xdiff}]
set tx3 [expr {$x - 2 * $xdiff}]
set ::id [.c create line $::ax $::ay $tx2 $::ay $tx3 $y $x $y \
-fill #494949 -width 3 -smooth bezier -splinesteps 24]
}


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From Newsgroup: comp.lang.tcl

On Thu, 12 Jun 2025 12:51:58 -0400
saito <saitology9@gmail.com> wrote:

When you run the following code, a canvas comes up and you can
click-hold your mouse and drag it around to create bezier curves.

The line is jagged and it has a double curve. I wonder if it is possible
to make it smooth and reduce it to a plain s shape. Any ideas?

If by "smooth" you mean antialias, then no, Tk canvas doesn't do antialias.
As I undertand it, the tkpath package does, but I can't confirm since never used it.

Regards
--
Emiliano
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From Newsgroup: comp.lang.tcl

On 6/12/2025 7:08 PM, Emiliano wrote:

If by "smooth" you mean antialias, then no, Tk canvas doesn't do antialias. As I undertand it, the tkpath package does, but I can't confirm since never used it.

Thanks for taking a look. This is a piece of very old code from someone
else and I need to decide whether to improve it or trash it.

I guess there is no escaping the jagged lines. That is OK. How about
just making it a simple curve? As it is currently, there are two points
where it curves making it look like an S. Any ideas to reduce it to
simple / relaxed sine curve?
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From Newsgroup: comp.lang.tcl


saito <saitology9@gmail.com> posted:

On 6/12/2025 7:08 PM, Emiliano wrote:

If by "smooth" you mean antialias, then no, Tk canvas doesn't do antialias. As I undertand it, the tkpath package does, but I can't confirm since never used it.

Thanks for taking a look. This is a piece of very old code from someone
else and I need to decide whether to improve it or trash it.

I guess there is no escaping the jagged lines. That is OK. How about
just making it a simple curve? As it is currently, there are two points where it curves making it look like an S. Any ideas to reduce it to
simple / relaxed sine curve?

This is the character of the Bezier smoothing. If you want a curve with
only a single "bent", you might consider to derive a midpoint from the
two endpoints and draw a smoothed line (-smooth 1) through THREE points.

A possible method:
- Take the vector from point A to point B
- Determine the normal n to that vector (note: choice of direction!)
- Use a fraction of the distance between A and B to calculate the new point:

new point = (A + B)/2 + fraction * distance(A,B) * normal vector

You have a number of choices here :).
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From Newsgroup: comp.lang.tcl

On Thu, 12 Jun 2025 20:49:44 -0400
saito <saitology9@gmail.com> wrote:

On 6/12/2025 7:08 PM, Emiliano wrote:

If by "smooth" you mean antialias, then no, Tk canvas doesn't do antialias. As I undertand it, the tkpath package does, but I can't confirm since never used it.

Thanks for taking a look. This is a piece of very old code from someone
else and I need to decide whether to improve it or trash it.

I guess there is no escaping the jagged lines. That is OK. How about
just making it a simple curve? As it is currently, there are two points where it curves making it look like an S. Any ideas to reduce it to
simple / relaxed sine curve?

See the "-smooth raw" option. With this option you can define the spline control points. In the following code, circles are knot points (coordinate pairs 0, 3, 6 ...) and the squares are control points (coordinate pairs 1, 2, 4, 5 ...). Try moving the points around.

Sample code: ==================================================
package require Tk

proc lexpr {args} {
lmap expr $args {uplevel 1 [list expr $expr]}
}

proc point-coords {x y} {
lexpr {$x-5} {$y-5} {$x+5} {$y+5}
}

proc move-node {c item idx x y} {
set x [$c canvasx $x]
set y [$c canvasy $y]
$c coords $item [point-coords $x $y]
$c rchars line {*}[lexpr {$idx*2} {1+2*$idx}] [list $x $y]
}

canvas .c -xscrollcommand [list .sx set] -yscrollcommand [list .sy set] ttk::scrollbar .sy -orient vertical -command [list .c yview]
ttk::scrollbar .sx -orient horizontal -command [list .c xview]
grid .c .sy -sticky news
grid .sx -sticky ew
grid columnconfigure . 0 -weight 1
grid rowconfigure . 0 -weight 1

set coords {50 150 100 250 150 50 200 150}
set idx 0
.c create line $coords -smooth raw -tags line -splinesteps 20
foreach {x y} $coords type {oval rectangle rectangle oval} {
set item [.c create $type [point-coords $x $y] -fill red]
.c bind $item <B1-Motion> [list move-node %W $item $idx %x %y]
incr idx
}
bind .c <ButtonRelease-1> {%W configure -scrollregion [%W bbox all]}
.c configure -scrollregion [.c bbox all] ==================================================

Regards
--
Emiliano
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From Newsgroup: comp.lang.tcl

On 6/13/2025 4:28 AM, Arjen wrote:

This is the character of the Bezier smoothing. If you want a curve with
only a single "bent", you might consider to derive a midpoint from the
two endpoints and draw a smoothed line (-smooth 1) through THREE points.

A possible method:
- Take the vector from point A to point B
- Determine the normal n to that vector (note: choice of direction!)
- Use a fraction of the distance between A and B to calculate the new point:

new point = (A + B)/2 + fraction * distance(A,B) * normal vector

You have a number of choices here :).

Thanks. I will give it a try. And I appreciate your confidence in my
geometry chops :-)


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From Newsgroup: comp.lang.tcl

On 6/13/2025 9:19 AM, Emiliano wrote:

See the "-smooth raw" option. With this option you can define the spline control points. In the following code, circles are knot points (coordinate pairs 0, 3, 6 ...) and the squares are control points (coordinate pairs 1, 2, 4, 5 ...). Try moving the points around.

Thanks for the code! Very interesting. I have to look at it in detail to understand what is going on.
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