TIP #415 Version 1.1: Enable Easy Creation of Circular Arc Segments

This is not necessarily the current version of this TIP.

Abstract

Creating a segment of a circular arc is unnecessarily difficult using the canvas arc. This TIP proposes a simple extension of the syntax to support the creation of circular arc segments in a natural way. A similar extension to support the more general elliptical arc segments is outside the scope of this TIP.

Rationale

There is scope to enhance arc creation to make it much more useful as was shown by a recent discussion on news:comp.lang.tcl. The proposal here is the simplest enhancement to enable creation of circular arc segments from a single parameter.

Proposal

Enhance arc creation to support a new -height option

canvas create arc x1 y1 x2 y2 -height h ?options?

This will create an arc from (x1,y1) to (x2,y2) with a mid-point a distance h from the chord. For h>0 the arc will be clockwise, and for h<0 the arc will be anti-clockwise. A value of 0 is not valid and is currently ignored so that the command reverts to it's previous behaviour.

If -height is present the style is automatically set to arc; the pieslice style option is not permitted.

If -height is present the -start and -extent options are ignored.

Note that in this version of the command (x1,y1) and (x2,y2) no longer form the corners of the bounding box but instead the start and end points of the arc's chord.

Example

A pretty pattern can be created with the following:

set c [canvas .c -width 700 -height 700 -bg grey]
pack $c -fill both -expand 1

$c create line 300 100 400 500 -fill magenta
array set colours {0 red 1 yellow 2 green 3 cyan 4 blue 5 magenta}
for {set i 1} {$i <= 24} {incr i} {
    set col [expr {$i % 6}]
    $c create arc 400 500 300 100 -height [expr {$i*10}] \
            -outline $colours($col) -tags a_$i
    $c create arc 400 500 300 100 -height [expr {-$i*10}] \
            -outline $colours($col) -tags b_$i
}

Reference Implementation

A reference implementation for the functionality is available, but handling the option combinations, manual updates and unit testing is still to do.

Copyright

This document has been placed in the public domain.

This is not necessarily the current version of this TIP.