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Re: Canvas arc comments

From: Ian Hickson <ian@hixie.ch>
Date: Fri, 13 Jun 2008 01:39:46 +0000 (UTC)
To: Philip Taylor <pjt47@cam.ac.uk>
Cc: HTML WG <public-html@w3.org>
Message-ID: <Pine.LNX.4.62.0806122310400.8559@hixie.dreamhostps.com>
On Sun, 27 Apr 2008, Philip Taylor wrote:
> 
> http://www.whatwg.org/specs/web-apps/current-work/multipage/section-the-canvas.html#arcx- 
> says:
> 
> "[...] The arc is the path along the circumference of this circle from 
> the start point to the end point ... . Since the points are on the 
> circle, as opposed to being simply angles from zero, the arc can never 
> cover an angle greater than 2π radians. If the two angles are equal, or 
> if the radius is zero, then the arc is defined as being of zero length 
> in both directions."
> 
> That definition would break either
>   ctx.arc(x, y, 0, 2*Math.PI, true);
> or
>   ctx.arc(x, y, 0, 2*Math.PI, false);
> since one of them would be drawn as very nearly zero length, depending on
> whether 2*Math.PI is <2π or >2π. People (at least me) expect both of those
> cases to draw an entire circle.
> 
> Suggested replacement:
> 
> "[...] If 'anticlockwise' is false and endAngle >= startAngle +
> 2π, or if 'anticlockwise' is true and startAngle >= endAngle + 2π, then the
> arc is the whole circumference of the circle. Otherwise, the arc is the path
> along the circumference of this circle from the start point to the end point
> ... [and the rest the same as before]"
> 
> That works as expected in the (0, 2*Math.PI+/-epsilon) case, and is tolerant
> of errors in the angles (since a small change in the input won't cause a big
> change in the output). It matches the arcs drawn by Firefox and Safari, with
> the only difference being that it does no overdraw for >2π arcs (which is
> usually an invisible effect anyway).

I used a different solution that should have the same effect. Let me know 
if I misunderstood what I was doing here.


On Sun, 27 Apr 2008, Philip Taylor wrote:
> 
> The behaviour of arcTo is undefined for radius=0. (In particular, "let 
> The Arc be the shortest arc given by circumference of the circle that 
> has radius radius, and that has one point tangent to [some line], and 
> that has a different point tangent to [some other line]" is impossible 
> to satisfy, since there aren't two different points on the circle.)
> 
> Suggested fix: Alter "If the point (x0, y0) is equal to the point (x1, 
> y1), or if the point (x1, y1) is equal to the point (x2, y2), then the 
> method must add the point (x1, y1) to the subpath, and connect that 
> point to the previous point (x0, y0) by a straight line." to include 
> "... or if radius is 0".
> 
> "if the direction from (x0, y0) to (x1, y1) is the same as the direction 
> from (x1, y1) to (x2, y2), then method must add the point (x1, y1) to 
> the subpath" - s/then/then the/

Fixed and fixed.

-- 
Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'
Received on Friday, 13 June 2008 01:40:29 UTC

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