Okay the behaviour for lineTo, quadraticCurveTo and bezierCurveTo
without an existing subpath (unsure about arcTo any "sane" response to
arcTo with an empty path results in one of those edge cases where
webkit, gecko, and presto all disagree) should probably be changed to
(worded better of course :D ):
* lineTo(x, y) is equivalent to moveTo(x, y) if the context has no
subpaths.
* The quadraticCurveTo(cpx, cpy, x, y) method is equivalent to
"moveTo(cpx, cpy); quadraticCurveTo(cpx, cpy, x, y);" if the context
has no subpaths
* The bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y) method is equivalent
to "moveTo(cp1x, cp1y); bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y);"
if the context has no subpaths
My rational for this change is that it is a relaxation of existing API
-- in the specified API these cases would become no-ops that feed into
subsequent calls, eg. lineTo(..);lineTo(..);lineTo(..) will draw
nothing as the path never becomes non-empty so none of the calls can
ever have an effect, whereas this re-specification would result in
subsequent operations drawing something.
--Oliver
On Jul 11, 2009, at 3:41 PM, Oliver Hunt wrote:
While investigating a compatibility issue with http://www.blahbleh.com/clock.php
I found that the spec behaviour on CanvasRenderingContext2D.lineTo
conflicts with what Gecko implements.
The current spec language is
"The lineTo(x, y) method must do nothing if the context has no
subpaths. Otherwise, it must connect the last point in the subpath
to the given point (x, y) using a straight line, and must then add
the given point (x, y) to the subpath."
Gecko appears to treat the empty path case as moveTo(x,y). I'm
going to do a bit more investigation into the behaviour of this and
the other path manipulation functions to see whether lineTo is
"special" or this logic effects every function (of course any Gecko
devs may be able to answer more quickly than i can manually
verify). On the *assumption* that my initial analysis is correct i
propose that the language be updated to something akin to:
The lineTo(x, y) method is equivalent to moveTo(x, y) if the context
has no subpaths. Otherwise, it must connect the last point in the
subpath to the given point (x, y) using a straight line, and must
then add the given point (x, y) to the subpath.
--Oliver
