Re: [whatwg] CanvasRenderingContext2D.lineTo compatibility problem
On Sat, 11 Jul 2009, 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. On Sat, 11 Jul 2009, Oliver Hunt wrote: 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 rationale 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. Fixed the spec as proposed (and also for arcTo(), though as you say, the behaviour there isn't very interoperable). -- Ian Hickson U+1047E)\._.,--,'``.fL http://ln.hixie.ch/ U+263A/, _.. \ _\ ;`._ ,. Things that are impossible just take longer. `._.-(,_..'--(,_..'`-.;.'
[whatwg] CanvasRenderingContext2D.lineTo compatibility problem
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
Re: [whatwg] CanvasRenderingContext2D.lineTo compatibility problem
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
