[whatwg] [Canvas] Behavior on non-invertable CTM
ian at hixie.ch
Thu Jul 18 10:49:14 PDT 2013
On Tue, 29 Jan 2013, Dirk Schulze wrote:
> The spec doesn't have any wording about the behavior on non-invertible
> CTMs on Canvas contexts. Is it still possible to add segments to the
> current path once a CTM is not invertible anymore? Does the path get
> rejected completely then? Implementations are fairly different.
> Here are two examples (code attached at the end of the mail as well):
> Note that the path is stroked after restoring the initial CTM in both
> The first one does scale(0), which should make the CTM non-invertibe,
> WebKit still applies lineTo and closePath for some reason. IE and FF
> refuse to draw anything.
scale(0) is invalid, and should throw an exception.
If you do scale(0,0), the browsers act the same as with your second test
that uses setTransform() with 6 zeros.
> The second does setTransform(0,0,0,0,0,0), which should reset the CTM to
> a zero matrix (again, not invertible). IE, Opera and FF draw a line to
> 0,0 and close the path afterwards (which kind of makes sense, since the
> universe is convoluted to one point). WebKit refuses the lineTo command
> and closes the path as expected.
WebKit seems to just be wrong here, and the others right.
> This is an edge case, but should still be clarified in the spec.
I don't understand what there is to clarify. In both cases, the behaviour
seems well-defined: if you're transforming everything to zero, that's what
the result will be. Zero. Firefox's behaviour is the right one.
Ian Hickson U+1047E )\._.,--....,'``. fL
http://ln.hixie.ch/ U+263A /, _.. \ _\ ;`._ ,.
Things that are impossible just take longer. `._.-(,_..'--(,_..'`-.;.'
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