[Webkit-unassigned] [Bug 48031] AffineTransform operator* reverses order of operands.

Sun Dec 19 15:07:07 PST 2010

https://bugs.webkit.org/show_bug.cgi?id=48031

--- Comment #12 from Shane Stephens <shanestephens at google.com>  2010-12-19 15:07:07 PST ---
Yes, this test is failing locally and it's probably failing as a result of 48215.  Unfortunately the bug I fixed in 48215 introduced another bug that's now evident in LayoutTests/svg/W3C-SVG-1.1/animate-elem-24-t.svg :(

The messy details:

SVGStyledTransformableElement instances have a TransformList and a supplementalTransform.  While AnimateTransform elements update values in the TransformList, AnimateMotion elements update the supplementalTransform.  Crucially, transform values provided in the transform attribute of an element also appear in the TransformList.

The order in which these matrices are accumulated matters - for example, a rotate followed by a translate does not produce the same result as a translate followed by a rotate.

The problem that I fixed in 48215 was caused by the AnimateMotion's transform being applied first, before anything in the TransformList; whereas the specification indicates that this transform should be applied after values in the transform attribute.

Unfortunately, what I did was move the application of AnimateMotion to the end - i.e. after everything in the TransformList.  From the animate-elem-24-t.svg test, it appears that this is also wrong.  In fact, it's possible that the supplementalTransform needs to apply after some TransformList values but before others, in which case it might make sense to remove the supplementalTransform altogether and use the TransformList for AnimateMotion as well.  I'll read the specification carefully and seek guidance on the svg-wg list if necessary to determine what's appropriate.

This current bug, however, is unrelated in that it doesn't (or at least, shouldn't) change the behaviour of any of the code - it's simply about fixing the order of operands in matrix multiplication.  So I'd like to propose filing the fact that animate-elem-24-t.svg fails as a separate bug.  While we're at it, it seems like we should be modifying or copying any animation tests in this directory so that we have versions that actually work with run-webkit-tests.  If this test was actually testing when I was working on 48215 I probably would have picked up the issue then.

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