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Re: SVG animateMotion specification clarification request

From: Shane Stephens <shans@google.com>
Date: Fri, 21 Jan 2011 09:41:49 +1100
Message-ID: <AANLkTimdSODDNoW7VCm8SXL+OF0GBgOdz35iRK5Jt2_j@mail.gmail.com>
To: www-svg@w3.org
Hi list,

Has anyone had a chance to look at this?  Hopefully I've missed
something, but if not there's an inconsistency between some of the
test files and the specification.


On Mon, Dec 20, 2010 at 3:59 PM, Shane Stephens <shans@google.com> wrote:
> Hi list,
> I'm trying to work out the correct behaviour for the <animateMotion>
> element as relates to other transforms on the same target element.
> The specification is not 100% clear about the correct behaviour, and
> if my reading is correct then at least one of the SVG Test files
> (http://www.w3.org/Graphics/SVG/Test/20061213/htmlEmbedHarness/full-animate-elem-24-t.html)
> is incorrect.
> Here's the relevant part of the specification:
> "The various (x,y) points of the shape provide a supplemental
> transformation matrix onto the CTM for the referenced object which
> causes a translation along the x- and y-axes of the current user
> coordinate system by the (x,y) values of the shape computed over time.
> Thus, the referenced object is translated over time by the offset of
> the motion path relative to the origin of the current user coordinate
> system. The supplemental transformation is applied on top of any
> transformations due to the target element's ‘transform’ attribute or
> any animations on that attribute due to ‘animateTransform’ elements on
> the target element.
> The ‘additive’ and ‘accumulate’ attributes apply to ‘animateMotion’
> elements. Multiple ‘animateMotion’ elements all simultaneously
> referencing the same target element can be additive with respect to
> each other; however, the transformations which result from the
> ‘animateMotion’ elements are always supplemental to any
> transformations due to the target element's ‘transform’ attribute or
> any ‘animateTransform’ elements."
> It's clear that the transformation matrix generated by animateMotion
> elements (the supplemental transformation) is intended to be
> independent of that generated by animateTransform elements and
> 'transform' attributes in the target element (the CTM).  The question
> I have is whether the supplemental transformation is post-multiplied
> onto the CTM or pre-multiplied.
> The relevant wording is "The supplemental transformation is applied on
> top of [the CTM]".  Unfortunately this isn't clear - elsewhere in the
> specification there is a sense of postMultiply == concatenate ==
> "applied after", but not "on top of".
> The following example illustrates (I think) that the correct approach
> is to postMultiply the supplemental transform:
> <g transform="translate(300, 30)">
>  <rect width="40" height="40" fill="green"></rect>
>  <animateMotion dur="1s" repeatCount="1" rotate="auto" path="M
> 100,250 C 100,50 400,50 400,250"></animateMotion>
> </g>
> If we were to preMultiply the supplementalTransform, then the observed
> path of the green rectangle varies wildly depending on whether
> rotate="auto" is specified or not - with rotate="auto", the rectangle
> appears to move along a path starting above the screen and rotating in
> roughly a semicircle moving predominantly downwards, as the
> supplementalTransform rotates as well as translates, and the rotation
> in turn effects the translation component of the CTM.  On the other
> hand, if the rotate attribute is not set to "auto", there is no
> rotation component to the supplementalTransform and the rectangle
> appears to move along a path starting at 400,280 (300,30 + 100,250)
> and moving in roughly a semicircle moving predominantly to the right.
> On the other hand, if we postMultiply the supplementalTransform then
> the green rectangle always moves on the path starting at 400,280 and
> moving to the right.  If rotate="auto" is set, then the rectangle
> rotates as it moves along the path, which I think is more in keeping
> with the intention of this attribute.
> However, postMultiplying the supplementalTransform causes
> animate-elem-24-t to render incorrectly.  The relevant part of this
> file is:
> <text id="TextElement" x="0" y="0" font-family="MyFont" font-size="20"
> fill="#f22" transform="rotate(-30)">It's alive!
>  <animateMotion path="M 0 0 L 50 180" begin="3s" dur="6s" fill="freeze"/>
>  <animateTransform attributeName="transform" attributeType="XML"
> type="rotate" from="-30" to="0" begin="3s" dur="6s" fill="freeze"/>
>  <animateTransform attributeName="transform" attributeType="XML"
> type="scale" from="1" to="3" additive="sum" begin="3s" dur="6s"
> fill="freeze"/>
> </text>
> In this case, postMultiplying the supplementalTransform causes the
> rotate and scale components of the CTM generated from the
> animateTransform elements to effect the size of the translation
> generated by the animateMotion - and this is clearly not expected by
> the author of the test as the indicated expected positions of the text
> at various times no long match the actual positions of the text.
> So which is correct?
> Cheers,
>    -Shane
Received on Thursday, 20 January 2011 22:42:19 UTC

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