[css-transforms-1] Neutral element for addition wrt animating transforms

[smfr]

https://lists.w3.org/Archives/Public/www-svg/2012May/0088.html
https://lists.w3.org/Archives/Public/www-svg/2012May/0090.html

[smfr]

We think that https://drafts.csswg.org/css-transforms/#neutral-element should be editing to use identity transform functions (scale(1) etc).

[svgeesus]

Neutral value for scale is 1, not zero. Neutral matrix is identity, not the null matrix. Editorial fixes.

[svgeesus]

@adanilo

The example 6 seems to indicate these are synthesized elements, not neutral. The example describes a rectangle starting at 0 size, animating to scale(1) at 5s so these don't appear to be no-op elements in the animation.

[svgeesus]

@birtles this concept comes from Web Animations, what did you mean here?

[birtles]

That's not something specifications need to define. Implementations can simply use a null value there. The only requirement is that when you add the neutral value for composition with the underlying value, you get the underlying value. (Since the addition happens before interpolation there's no use for the value beyond the addition step.)

[dirkschulze]

@birtles Is the null value an official term? Anyway, for CSS Transforms I added this on request of the WebAnimations team. Do we still need the "Neutral element for addition" section linked above? Or do we need to say that the neutral element for addition is the null value and implementations will know what to do? Or should we leave it as is?

@svgeesus Neutral always depends on the context. Neutral element for number addition is 0 and for number multiplication is 1. In this case it was specifically meant to be the null value.

[birtles]

Sorry, looking at the mails linked to in this thread, this issue appears to be referring to SMIL. I argued about a decade ago that the underlying value for scale should be 1 in SMIL. The SVG 1.2 Tiny test suite, however, used zero so I made Firefox follow that. I don't know if it's worth trying to fix SMIL now, or at least not <animateTransform>. We could try to fix it for <animate> I guess.

The "neutral value for composition" is something internal to Web Animations. It's just a roundabout way of saying, "use the underlying value". It should probably be dropped and it doesn't need to be defined here. I would just say that for SVG animation, the underlying value used for a by-animation of transform is a zero matrix.

[dirkschulze]

@birtles Thanks. Suggested text in replacement of the current spec text:

The used value for a by-animation of the transform property is a zero matrix.