SMIL:CSS Animation

Current status

There's an existing smil_css patch in dholbert's smil-patches patch-queue. This patch applies on top of the other patches in the queue, and it makes use of a simple "getOverrideStyle" implementation in order to set styles.

The smil_css patch is not complete yet -- it currently has these limitations:

• It can only animate CSS properties of type nsCSSValue. (not nsCSSValuePair / nsCSSValueRect / etc)
• It can only currently interpolate between fixed-length units. (inch, mm, pt, etc). (unit-conversion code needs a reference to the PresContext & font-metrics to interpolate/interconvert between fixed-length units / display pixels / em-units / ex-units.)

Overview of Proposed Strategy

Animating CSS Style

As suggested in the documentation on the "SMIL Sandwich Model" in the smil-animation spec, I'm proposing that we use an implementation backed by getOverrideStyle().

Here's an overview of the steps required to animate a specific CSS property on a specific element, using getOverrideStyle(), during a SMIL animation sample:

1. Call getOverrideStyle() on the element, and from this, clear any existing value for the CSS property in question. (This ensures that getComputedStyle() won't reflect any animation effects, so it can be used as a 'base value' for animation in the next step)
2. Perform SMIL interpolation & compositing for the CSS property, using getComputedStyle() as the base value.
3. Store the final composited value of the property in getOverrideStyle().

Implementing getOverrideStyle()

Note that Gecko doesn't currently implement getOverrideStyle(). Here's a basic outline of how we could implement this:

1. Add a new nsICSSStyleRule member variable on each nsStyledElement, which is used as a backing store for the override style.
2. Internally, this should mostly behave like SVG's content style rule. (See mContentStyleRule in nsSVGElement.h and nsSVGElement.cpp )