static void
SampleValue(float aPortion, Animation& aAnimation, StyleAnimationValue& aStart,
            StyleAnimationValue& aEnd, Animatable* aValue)
{
  StyleAnimationValue interpolatedValue;
  NS_ASSERTION(aStart.GetUnit() == aEnd.GetUnit() ||
               aStart.GetUnit() == StyleAnimationValue::eUnit_None ||
               aEnd.GetUnit() == StyleAnimationValue::eUnit_None,
               "Must have same unit");
  StyleAnimationValue::Interpolate(aAnimation.property(), aStart, aEnd,
                                aPortion, interpolatedValue);
  if (aAnimation.property() == eCSSProperty_opacity) {
    *aValue = interpolatedValue.GetFloatValue();
    return;
  }

  nsCSSValueSharedList* interpolatedList =
    interpolatedValue.GetCSSValueSharedListValue();

  TransformData& data = aAnimation.data().get_TransformData();
  nsPoint origin = data.origin();
  // we expect all our transform data to arrive in css pixels, so here we must
  // adjust to dev pixels.
  double cssPerDev = double(nsDeviceContext::AppUnitsPerCSSPixel())
                     / double(data.appUnitsPerDevPixel());
  gfxPoint3D transformOrigin = data.transformOrigin();
  transformOrigin.x = transformOrigin.x * cssPerDev;
  transformOrigin.y = transformOrigin.y * cssPerDev;
  gfxPoint3D perspectiveOrigin = data.perspectiveOrigin();
  perspectiveOrigin.x = perspectiveOrigin.x * cssPerDev;
  perspectiveOrigin.y = perspectiveOrigin.y * cssPerDev;
  nsDisplayTransform::FrameTransformProperties props(interpolatedList,
                                                     transformOrigin,
                                                     perspectiveOrigin,
                                                     data.perspective());
  gfx3DMatrix transform =
    nsDisplayTransform::GetResultingTransformMatrix(props, origin,
                                                    data.appUnitsPerDevPixel(),
                                                    &data.bounds());
  gfxPoint3D scaledOrigin =
    gfxPoint3D(NS_round(NSAppUnitsToFloatPixels(origin.x, data.appUnitsPerDevPixel())),
               NS_round(NSAppUnitsToFloatPixels(origin.y, data.appUnitsPerDevPixel())),
               0.0f);

  transform.Translate(scaledOrigin);

  InfallibleTArray<TransformFunction> functions;
  functions.AppendElement(TransformMatrix(ToMatrix4x4(transform)));
  *aValue = functions;
}
static void
InvertSign(StyleAnimationValue& aValue)
{
  switch (aValue.GetUnit()) {
    case StyleAnimationValue::eUnit_Coord:
      aValue.SetCoordValue(-aValue.GetCoordValue());
      break;
    case StyleAnimationValue::eUnit_Percent:
      aValue.SetPercentValue(-aValue.GetPercentValue());
      break;
    case StyleAnimationValue::eUnit_Float:
      aValue.SetFloatValue(-aValue.GetFloatValue());
      break;
    default:
      NS_NOTREACHED("Calling InvertSign with an unsupported unit");
      break;
  }
}