[scroll-animations-1] Define current time of scroll-driven timelines. w3c#7778

Author: fantasai

scroll-animations-1/Overview.bs

@@ -31,6 +31,7 @@ spec:web-animations-1;
 	type:interface; text:AnimationTimeline
 	type:attribute; text:currentTime
 	type:dfn;
+		text:current time
 		text:active interval
 		text:start delay
 		text:end delay
@@ -128,9 +129,6 @@ spec: cssom-view-1; type: dfn;
 	along a particular axis.
 	The startmost scroll position represents 0% progress
 	and the endmost scroll position represents 100% progress.
-	If the 0% position and 100% position coincide
-	(i.e. the [=scroll container=] has no overflow to scroll),
-	the timeline is [=inactive timeline|inactive=].
 
 	[=Scroll progress timelines=] can be referenced in 'animation-timeline'
 	anonymously using the ''scroll()'' [=functional notation=]
@@ -139,6 +137,18 @@ spec: cssom-view-1; type: dfn;
 	In the Web Animations API,
 	they can be represented anonymously by a {{ScrollTimeline}} object.
 
+## Calculating Progress for a Scroll Progress Timeline ## {#scroll-timeline-progress}
+
+	Progress (the [=current time=]) for a [=scroll progress timeline=]
+	is calculated as:
+	<var>[=scroll offset=]</var> &div;
+	(<var>[=scrollable overflow=] size</var> &minus;
+		<var>[=scroll container=] size</var>)
+
+	If the 0% position and 100% position coincide
+	(i.e. the denominator in the [=current time=] formula is zero),
+	the timeline is [=inactive timeline|inactive=].
+
 ## Anonymous Scroll Progress Timelines ## {#scroll-timelines-anonymous}
 
 ### The ''scroll()'' notation ### {#scroll-notation}
@@ -368,7 +378,8 @@ spec: cssom-view-1; type: dfn;
 	(or more precisely, the relevant [=view progress visibility range=]
 	of that [=scrollport=]).
 	The startmost such scroll position represents 0% progress
-	and the endmost such scroll position represents 100% progress.
+	and the endmost such scroll position represents 100% progress,
+	see [[#view-timeline-progress]].
 
 	Note: The 0% and 100% scroll positions are not always reachable,
 	e.g. if the box is positioned
@@ -465,6 +476,26 @@ spec: cssom-view-1; type: dfn;
 	is used when identifying 100% progress.
 	[[CSS-POSITION-3]] [[CSS-TRANSFORMS-1]]
 
+## Calculating Progress for a View Progress Timeline ## {#view-timeline-progress}
+
+	Progress (the [=current time=]) in a [=view progress timeline=]
+	is calculated as:
+	|distance| &div; |range|
+	where:
+	* |distance| is the current [=scroll offset=]
+		minus the [=scroll offset=] corresponding to
+		the start of the ''animation-timeline-range/cover'' range
+	* |range| is
+		the [=scroll offset=] corresponding to
+		the start of the ''animation-timeline-range/cover'' range
+		minus
+		the [=scroll offset=] corresponding to
+		the end of the ''animation-timeline-range/cover'' range
+
+	If the 0% position and 100% position coincide
+	(i.e. the denominator in the [=current time=] formula is zero),
+	the timeline is [=inactive timeline|inactive=].
+
 ## Anonymous View Progress Timelines ## {#view-timelines-anonymous}
 
 ### The ''view()'' notation ### {#view-notation}