Hi,
This is possibly only answerable by someone on the Adobe Dev team, but here's my puzzle:
I have been working a lot with the Paint Effect lately and have been trying to understand exactly how the brush strokes are drawn on when the End property is animated from 0 to 100%. (as described in the Adobe help pages: http://help.adobe.com/en_US/aftereffects/cs/using/WS3878526689cb916558 66c1103a9d3c597-7c84a.html#WS3878526689cb91655866c1103a9d3c597-7c66a)
If I copy the path from a Brush Stroke to the position of a null (a default durations of 2 seconds) and drag the keyframes out to the same duration as the 0-100% End Property keyframes, I am expecting the null to move in relation to the end of the Stroke - appearing to draw it on. However, the temporal interpolation of the Stroke being drawn on appears to be non-linear but I can't figure out how a the percentage is determined if not by path length. The null position and the End of the stroke appear to be moving at two different paces.
By shift-clicking it is possible to draw paths with straight lines and with vertices further apart than the sampling rate places them when drawing with a stylus. I figured that a path with only a few vertices would be simpler to analyse. I assumed that the stroke length was known (by measuring and adding the length of each segment between vertices), and that the percentage (on the End property) was directly related to distance along the path. However with longer straight segments the "write-on" interpolation becomes even more perplexing. If a Stroke is selected in the Layer viewer, you can see the path being drawn on and the resulting Paint stroke revealed. If I copy this path to a position property, not only do the path End and position vary in position, but the revealed Paint stroke doesn't necessarily match the end of the path either!
I'm at a bit of a loss and was hoping that there was an explanation (perhaps a definitive temporal/spatial relationship I'm missing). If anyone has any ideas on how to do some data modelling on the behaviour of the Stroke End property percentage I'm open to suggestions. I am looking to script a solution and even tried setting positions vertex-by-vertex with keyframe time based on the percentage along the length of the path ... but I get identical keyframes to copying and pasting the path to a Position property.
Thanks for reading,
Laurence.