I'm currently working on a project where i need to control the stroke end percentage based on a motion of a hand. I stroke animated the strings of a guitar in a footage where the person moves his hands over the frets.. now i want to animate in such a way that the guitar strings ( which are strokes.. ) follow the motion of the hand, [which i want to achieve through changing the value of end % of a stroke from (50% - 100%)] ? is there nyway to achieve this? ny expression?
I'm not sure about doing this through expressions via percentages but one way I could think to do this would be using the beam effect with motion tracking.
You could track the hand motion in something like mocha. Then using the beam effect link the beam ending or starting point x&y via an expression direct link to the position of the tracked hand via a null or shape layer. You wouldn't need to deal with percentages this way, it would all be based on the actual position values. The beam effect gives a stroked look similar to your image although if you need to add an extra look to it you can stack effects on top or below it.
I would think the hardest part would be getting the track down if the hand moves a lot & getting the values to line up right. But after that it would be all automatic.