[Iñaki San miguel]

Finally someone else sent me this tip. It is about masking the drawing with another nonstop path.
I think I will do it like this. Anyway it is fantastic to have people like you online and I bet also in live.
Thank you very much and I will have a beer in your honour, today! Big hug!

Nathan Dean Re: how to make whiteboard “scribe” videos
by Nathan Dean on Mar 5, 2013 at 12:40:46 am
Last Edited By Nathan Dean on Sep 2, 2014 at 6:15:28 pm

I was able to produce this video using a graphic I drew in illustrator and the “Stroke” effect. Essentially this process requires three steps: prepping art in illustrator, revealing the art in AE via stroke effect, and mapping a hand to draw it using a motion path. This does require some advanced knowledge of keyframes and a basic understanding of expressions.

REVEAL PATH
Essential to making this technique work…
In illustrator create an additional “reveal path” for each object you want animated separately: One continuous line that covers the object… drawn in the order I want it revealed. The reveal path serves two purposes…
––A layer mask that drives the stroke effect to reveal art layer
––the hand layer’s motion path

Reveal paths MUST BE CONTINUOUS, but don’t always have to be exact. Depending on your brush size in the stroke effect you don’t always need to draw over every single detail either. Sometimes scribbling over it is good enough.

REVEAL ART LAYER
import your saved artwork into AE
NEVER SCALE THE ART LAYERS IN AFTER EFFECTS (see step “Align the Hand”)
switch back to illustrator, select and copy the reveal path of that object
switch to AE, paste copied reveal path on art layer to create a layer mask
add effect Generate > Stroke, set path options to “All masks” and check “Stroke Sequentially”
under effect options set Paint Style to “Reveal Original Image”

MAP HAND MOVEMENTS
add a picture of a hand holding a marker to your main comp and then precomp it
set anchor point to where the marker tip is
click the position stopwatch and paste previously copied reveal path to create a motion path

 
SYNC KEYFRAME TIMING
The hand position has all roving keyframes except the beginning and end. Select the last keyframe and adjust to line up with the art’s reveal keyframes (@ 3:23 in screencast)

ALIGN THE HAND
The art layer MUST BE 100% scale, otherwise the motion path won’t match the art layer’s mask. After creating the motion path in the hand precomp, you’ll need to reposition the group of keyframes so it aligns with the art layer’s mask. Select the keyframes you just pasted, place your playhead on the first or last keyframe, drag the hand layer into place (3:28). This should reposition the group of keyframes instead of just one.

To add realism I randomized the rotation of the arm by 1-4 degrees. In the precomp I added puppet-warp on the hand layer. Each frame had a slight variation of the warp but kept the markertip in the same position. Then in the main comp I used the time-remapping effect with an expression to choose a random frame from the hand precomp.

For faster sequences you can get away with faking this using manual keyframes. But for detail drawings and separate sequential elements this is the way to go.

[Iñaki San miguel]

Thank you very much for the answer. But, I have no clue about linking the null with the trim path.

I also tried with this but no way. I get lost: https://www.reddit.com/r/AfterEffects/comments/1inivq/

If you have any more news… let me know.

Thank you again!

Is there any way to link a nul object to a trimming path?
byu/d3rfu5 inAfterEffects