This is one of those projects that can be accomplished in multiple ways, but here’s one method you could use.
What you need is a graphics with multiple layers, chalk layer(top layer) and chalkboard/pictures for the background(bottom layer). So first set up your work in Photoshop and save as layered .psd file.
Import your graphic into AE as a composition(that option is in the bottom left corner of the import dialogue) and open up your composition.
On the top layer(chalk line) add the effect Generate -> Stroke. Now use the pen tool(“G”) to create a path for the stroke effect to follow. Start from where you want it to start revealing to the end. In the effect window, for paths choose “Mask 1″(assuming you haven’t created any other masks in this composition. For Paint Style choose “Reveal Original Image”.
Move the Current Time Indicator to where you want your animation to finish and press the Stroke “End” stop watch to set a key frame. Now move to where you want the animation to begin and change the percentage to 0%. This will set another key frame and if you scrub the time line you should see your chalk line animate on.
At this point you may need to thicken the brush size. If the stroke line is a little too smooth you could also add a “Stylize -> Roughen Edges” to it, that might make it look a little less smooth, but if the animation is fast enough, nobody is going to notice that.
Johnny Cuevas, Editor
Thinkck.com
“I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work.”
—THOMAS EDISON on inventing the light bulb.