The concept behind this After Effects tutorial is using an animated Alpha Track Matte to reveal another image.
1) make a new solid (CTRL or CMD + Y)
2) draw a path on solid using the Pen tool
3) add Stroke effect to solid
4) adjust brush size
5) keyframe ‘End’ attribute
6) change Paint Style to ‘On Transparent’
7) make sure the 'Stroke' layer is on top of the footage layer you want to reveal
8) use the TrkMatte drop down menu and choose ‘Alpha Matte [Stroke]’
(if TrkMatte column is hidden hit F4 or click middle icon at lower left hand corner)
The footage layer should be revealed by the animated stroke.
For more information on Alpha Track Mattes see the 'Alpha Track Matte Tutorial'.
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