To Rotoscope use the Pen Tool
G = hotkey to toggle through various pen tools (don't use any tool other than the basic pen tool - the other tools are outdated and will handicap your workflow)
While in the pen tool use these Hotkeys:
CTRL (while hovered over a mask point) = change temporarily to Selection Tool
ALT (while hovered over a mask point) = add Bezier
CTRL T = free transform selected mask points
= zoom in and out
Spacebar = hand tool to move canvas
M = expose Mask Path attribute. After closing off any mask use the hotkey M to see the Mask Path attribute. Hopefully this will remind you to set a Mask Path keyframe!
Happy Roto-ing!
Use F4 to hide or reveal the 'TrkMat' column.
When you choose 'Alpha Matte' from the drop down menu, After Effects will use the layer above as a 'matte'. This will mask off the bottom layer, using the top layer's Alpha Channel as a 'cookie cutter'.
By default, 'matte' layers become invisible, but the eyeball can be manually turned back on if necessary (usually to temporarily view the matte itself).
I personally always color code 'matte' layers RED. The layers that 'use track mattes' I color code YELLOW. This way, at a quick glance I can tell everything is correct when RED layers are on top of YELLOW ones.
Thanks to Demian Dunkley for inspiring me to make this video.
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'.