Tracking masks to moving subjects in Video by Zaweba
Track a mask forward or back in Video by Zaweba so it follows the picture, add rotation and scale, or pin it in perspective to a sign or screen.
Masking a still subject is easy. A mask on something that moves, like a person walking, a car passing or a phone in someone's hand, needs to move too, and keying its position by hand on every frame is tiresome work. Video by Zaweba tracks masks for you, following what is inside the mask and keying the mask as it goes.
Basic tracking
Each mask has Track back and Track forward. Draw a mask round the thing you want to follow, on a frame where it is clear. Press Track forward and the app follows the picture inside the mask, frame by frame, keying the mask's position on every frame. Track back does the same going backwards from the current frame.
Starting in the middle of a shot and tracking both ways is often the best approach, because you can begin on the clearest frame and track out towards the harder parts.
Following rotation and scale
A later mode follows two detailed spots instead of one area. From the way the two spots move relative to each other, the app works out rotation and scale as well as position, and keys all three. This suits subjects that turn or move closer and further away, like a face turning, or a product on a turntable.
Pick spots with plenty of detail, such as the corner of an eye or a printed mark, rather than flat areas. Detail gives the tracker something clear to follow.
Following in perspective
"Follow in perspective" tracks four spots and pins the mask's corners to them. As the surface tilts away, the mask's shape changes with it, so it stays on a sign or a screen even as the camera moves round it.
This is the mode for flat surfaces seen at an angle: a sign on a building, a laptop screen, a book cover, a poster. The four corners are followed separately, so the mask keeps the right perspective as the angle changes.
Jobs tracking helps with
Blurring a moving face or number plate. Mask the area on a blurred copy and track it. Add feather so the edge is soft.
Highlighting a person. Track a mask round someone in a crowd and brighten the inside or darken the outside to draw the eye.
Replacing a screen or sign. Use perspective tracking to keep a mask on a screen as the camera moves, then use the mask to place new content there, perhaps with a track matte.
Colour work on a moving area. A graded copy masked to a moving subject, with the mask tracked, lets you correct one part of the shot without affecting the rest.
Getting good tracks
Start on a clear frame. Track in short stretches on difficult shots, checking as you go, rather than running a whole minute in one go.
Give the tracker detail. Smooth, plain areas or areas with repeating patterns are hard to follow. Spots with clear edges and texture work best.
Watch for things passing in front. When a hand or another person crosses the tracked area, the tracker can jump. Stop before the problem, then continue from the other side, or fix the few frames by hand.
Check the result by playing it through. A track that looks fine on each frame can still shake slightly in motion.
Since tracking makes ordinary keys, you can fix a bad stretch in the usual ways. Delete the keys on frames where it went wrong and track again from a good frame, or adjust a few keys by hand. The keyframe graph helps smooth out small jitters.
Tracking with the other mask tools
Tracking is what makes masks practical on real footage. Combined with feathering, combining modes, colour masks and SVG masks, it lets Video by Zaweba handle jobs like blurring, highlighting and screen replacement that would otherwise need a separate effects program.
