by 
Watch the tutorial
Step 1: Find the right frame
Scrub through the first clip and look for a frame where the person is just starting to walk into the picture and there is empty space behind them. In the example the legs of a passer by enter from the left side of a street shot. This is the moment where your mask begins.
Step 2: Draw a mask with the Pen tool
Select the clip and open Effect Controls. Under Opacity, click the Pen tool (the tooltip says Free draw bezier). Now click around the empty space behind the person's leg in the Program Monitor to draw the mask. Make sure you connect the first and the last point to close the shape.
Step 3: Invert the mask
At first the mask shows the empty space and hides everything else. Because the mask should reveal the other video, tick Inverted in Effect Controls. Now the empty area becomes the transparent black spot and the person stays visible.
Step 4: Track the mask forward
To make the mask follow the person, click the stopwatch next to Mask Path to start keyframing it. Then click the Track Selected Mask Forward button next to it. Move forward one frame at a time and adjust the mask points so the shape still covers the empty space behind the leg. If you need more detail, add extra points by clicking on the blue outline of the mask.
Step 5: Repeat through the clip
Keep tracking forward and adjusting the mask until the person has walked through the whole frame. This takes a little patience, but the result looks much cleaner than a quick shortcut. If the mask grows beyond the visible picture, zoom out in the Program Monitor (for example to 10%) so you can still see all the blue points. On the last frame, no part of the person should be visible anymore, so the whole screen is masked out and black.
Step 6: Set the first keyframe
Now move the playhead to the first keyframe you created, step one frame back with the Left Arrow key and drag the mask completely out of the screen. That way the mask appears smoothly as the person enters, instead of popping up from nowhere.
Step 7: Reveal the second clip
Go to the last keyframe, where the black space covers the picture. Drag the second clip up to the track above (or place it under, depending on your setup) and align its start with the playhead so that it shows through the masked area. Play the sequence and the person walks by and uncovers the new scene. For a related technique, see our guide to object mask transitions in Premiere Pro.
Tips
- Shoot for it. The effect works best with a locked off camera and a person who walks close to the lens.
- Add a little feather. A small Mask Feather value softens the edge of the mask so it blends in with motion blur.
- Add a whoosh. A soft footstep or whoosh sound at the moment of the walk by makes it feel real.
FAQ
Which clips work best for a walk by transition?
Clips where something walks close to the camera, such as a person, a car or a hand, and covers a large part of the frame.
Why does my mask drift off the subject?
Automatic tracking can slip, so check the mask every frame or two and adjust the points by hand as shown in the video.
Why do I need to invert the mask?
The mask should cut out the empty space so the clip under it can show through, which is what Inverted does.
Want more ready-made transitions? Take a look at our free glitch transitions with sound.
