Substituting a new face onto someone else's body in a video can be fast and easy or extremely time-intensive, depending on the tools you have and how realistic you need the effect to be. For best results, you should use two videos and a video editor that has motion and rotation tracking abilities. For less professional but perhaps funnier results, you can use a few still photos and paste these onto the video.
Software for Near-Professional Results
While some software programs with motion and rotation tracking abilities can cost thousands of dollars, you can achieve a professional effect using Adobe After Effects and a plugin like Mocha or Camera Tracker. Adobe After Effects is available as a free 30-day trial and comes with the Mocha plugin included. You should then study the face in the original video and record a second video of yourself or a friend matching the same movements and the same times as closely as possible. You can then paste the new face over the old one.
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Using Motion Tracking and Masks
In order to make the replaced face on your video look realistic, create a mask on the face of the original video, centered around the eyes and nose but not the mouth. When the person speaks, the movement of the mouth can throw off the tracking software. Once you have this mask in place, create a second mask on the replacement face and insert it over the original video. Make the second mask semi-opaque and then position it and resize it as needed. The motion tracking and image rotation options in the software will move the second face to keep it aligned with the mask in the original video.
Lighting, Color and Realism
To achieve a realistic effect, the lighting and color of the replacement face should be as close as possible to the face in the original video. The replacement face should also be the same size as the face in the original video or larger. If it's smaller, you can scale the replacement face, but too much scaling can result in blurred details or pixelation. When creating the mask for the replacement face, trace carefully around the hairline and jaw. if the person turns his head, it's best to create a new mask and trace it over again rather then letting the mask cut across the cheeks or cut details from the background that would obscure the original video.
Fun, Less Professional Alternatives
Using a few well-chosen photos, you can substitute a face on a video using a variety of inexpensive apps on your computer or smartphone. You can also achieve the same effect if you have a video editor that allows you to paste still images onto a video. Just use an image editing tool like Photoshop, GIMP or Paint.Net to cut a face from a photo and then insert it into the video. Make sure you adjust the photo's position or use a photo taken at a different angle for frames where the person in the video is moving his head. Since there are usually 30 frames in each second of video, this process may take some time to finish even for a short five-minute video.