How to Use FFmpeg xfade with Multiple Videos

Applying transitions between multiple videos in FFmpeg can be challenging because the xfade filter only accepts two video inputs at a time. This article provides a clear, step-by-step guide on how to chain multiple xfade filters together to transition three or more videos, including how to calculate the correct offsets and transition durations for seamless video editing using the command line.

The Logic of Chaining xfade

Because xfade only transitions between two inputs, to merge three or more videos you must chain the filters sequentially. This means you transition Video 1 and Video 2 to create a temporary video stream, and then transition that temporary stream with Video 3, and so on.

To do this successfully, you must calculate the exact start time (offset) for each subsequent transition.

The Offset Calculation Formula

The formula to calculate the offset for any transition in the chain is:

\[\text{Offset}_n = \text{Offset}_{n-1} + \text{Duration of the incoming video} - \text{Transition Duration}\]

For the very first transition, the offset is simply:

\[\text{Offset}_1 = \text{Duration of Video 1} - \text{Transition Duration}\]

Step-by-Step Example with Three Videos

Let’s assume you have three videos, each exactly 10 seconds long, and you want a 1-second fade transition between them.

1. First Transition (Video 1 + Video 2):
   • Video 1 duration = 10s
   • Transition duration = 1s
   • Offset 1 = \(10 - 1 = 9\) seconds.
   • This creates a merged stream of \(10 + 10 - 1 = 19\) seconds.
2. Second Transition (Merged Stream + Video 3):
   • Merged stream duration before this transition starts = 19s
   • Incoming Video 3 duration = 10s
   • Transition duration = 1s
   • Offset 2 = \(\text{Offset 1} + \text{Video 2 duration} - \text{Transition duration}\)
   • Offset 2 = \(9 + 10 - 1 = 18\) seconds.

The FFmpeg Command

Here is the FFmpeg command to execute this three-video chain using filter_complex:

ffmpeg -i video1.mp4 -i video2.mp4 -i video3.mp4 -filter_complex \
"[0:v][1:v]xfade=transition=fade:duration=1:offset=9[v01]; \
 [v01][2:v]xfade=transition=fade:duration=1:offset=18[outv]" \
-map "[outv]" output.mp4

How the Command Works

• -i video1.mp4 -i video2.mp4 -i video3.mp4: Imports the three source video files as streams 0:v, 1:v, and 2:v.
• [0:v][1:v]xfade=...[v01]: Fades Video 1 into Video 2 at the 9-second mark for 1 second. The output of this transition is labeled as a temporary stream [v01].
• [v01][2:v]xfade=...[outv]: Takes the temporary stream [v01] and transitions it into Video 3 at the 18-second mark. The final video output is labeled [outv].
• -map "[outv]": Tells FFmpeg to write the final processed video stream to the output file.

Scaling to Four or More Videos

To add a fourth 10-second video with a 1-second transition, you would add a third xfade step. The offset would be \(18 + 10 - 1 = 27\) seconds.

ffmpeg -i video1.mp4 -i video2.mp4 -i video3.mp4 -i video4.mp4 -filter_complex \
"[0:v][1:v]xfade=transition=fade:duration=1:offset=9[v01]; \
 [v01][2:v]xfade=transition=fade:duration=1:offset=18[v02]; \
 [v02][3:v]xfade=transition=fade:duration=1:offset=27[outv]" \
-map "[outv]" output.mp4

By following this mathematical progression, you can chain an unlimited number of videos using the FFmpeg xfade filter.