Replace Color in Video with FFmpeg Chromakey

This article explains how to replace a specific color in a video with another color using FFmpeg. By combining the chromakey filter with a synthetic background generated by the color filter, you can isolate an existing color, make it transparent, and layer the video over a solid replacement color using the overlay filter.

The FFmpeg Command

FFmpeg's chromakey filter removes a specified color by converting it into an alpha channel (transparency). To replace that color with a new one, you must generate a solid color background and overlay the keyed video on top of it.

Run the following command in your terminal:

ffmpeg -i input.mp4 -filter_complex \
"[0:v]chromakey=color=0x00FF00:similarity=0.3:blend=0.1[keyed]; \
 color=c=0xFF0000[bg]; \
 [bg][keyed]scale2ref[bg][keyed]; \
 [bg][keyed]overlay=shortest=1[v]" \
-map "[v]" -map 0:a? -c:a copy output.mp4

How the Filtergraph Works

  1. [0:v]chromakey=color=0x00FF00:similarity=0.3:blend=0.1[keyed]
    • color: The hex code of the color you want to remove (in this example, pure green 0x00FF00).
    • similarity: A value between 0.01 and 1.0 that controls the tolerance. Higher values key out a wider range of similar shades.
    • blend: A value between 0.0 and 1.0 that softens the edges of the keyed area to prevent harsh outlines.
  2. color=c=0xFF0000[bg]
    • Generates a solid background canvas using the replacement color (in this example, pure red 0xFF0000). You can specify hex codes (0xRRGGBB) or standard color names like blue, black, or yellow.
  3. [bg][keyed]scale2ref[bg][keyed]
    • Dynamically scales the solid color background [bg] to match the exact dimensions of the input video [keyed] without needing to manually specify resolutions.
  4. [bg][keyed]overlay=shortest=1[v]
    • Places the keyed video over the solid background. The transparent sections reveal the new color underneath. shortest=1 ensures the output ends when the main video ends.
  5. -map "[v]" -map 0:a? -c:a copy
    • Maps the processed video stream [v] and copies the existing audio stream directly without re-encoding.

Fine-Tuning the Results

  • Spill and Artifacts: If parts of the subject are disappearing, lower the similarity parameter (e.g., to 0.2 or 0.15). If remnants of the old color remain visible, raise the similarity parameter slightly.
  • Rough Edges: Increase the blend parameter (e.g., from 0.1 to 0.2) to create smoother transitions along the borders of keyed objects.