How to use custom transparency in ffmpeg to overlay png images on videos？

Overlaying a PNG image with custom transparency on a video using FFmpeg is a common task, often used for adding watermarks or graphical elements to videos. Here is a step-by-step guide and command-line example to achieve this functionality.

Step 1: Prepare Materials

Ensure you have a video file and a PNG image file. The PNG image should have a transparent background so that only the desired areas are overlaid on the video.

Step 2: Use FFmpeg Commands

Use FFmpeg's overlay filter to overlay the image on the video. If your PNG image lacks built-in transparency or you want to adjust its transparency, you can use the format and colorchannelmixer filters.

Here is a command-line example. Suppose you want to overlay logo.png on the video input.mp4 and set the PNG image's transparency to 50%:

bash
ffmpeg -i input.mp4 -i logo.png -filter_complex "[1]format=rgba,colorchannelmixer=aa=0.5[logo];[0][logo]overlay=10:10" -codec:a copy output.mp4

Detailed Command Explanation

• -i input.mp4 and -i logo.png specify the input video and image files respectively.
• filter_complex is used to define a complex filter chain.
  • [1]format=rgba ensures the image is in RGBA format (including the alpha channel).
  • colorchannelmixer=aa=0.5 sets the alpha channel value, where 0.5 represents 50% transparency.
  • [logo] is the label for the processed image, used to reference it within the filter chain.
  • [0][logo]overlay=10:10 overlays the processed image on the video, with 10:10 defining the top-left position on the video.
• -codec:a copy copies the original video's audio to the output file without transcoding.
• output.mp4 is the name of the output file.

Step 3: Check Output

After running the command, verify the output.mp4 file to confirm that the image is correctly overlaid and the transparency matches your expectations.

Using this method, you can flexibly overlay any PNG image with any transparency level on a video, making it ideal for creating professional-looking video content.