How to scale overlay image in ffmpeg

When using FFmpeg for video editing, especially when overlaying images, a common requirement is to adjust the size of the overlay image. The following steps will help you achieve this.

1. Determine the target size for the overlay image

First, you need to determine the target dimensions for the image you wish to overlay. This typically depends on the resolution of the main video and the spatial placement you want the overlay to occupy within the video.

2. Adjust the image size using FFmpeg's scale filter

FFmpeg provides a filter named scale for resizing the image. The following is a basic command-line example demonstrating how to use the scale filter:

bash
ffmpeg -i overlay.png -vf "scale=320:240" resized_overlay.png

In this example, overlay.png is the original image to be scaled, scale=320:240 specifies the new width and height, and resized_overlay.png is the output file of the scaled image.

3. Overlay the scaled image onto the video

After scaling the image, the next step is to overlay it onto the video. This can be achieved using the overlay filter. The following is an example command demonstrating how to overlay the scaled image onto the video:

bash
ffmpeg -i video.mp4 -i resized_overlay.png -filter_complex "overlay=10:10" output.mp4

In this command:

• video.mp4 is the source video file.
• resized_overlay.png is the previously scaled image file.
• overlay=10:10 specifies the position where the image is overlaid on the video, where 10:10 indicates the coordinates of the top-left corner of the image on the video.

4. Adjustments and Optimization

Depending on your needs, you may also want to adjust other parameters, such as the transparency of the overlay image. This can be done using additional options of the overlay filter, for example:

bash
ffmpeg -i video.mp4 -i resized_overlay.png -filter_complex "overlay=10:10:enable='between(t,0,20)':alpha=0.5" output.mp4

In this command, enable='between(t,0,20)' indicates that the image is displayed only during the first 20 seconds of the video, and alpha=0.5 sets the transparency to 50%.

By following these steps, you can effectively scale and overlay images in FFmpeg to meet your video editing requirements. These techniques are very useful for creating video tutorials, advertisements, or any project that requires image enhancement.