How to Add Font size in subtitles in ffmpeg video filter

In FFmpeg, you can utilize the subtitle filter (subtitles) to add subtitles to a video stream. This filter enables you to specify a subtitle file and render it onto the video. You can adjust the font size and other style properties by configuring filter options.

To modify the font size of subtitles, use the force_style parameter within the subtitles filter. Within this parameter, specify the FontSize value to adjust the font size.

The following example demonstrates using FFmpeg to add and adjust subtitle font size within a video filter:

bash
ffmpeg -i input_video.mp4 -vf "subtitles=subtitles.srt:force_style='FontSize=24'" -c:a copy output_video.mp4

In this command:

• -i input_video.mp4 specifies the input video file.
• -vf defines the video filter chain.
• subtitles=subtitles.srt specifies the subtitle file.
• force_style='FontSize=24' sets the subtitle font size to 24 via the force style option.
• -c:a copy indicates that audio encoding is copied without re-encoding.
• output_video.mp4 is the output video file with subtitles added.

Please note that the subtitle file used (as shown in this example, subtitles.srt) must be in the correct SRT or ASS format. If you are using an ASS subtitle file, it typically includes style information such as font size. In such cases, you may not require the force_style option unless you intend to override the styles specified in the ASS file.

Furthermore, the FFmpeg version, compilation settings, and the subtitle library used (e.g., libass) can impact the availability and functionality of the subtitles filter. Ensure you have the latest version of FFmpeg installed and that it was compiled with subtitle-supporting options.