How to Use the FFmpeg sendcmd Filter

The sendcmd filter in FFmpeg is a powerful tool that allows you to change filter parameters dynamically during video processing without having to split your video into multiple segments. This article provides a straightforward guide on how to use the sendcmd (video) and asendcmd (audio) filters, detailing their syntax, command file formatting, and practical examples for real-time parameter manipulation.

Understanding sendcmd Syntax

The sendcmd filter works by reading a list of commands and dispatching them to other filters in the filtergraph at specific timestamps. You can pass these commands either through an external text file or directly as an inline string.

The basic parameters for the filter are: * f: The path to the text file containing the commands. * c: The inline command string (used if you do not want to use an external file).

Command Format

Whether in a file or inline, commands must follow a specific syntax:

time [target_filter_name] command_name command_arguments;

• time: The timestamp when the command should trigger. You can write this in seconds (e.g., 5.0) or in HH:MM:SS.msec format (e.g., 00:01:15.500). You can also use intervals like start-end.
• target_filter_name: The name of the filter you want to control (or its instance label).
• command_name: The specific command supported by the target filter (usually reinit for changing parameters).
• command_arguments: The new key-value pairs for the filter parameters.

Example 1: Changing Text Dynamically with drawtext

A common use case for sendcmd is changing the text displayed by the drawtext filter at specific intervals.

Step 1: Create the Command File

Create a text file named commands.txt with the following content:

2.0 drawtext reinit text='First Subtitle';
5.0 drawtext reinit text='Second Subtitle';
8.0 drawtext reinit text='Final Subtitle';

Step 2: Run the FFmpeg Command

Run FFmpeg, linking the command file to the sendcmd filter and passing the commands to a labeled drawtext filter:

ffmpeg -i input.mp4 -vf "sendcmd=f=commands.txt,drawtext=fontfile=Arial.ttf:text='Initial Text':fontsize=24:fontcolor=white:x=(w-tw)/2:y=h-100" -c:a copy output.mp4

Example 2: Inline Commands (No File Required)

If you only have one or two simple commands, you can write them inline using the c parameter. This eliminates the need for an external text file.

The following command changes the brightness of a video using the eq filter at the 3-second mark:

ffmpeg -i input.mp4 -vf "sendcmd=c='3.0 eq reinit brightness=0.5',eq=brightness=0" -c:a copy output.mp4

Note: When writing inline commands, pay close attention to nested quotation marks to avoid command-line parsing errors.

Example 3: Dynamic Audio Volume Control (asendcmd)

To change audio parameters, use the audio equivalent: asendcmd. This example changes the volume of an audio stream dynamically.

Step 1: Create the Audio Command File

Create a file named audio_commands.txt:

0.0 volume reinit volume=1.0;
4.0 volume reinit volume=0.2;
8.0 volume reinit volume=1.0;

Step 2: Run the FFmpeg Command

Apply the commands to the volume filter:

ffmpeg -i input.mp4 -af "asendcmd=f=audio_commands.txt,volume=1.0" -c:v copy output.mp4

Supported Filters

Not all FFmpeg filters support dynamic parameter modification via sendcmd. To check if a filter supports commands, you can look for the Timeline support and Command support flags in the official FFmpeg documentation for that specific filter. Some of the most commonly used compatible filters include: * drawtext (for text properties) * eq (for brightness, contrast, saturation) * volume (for audio level adjustments) * hue (for hue and saturation adjustments)