"drawtext" and "drawbox" using "sendcmd" manifest

The sendcmd filter in FFmpeg allows dynamic updates to video filters (e.g., drawtext and drawbox) at specific time intervals during playback or processing. Each sendcmd instruction follows the syntax:

startTime-endTime [enter|leave] COMMANDS;

• startTime-endTime: The time period in seconds during which the command applies.
• [enter|leave]: Indicates whether the command is executed when entering or leaving the specified time period.
• COMMANDS: A comma-separated list of commands, specific to the filter being controlled.

In this example, the reinit command for the drawtext filter is used to dynamically reinitialize its arguments (e.g., position and text). Each instruction ends with a ; symbol.

For detailed syntax and available commands, refer to the FFmpeg Commands Syntax Documentation.

Example sendcmd script:

0.0 [enter] drawtext reinit 'text=Person:x=0.165*W:y=0.139*H-30';
0.5 [enter] drawtext reinit 'text=Person:x=0.165*W:y=0.138*H-30';
1.0 [enter] drawtext reinit 'text=Person:x=0.166*W:y=0.136*H-30';
1.5 [enter] drawtext reinit 'text=Person:x=0.200*W:y=0.126*H-30';
2.0 [enter] drawtext reinit 'text=Person:x=0.230*W:y=0.125*H-30';
2.5 [enter] drawtext reinit 'text=Person:x=0.169*W:y=0.119*H-30';
# ...

In this script:

• The drawtext filter is updated at different timestamps with new x and y values, calculated dynamically from the detection data.
• The reinit command ensures the filter parameters are updated without restarting the entire filter.

A full implementation to generate such a script dynamically can be found on this GitHub repository.