FFmpeg中sendcmd适配drawtext，为何无法兼容scale、rotate等滤镜？

Great question! Let’s break this down clearly, like we’re troubleshooting together:

1. How sendcmd actually works #

The sendcmd filter’s whole job is to send control commands to filters that come after it at specific timestamps. But here’s the critical catch: the target filter has to explicitly support the command you’re sending. It can’t magically modify any filter’s parameters out of the box.

2. Why drawtext plays nice with sendcmd #

The drawtext filter was built to handle runtime parameter changes—it implements a reinit command that lets you reinitialize its settings (like text content, font size, or position) while the video is processing.

Your working example uses exactly this functionality:

ffmpeg -i testIN.mp4 -filter_complex "[0:v]sendcmd=f=testTXT.cmd,drawtext=fontfile=PF.ttf:text='': fontcolor=white:fontsize=150" testOUT.mp4

And your testTXT.cmd sends valid reinit commands that drawtext understands perfectly:

0 drawtext reinit 'text=111';
1 drawtext reinit 'text=222';
2 drawtext reinit 'text=333';

When sendcmd fires these commands, drawtext knows exactly how to respond by updating its text parameter on the fly.

3. Why scale/rotate don’t work by default #

Filters like scale and rotate don’t have the same built-in command interface as drawtext. Their core parameters (like w/h for scale, angle for rotate) are usually locked in when the filter initializes, and there’s no native reinit or similar command to modify them mid-processing.

If you tried sending a command like scale reinit 'w=640:h=360', the scale filter would have no idea what to do with that input—it would just ignore it entirely.

4. Workaround: Use variables with expression-supporting filters #

Luckily, many FFmpeg filters (including scale and rotate) support expression-based parameters. You can combine this with sendcmd’s setvar command to dynamically update parameters at runtime.

Example: Dynamic scaling #

First, create a scale.cmd file:

0 setvar target_w 640; setvar target_h 360;
2 setvar target_w 1280; setvar target_h 720;
4 setvar target_w 320; setvar target_h 180;

Then run FFmpeg with this filter chain:

ffmpeg -i testIN.mp4 -filter_complex "[0:v]sendcmd=f=scale.cmd,scale=w='${target_w}':h='${target_h}'" testOUT.mp4

Here, scale uses the ${target_w} and ${target_h} variables, and sendcmd updates those variables at the specified timestamps to resize the video dynamically.

Example: Dynamic rotation #

Create a rotate.cmd file:

0 setvar rot_angle 0;
1 setvar rot_angle PI/4; # 45 degrees
2 setvar rot_angle PI/2; # 90 degrees

Then run:

ffmpeg -i testIN.mp4 -filter_complex "[0:v]sendcmd=f=rotate.cmd,rotate=angle='${rot_angle}':ow='hypot(iw,ih)':oh='ow'" testOUT.mp4

The rotate filter uses the ${rot_angle} variable, and sendcmd adjusts it over time. The ow/oh parameters ensure the output frame size accommodates the rotated video without cropping.

5. What if the filter doesn’t support expressions? #

For filters that don’t accept expression parameters, you’ll need a more complex filtergraph:

• Split the video stream into multiple branches, each with a different filter configuration
• Use the select filter to switch between branches at specific timestamps
• Merge the branches back together

This is less clean than the variable approach, but it works for cases where expressions aren’t supported.

内容的提问来源于stack exchange，提问作者Makes