I am constructing complexFilters to procedurally generate audio with ffmpeg via fluent-ffmpeg. The interesting part of my fluent-ffmpeg complex filters look like:

{
   "filter": "volume",
   "options": {
       "volume": "min(1, max(0, ((cos(PI * t * 1 / 13) * 1 + cos(PI * t * 1 / 7) * 0.5 + cos(PI * t * 1 / 3) * 0.25) +
-0.5 ) * 0.75 * -1 + 0.5))",
       "eval": "frame"
    },
    "inputs": "track_1_output",
    "outputs": "track_1_adjusted"
},

Note that I am using an expression to determine the volume dynamically. To simplify my tests, I’m constructing complexFilters on the command line to understand the mysteries of some of the lightly-documented ffmpeg filters. Here’s a working CLI test of using the volume filter to fade the sources in and out according to a time-based expresssion:

# working fade in/out
ffmpeg -i knox.mp3 -i static.mp3 -filter_complex 
"[0:a] 
    volume = 'min(1, max(0, 0.5 + 0.5 * cos(PI * t / 5)))': eval=frame 
    [a0]; 
[1:a] 
    volume = 'min(1, max(0, 0.5 - 0.5 * cos(PI * t / 5)))': eval=frame 
    [a1]; 
[a0][a1]
    amix = inputs=2: duration=shortest" 
-c:a libmp3lame -q:a 2 output.mp3

Now I want to apply lowpass, highpass, aphaser, and/or aecho to the transitions using a time-based expression. For instance, looking at lowpasss, according to ffmpeg -filters it says that it has time-based support:

  T.. = Timeline support
 TSC lowpass           A->A       Apply a low-pass filter with 3dB point frequency.

How can I similarly apply a time-based expression to lowpass, highpass, aphaser, or aecho? I thought the mix option might be the key, but I couldn’t construct a working example and couldn’t find any examples.

# applying a lowpass filter
ffmpeg -i knox.mp3 -filter_complex 
"[0:a] 
    lowpass=f=3000:mix='0.5 + 0.5 * cos(PI * t / 5)';" 
-c:a libmp3lame -q:a 2 output.mp3

Or if you prefer the fluent-ffmpeg filter:

const ffmpeg = require('fluent-ffmpeg');

ffmpeg('knox.mp3')
    .complexFilter([
        {
            filter: 'lowpass',
            options: {
                f: 3000,
                mix: '0.5 + 0.5 * cos(PI * t / 5)'
            },
            inputs: '[0:a]',
        }
    ])
    .audioCodec('libmp3lame')
    .audioQuality(2)
    .save('output.mp3')
    .on('end', () => {
        console.log('Processing finished successfully');
    })
    .on('error', (err) => {
        console.error('Error: ' + err.message);
    });

With the resultant error:

[Parsed_lowpass_1 @ 0x7f9a72005c00] [Eval @ 0x7ff7bec423d8] Undefined constant or missing '(' in 't/5)'
[Parsed_lowpass_1 @ 0x7f9a72005c00] Unable to parse option value "0.5 + 0.5 * cos(PI * t / 5)"
Error applying option 'mix' to filter 'lowpass': Invalid argument

What complicated fffmpeg faerie magic is needed to make some of the filters other than volume controlled by a time-based expr?