Using updatable flags

[avidseeker]

$ avifenc --duration:u 2 "$file" out-"$file"

Results in

Unrecognized file format for input file:
Cannot read input file:

Am I understanding this correctly?

   UPDATABLE OPTIONS
       The following options can optionally have a :u (or :update) suffix like -q:u Q, to apply only to input files appearing after the option:

       -q, --qcolor Q
              Quality for color in 0..100 where 100 is lossless.

       --qalpha Q
              Quality for alpha in 0..100 where 100 is lossless.

       --qgain-map Q
              Quality for the gain map in 0..100 where 100 is lossless.

Basically I have an encoded animated avif and I want to update (re-adjust) the duration between frames. Can avifenc do that for me or do I need to manually decode avif to frames then encode them back to avif again?

Same question for updating the quality of an avif (e.g: 100% to 50%)?

[y-guyon]

Am I understanding this correctly?

The "updatable options" mean they can be changed for each non-AVIF input frame in the same command line. It does not mean avifenc can update these properties in an existing AVIF file.

Basically I have an encoded animated avif and I want to update (re-adjust) the duration between frames.

I usually call that "muxing", meaning editing container-level metadata without decompressing and recompressing codec-level pixels. This is faster and avoids introducing recompression artifacts.
I would suggest looking at tools such as ffmpeg for that (I could not find a working example though).

Can avifenc do that for me

No. avifenc is designed as an AVIF encoding tool that decompresses non-AVIF input and compresses it as AVIF output.
AVIF being AV1 wrapped in ISOBMFF and HEIF, maybe there are more generic video editing tools that offer that feature (I am thinking about MP4Box for example).

do I need to manually decode avif to frames then encode them back to avif again?

As of today, with the provided libavif tools, yes. It can be done with the following:

avifdec in.avif --index all out.png
avifenc --timescale 1 --duration 1 out-* out.avif

Note that pixels are recompressed with loss. Use --lossless to avoid that but be aware that it could generate large file sizes.

Same question for updating the quality of an avif (e.g: 100% to 50%)?

This needs recompression anyway, so this is fine:

avifdec in.avif --index all out.png
avifenc -q 50 out-* out.avif

Note that frame durations are lost that way and must be specified, otherwise the default frame rate applies.
Unfortunately avifdec currently only supports outputting a single frame in the same file, even in y4m format.