shrivel

A small cross-platform command-line tool that transcodes videos to H.265/HEVC or AV1 by orchestrating FFmpeg. HEVC is the default; select AV1 with --codec av1. Point it at a folder of videos and it shrinks them, picking a quality target per file based on the source bitrate.

Requirements

• ffmpeg and ffprobe available in your PATH.
• Rust (2024 edition) to build from source.

Installation

cargo install --path .

Or build without installing:

cargo build --release
# binary: target/release/shrivel

Usage

# Recursively convert every .mp4 in ./input into ./output
shrivel

# Custom folders and quality
shrivel --input ~/Videos/raw --output ~/Videos/small --cq 28

# Convert one file into the output directory
shrivel --input ~/Videos/clip.mp4 --output ~/Videos/converted

# Resample to 30 fps, denoise, and skip files already using the selected codec
shrivel --fps 30 --denoise --skip-same-codec

# Scale to 1280 pixels width, resample, and denoise
shrivel --scale 1280 --fps 30 --denoise

# Process several container types, forcing the CPU encoder
shrivel --ext mp4,mkv,mov --encoder software

# Encode to AV1 (auto-select hardware when usable, otherwise SVT-AV1 on CPU)
shrivel --codec av1

# Preview the FFmpeg commands without running them
shrivel --dry-run

When the input is a directory, it is searched recursively. Converted files keep their paths relative to the input folder, and the corresponding subdirectories are created inside the output folder. For example, input/trips/day1.mp4 is written to output/trips/day1.mp4. When the input is a single video file, it is written directly inside the output directory with the same filename.

Options

The process exits with a non-zero status if at least one file failed to convert.

How it works

For each input file shrivel:

1. Runs ffprobe to read the video codec, bitrate and frame rate. If the video stream has no bitrate (common in Matroska files), the container bitrate is used instead.

2. Optionally skips the file when it already uses the selected codec (--skip-same-codec).

3. Adapts the quality value to the source bitrate. Heavily compressed sources gain little from a strict target, so the value is relaxed to avoid inflating the output:

   Source bitrate	Effective CQ
   unknown	base
   below 3000 kbit/s	base + 4
   3000 to 5999 kbit/s	base + 2
   6000 kbit/s or more	base

   The result is capped at 51.

4. Builds the video filter chain (denoise, then frame-rate change). A frame-rate change is applied only when the source differs from the target by more than 0.5 fps, and it forces constant-frame-rate output to avoid leftover variable-frame-rate drift.

5. Runs FFmpeg, mapping the first video stream and all audio streams. Audio is copied without re-encoding. Other streams (subtitles, data) are dropped.

6. Reports the input and output sizes. If FFmpeg fails, the partial output file is removed and the first lines of FFmpeg's error output are shown.

HEVC files in .mp4, .m4v and .mov outputs are tagged hvc1 so they play on Apple devices. AV1 outputs do not receive this HEVC-specific tag.

Encoders

With --encoder auto (the default), shrivel tries each hardware encoder supported by the selected codec in turn by encoding one tiny synthetic frame, and uses the first one that works. If none works it uses software x265 for HEVC or SVT-AV1 for AV1. An encoder being listed by ffmpeg -encoders is not enough, because hardware encoders are often compiled in but unusable on the current machine.

Quality values are not directly comparable between codecs or encoders. --cq is a shared starting point (0-51); check a short clip when switching backend or codec. SVT-AV1 uses preset 6 by default. Hardware AV1 encoding is available only when both FFmpeg and the system hardware/runtime support that encoder.