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VideoTools/docs/Conversion_Settings.md

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Conversion Settings

This file documents the default FFmpeg parameters used in video-tools.sh and explains why they are chosen.

Output Format

All conversions produce:

Format: MP4
Container: MPEG-4 Part 14
Video Codec: H.264 (libx264)
Audio Codec: AAC

MP4 was chosen because:

  • It’s universally supported across modern devices.
  • It balances size and compatibility well.
  • H.264 encoding provides excellent quality at smaller bitrates.

Default Parameters

Parameter Value Purpose
-c:v libx264 H.264 video encoding Modern, efficient codec with wide hardware support
-crf 18 Constant Rate Factor Maintains near-lossless visual quality; lower = better
-preset slow Encoding preset Improves compression efficiency; trades some speed
-c:a aac Audio codec Standard high-quality stereo audio
-b:a 192k Audio bitrate Balances fidelity and file size
-movflags +faststart MP4 optimization Enables faster playback start in players or web streams
-fflags +genpts Timestamp repair Regenerates timestamps on older AVI/MPEG inputs

Quality vs. File Size

The CRF scale (used by FFmpeg) defines quality and compression balance:

CRF Description Typical Use
14–16 Near lossless Archival or professional mastering
18–20 High quality Everyday use, visually lossless
21–24 Medium quality Small file sizes, light compression
25+ Low quality Fast compression, heavy size reduction

The default of CRF 18 keeps visual clarity virtually identical to the original while cutting most AVI or MPG files to 40–60% smaller sizes.

Why Preset "slow"

The preset defines encoding effort vs. compression efficiency.
Options range from ultrafast → veryslow.

  • slow provides strong compression without extreme CPU time.
  • Encoding speed is roughly 2–3× real-time on a midrange CPU.
  • File sizes are typically 10–20% smaller than medium preset at the same quality.

Audio Strategy

AAC at 192k is chosen for:

  • Wide playback compatibility (phones, TVs, players).
  • Transparent stereo sound for most sources.
  • Reasonable storage size (~2MB/minute of stereo audio).

If future needs arise, the tool can later support:

  • -c:a copy for untouched audio streams.
  • -b:a 320k for high-fidelity preservation.

Color Space & Scaling

No scaling is applied by default.
The video retains original resolution and color profile.

Upscaling and filtering will be handled in a separate command (upscale-video) in future versions, with support for:

  • FFmpeg’s scale and zscale filters.
  • lanczos resampling (for sharp, clean upscale).
  • Optional integration with ML-based models (Real-ESRGAN, waifu2x).

Future Additions

Planned Setting Description
upscale-mode Default scaling filter for upscaling tasks
hevc-mode Switch to H.265 (libx265) for smaller files
audio-pass Option to skip audio re-encoding
config.json User-editable file to override default values

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