AVI Frame Rate Changer: Preserve Quality While Changing FPS

Batch AVI Frame Rate Changer for Smooth Slow‑Motion and Time‑Lapse

Changes the playback frame rate of many AVI files at once (batch processing).
Two main approaches:
- Edit AVI headers (change fps metadata) — fast, no re-encode, changes playback speed only.
- Re-encode or interpolate frames (frame‑generation) — creates additional frames for smooth slow‑motion or compresses frames for time‑lapse; preserves visual smoothness.

Header edit: use when original frames represent the motion you want but the container’s fps is wrong (quick fixes, no quality loss).
Re-encode/interpolate: use for true slow‑motion (smooth motion from high‑fps source) or for high‑quality time‑lapse (compress long recording into shorter duration).

Header edit / batch metadata:
- AVIFrate / avifratecmd (simple, free, direct header edits)
- ffmpeg (remux with changed timestamps; fast)
Re-encode / interpolation / high‑quality slow‑mo:
- VirtualDub2 + MVTools (AviSynth/VS scripts) — very flexible, good quality
- ffmpeg with minterpolate or blend filters — command-line interpolation
- Commercial: Twixtor, Optical Flow plugins (higher quality, paid)

Quick batch header change (no re-encode):
- Tool: avifratecmd or ffmpeg remux.
- Example (ffmpeg): for each file:
```
Code
ffmpeg -i input.avi -c copy -video_track_timescale  output.avi 
```
- Result: playback speed changes instantly; file quality unchanged.
Batch re-encode with interpolation for smooth slow‑motion:
- Tool: VirtualDub2 + MVTools or ffmpeg minterpolate.
- Example (ffmpeg) to double frame‑rate using motion interpolation:
```
Code
ffmpeg -i input.avi -vf minterpolate=‘mi_mode=mci:mc_mode=aobmc:vsbmc=1:fps=60’ -c:v libx264 -crf 18 output.avi 
```
- Repeat in a script to process a folder.
Batch time‑lapse (speed up):
- Tool: ffmpeg (frame dropping / setpts).
- Example (ffmpeg) 4