Ffmpeg Speed Time

Ffmpeg Speed Time

Context: $ARGUMENTS

Quick start

• Constant 2x speed (V+A): → Step 1 → speed mode with factor 2.0
• Smooth 0.25x slow-motion (motion-interpolated): → Step 1 → speed --smooth minterpolate --fps 120
• Reverse entire clip: → Step 1 → reverse mode
• Freeze last frame for 3s: → Step 1 → freeze --end 3
• Loop clip 5x: → Step 1 → loop --count 5
• Timelapse (keep every Nth frame): → Step 1 → timelapse --every 10
• Speed ramp 1x → 0.25x: → Step 1 → ramp (splits + concats)

When to use

• Slow a clip to 0.5x / 0.25x with either simple PTS scaling or smooth motion interpolation.
• Speed up a lecture/recording 2x–4x without pitching the voice up.
• Play a clip in reverse (visually and/or audio).
• Hold on the first or last frame for N seconds (title cards, outros).
• Ramp speed mid-clip (cinematic slow-down into a highlight).
• Loop a short clip, build a time-lapse, or freeze a mid-clip frame.

Step 1 — Pick an operation

Decision shortcuts:

• Pitch-preserving speed change → always pair setpts with atempo (never asetrate).
• Pitch-shift for effect (chipmunk / deep voice) → asetrate=48000*K,aresample=48000 (changes speed AND pitch).
• Slow-mo below 0.5x and smooth motion matters → minterpolate (slow render, but no juddery frame repeats).
• Slow-mo below 0.5x and render speed matters → plain setpts (frames simply held longer).
• Reverse > ~60s clip → split into chunks, reverse each, concat in reverse order (otherwise RAM explodes).

Step 2 — Build the V+A filter graph

Speed change must apply to BOTH streams or video and audio drift. Core identity:

video: setpts = K * PTS    (K < 1 → faster, K > 1 → slower)
audio: atempo = 1 / K      (must be inverse of K)

atempo per-instance range is 0.5 – 100.0. For factors outside that range, chain:

0.25x  (K=4)   →  atempo=0.5,atempo=0.5
0.1x   (K=10)  →  atempo=0.5,atempo=0.5,atempo=0.4
4x     (K=0.25)→  atempo=2.0,atempo=2.0
8x     (K=0.125)→ atempo=2.0,atempo=2.0,atempo=2.0

2a. Constant speed (2x, pitch preserved)

ffmpeg -i in.mp4 \
  -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]" \
  -map "[v]" -map "[a]" out.mp4

2b. Constant speed (0.5x, pitch preserved)

ffmpeg -i in.mp4 \
  -filter_complex "[0:v]setpts=2.0*PTS[v];[0:a]atempo=0.5[a]" \
  -map "[v]" -map "[a]" out.mp4

2c. Smooth slow-motion 0.25x at 120 fps output

ffmpeg -i in.mp4 -filter_complex \
  "[0:v]minterpolate='mi_mode=mci:mc_mode=aobmc:me_mode=bidir:vsbmc=1:fps=120',setpts=4.0*PTS[v];\
   [0:a]atempo=0.5,atempo=0.5[a]" \
  -map "[v]" -map "[a]" out.mp4

minterpolate fps= is the OUTPUT fps the filter generates synthesized frames at — independent of the setpts time-scaling that follows. Apply setpts AFTER minterpolate.

2d. Reverse (short clips only — buffers whole stream in RAM)

ffmpeg -i in.mp4 -vf reverse -af areverse out.mp4

2e. Freeze LAST frame 3s (V+A)

ffmpeg -i in.mp4 \
  -vf "tpad=stop_mode=clone:stop_duration=3" \
  -af "apad=pad_dur=3" \
  -shortest out.mp4

2f. Freeze FIRST frame 2s

ffmpeg -i in.mp4 \
  -vf "tpad=start_mode=clone:start_duration=2" \
  -af "adelay=2s:all=1" \
  out.mp4

2g. Mid-clip freeze (hold frame at t=5s for 2s, source is 30 fps)

Frame index = 5s * 30fps = 150. Hold 2s = 2 * 30 = 60 extra frames:

ffmpeg -i in.mp4 -vf "loop=loop=60:size=1:start=150" -c:a copy out.mp4

2h. Loop entire clip 5x

ffmpeg -stream_loop 4 -i in.mp4 -c copy out.mp4

-stream_loop 4 = play input 5 times total (0-based). Faster than loop filter when stream-copy works. Otherwise:

ffmpeg -i in.mp4 -filter_complex "[0:v]loop=loop=4:size=9999:start=0[v];[0:a]aloop=loop=4:size=2e9:start=0[a]" \
  -map "[v]" -map "[a]" out.mp4

2i. Timelapse — keep every 10th frame, output 30 fps

ffmpeg -i in.mp4 -vf "setpts=PTS/10,fps=30" -an out.mp4

or explicit frame selection:

ffmpeg -i in.mp4 -vf "select='not(mod(n,10))',setpts=N/FRAME_RATE/TB" -an out.mp4

2j. Pitch-shifted speed (chipmunk effect)

ffmpeg -i in.mp4 \
  -filter_complex "[0:v]setpts=(2/3)*PTS[v];[0:a]asetrate=48000*1.5,aresample=48000[a]" \
  -map "[v]" -map "[a]" out.mp4

asetrate multiplies both playback rate and pitch (like tape speedup). Follow with aresample to restore the container's expected sample rate.

Step 3 — Run and verify duration

Expected output duration:

new_duration = source_duration * K            (K from setpts=K*PTS)

Verify:

ffprobe -v error -show_entries format=duration -of default=nw=1:nk=1 out.mp4

For speed 2x on a 60s source you should see ~30. For 0.5x, ~120. If the number disagrees, you probably forgot to match setpts with atempo or one of the audio chains exceeded per-instance range.

Available scripts

• scripts/time.py — subcommand-based wrapper around the recipes above. Stdlib only. --dry-run prints the exact ffmpeg command.

# 2x speed
uv run ${CLAUDE_SKILL_DIR}/scripts/time.py speed --input in.mp4 --output out.mp4 --factor 2.0

# 0.25x smooth slow-mo at 120 fps
uv run ${CLAUDE_SKILL_DIR}/scripts/time.py speed --input in.mp4 --output out.mp4 --factor 0.25 --smooth minterpolate --fps 120

# Reverse
uv run ${CLAUDE_SKILL_DIR}/scripts/time.py reverse --input in.mp4 --output out.mp4

# Freeze last 3s
uv run ${CLAUDE_SKILL_DIR}/scripts/time.py freeze --input in.mp4 --output out.mp4 --end 3

# Freeze mid-clip at t=5s for 2s
uv run ${CLAUDE_SKILL_DIR}/scripts/time.py freeze --input in.mp4 --output out.mp4 --at 5 --duration 2

# Loop 5x
uv run ${CLAUDE_SKILL_DIR}/scripts/time.py loop --input in.mp4 --output out.mp4 --count 5

# Timelapse
uv run ${CLAUDE_SKILL_DIR}/scripts/time.py timelapse --input in.mp4 --output out.mp4 --every 10 --fps 30

# Ramp 1.0x → 0.25x starting at t=10s over 2s
uv run ${CLAUDE_SKILL_DIR}/scripts/time.py ramp --input in.mp4 --output out.mp4 --from 1.0 --to 0.25 --start-time 10 --duration 2

Reference docs

• Read references/filters.md for full option tables (setpts expressions, minterpolate modes, atempo vs asetrate, recipe gallery).

Gotchas

• setpts and atempo are independent. Change video PTS without changing audio tempo and the two streams drift — 0.5x video over a 60s clip + untouched audio = 60s of video playing at half speed with only 60s of audio, so audio ends at the 50% mark (or the mux silently truncates one stream).
• atempo range is 0.5–100.0 per instance. For 0.25x chain atempo=0.5,atempo=0.5. For 4x chain atempo=2.0,atempo=2.0.
• setpts scales ALL timestamps uniformly. It cannot express "fast then slow" on its own. For variable speed, split the source into segments, speed-change each, then concat.
• reverse and areverse buffer the entire stream in RAM. A 10-minute 4K clip will OOM. Split first, reverse each chunk, concat chunks in reverse order.
• minterpolate is expensive. 1 fps render throughput is common at 1080p. Use on short clips (<30s) or pre-trim.
• minterpolate fps=X is the OUTPUT fps of the synthesis stage and is independent of the subsequent setpts time scaling. Setting fps=120 does not itself slow the video — setpts=4.0*PTS after it does.
• tpad can only pad, never trim. To freeze the FIRST frame, tpad=start_mode=clone:start_duration=N on video PLUS matching audio delay (adelay=Ns:all=1) or silence pad (apad=pad_dur=N combined with a pre-rolled silent source).
• Freezing the LAST frame needs matching audio pad. Use apad=pad_dur=N alongside the video tpad. Without it, audio ends early and -shortest truncates the frozen video back.
• loop filter is VIDEO only. Use aloop for audio, or prefer -stream_loop N at the input level when stream copy is viable.
• select + setpts=N/FRAME_RATE/TB is the portable idiom for dropping frames. The N/FRAME_RATE/TB rewrites the N-th surviving frame's PTS to the correct integer tick; without it, output timestamps will be garbage.
• asetpts=N/SR/TB is the audio equivalent (SR = sample rate), used after aselect.
• asetrate changes pitch AND speed. Use atempo for pitch-preserving speed. Use asetrate only when you WANT the pitch shift (chipmunk, demonic slowdown).
• Dropping frames via fps=10 is lossy — you throw away the intervening frames entirely. For smooth slow-down from existing frames, use minterpolate instead.
• Pixel format after minterpolate may need forcing. Append ,format=yuv420p before encoding for broad MP4 compatibility.
• loop=size= is a MAX frame count to buffer for the loop. Set it to a value larger than your clip's frame count (9999 for short clips; use 2e9 cap on aloop since it counts samples).
• Output duration can drift with variable-frame-rate sources. Force -vsync cfr or a fps= filter early if downstream tools require constant frame rate.

Examples

Example 1: 2x lecture speed with natural-sounding audio

ffmpeg -i lecture.mp4 \
  -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]" \
  -map "[v]" -map "[a]" -c:v libx264 -crf 20 -c:a aac -b:a 128k \
  lecture_2x.mp4

Duration of a 60-minute lecture → ~30 minutes. Audio remains at original pitch because atempo preserves it.

Example 2: Cinematic 0.25x slow-mo of a 5-second action shot

ffmpeg -i hit.mp4 -filter_complex \
  "[0:v]minterpolate='mi_mode=mci:mc_mode=aobmc:me_mode=bidir:vsbmc=1:fps=120',setpts=4.0*PTS,format=yuv420p[v];\
   [0:a]atempo=0.5,atempo=0.5[a]" \
  -map "[v]" -map "[a]" -c:v libx264 -crf 18 -preset slow hit_slowmo.mp4

5s input → 20s output at 120 fps with synthesized intermediate frames.

Example 3: Freeze last frame 3s for an outro

ffmpeg -i clip.mp4 \
  -vf "tpad=stop_mode=clone:stop_duration=3" \
  -af "apad=pad_dur=3" \
  -shortest -c:v libx264 -crf 20 -c:a aac \
  clip_outro.mp4

Example 4: Speed ramp 1x → 0.25x

# Split at t=10 and t=12, speed-change middle chunk, concat
ffmpeg -i in.mp4 -t 10 -c copy seg1.mp4
ffmpeg -i in.mp4 -ss 10 -to 12 -filter_complex \
  "[0:v]setpts=4.0*PTS[v];[0:a]atempo=0.5,atempo=0.5[a]" \
  -map "[v]" -map "[a]" seg2.mp4
ffmpeg -i in.mp4 -ss 12 -c copy seg3.mp4
printf "file 'seg1.mp4'\nfile 'seg2.mp4'\nfile 'seg3.mp4'\n" > list.txt
ffmpeg -f concat -safe 0 -i list.txt -c copy ramped.mp4

(Concat-copy requires same codec params in all three; re-encode the concat step if needed. scripts/time.py ramp automates this.)

Example 5: Timelapse, 1 frame every 10 → 30 fps output

ffmpeg -i timelapse_src.mp4 -vf "setpts=PTS/10,fps=30" -an timelapse.mp4

Troubleshooting

Error: Value X for parameter 'tempo' out of range [0.5 - 100]

Cause: atempo instance fed a factor outside its single-stage range. Solution: Chain stages: atempo=0.5,atempo=0.5 for 0.25x; atempo=2.0,atempo=2.0 for 4x.

Output has audio but no video after setpts change

Cause: You forgot to -map "[v]" — ffmpeg picked a default video stream from the input, not the filtered one. Solution: Explicit -map "[v]" -map "[a]" when using -filter_complex with named outputs.

minterpolate runs at 0.5 fps and never finishes

Cause: Expected — minterpolate is CPU-heavy at high resolutions. Solution: Downscale first (scale=1280:-2) or trim the clip before interpolating. No GPU path for minterpolate; only CPU.

Reverse command killed / OOM

Cause: reverse/areverse buffer the entire stream in RAM (uncompressed frames). Solution: Split into 5–10 second segments, reverse each, concat in reverse order of segments.

Freeze-frame on last frame produces a frozen video with early-cutting audio

Cause: tpad extends video but -shortest or missing apad caps duration at the audio end. Solution: Add -af "apad=pad_dur=N" matching the stop_duration, and keep -shortest (or drop it).

Audio and video drift after speed change

Cause: Only one of setpts/atempo was applied, or atempo factor is not 1/K. Solution: Verify: setpts=K*PTS pairs with atempo=1/K. Check ffprobe duration of output vs source_duration * K.

Loop filter output is only one iteration

Cause: loop=size= was smaller than the clip's frame count, so only the first size frames got looped. Solution: Set size greater than total frame count (9999 for short clips), or prefer -stream_loop N at the input level.