[FFmpeg] How to speed up some segments in your video

I am making scenic trail ride videos. Those videos have some repetitive segments in them. It can be very boring to viewers. So, I wanted to speed up those segments with FFmpeg. I’ve given requirements to ChatGPT to create one Bash script for it. Since ChatGPT does not know the environments of my machine, it took much longer to try and debug those scripts generated by ChatGPT. After around two days of try and error. I could make this video with that Bash script Chat GPT created for me.

The usage of the scripts goes as below.

speedup.sh video.mp4 ranges.txt

speedup.sh is the name of the script you saved. ranges.txt will look like as below. Those ranges in the file will be in normal speed. All segments between those ranges will be speed up 10 times.

00:00:00 00:00:40
00:03:45 00:04:40
00:05:30 00:10:20
00:12:40 00:13:55
00:15:16 00:16:10

You can edit the script below to change magnitude of speed up or so. If you feel lazy, you can throw this to ChatGPT and ask to modify as you needed. I hope this helps.

#!/bin/bash

# Input parameters
INPUT_FILE="$1"
RANGES_FILE="$2"
FRAMERATE=30

if [ -z "$INPUT_FILE" ] || [ -z "$RANGES_FILE" ]; then
    echo "Usage: $0 <input_file> <ranges_file>"
    exit 1
fi

TEMP_DIR=$(mktemp -d)
if [ ! -d "$TEMP_DIR" ]; then
    echo "Failed to create temporary directory"
    exit 1
fi

OUTPUT_FILE="output.mp4"
FINAL_CONCAT_LIST="$TEMP_DIR/final_concat_list.txt"

# Empty the concat list file
> "$FINAL_CONCAT_LIST"

# Function to process a normal speed segment
process_normal_segment() {
    local start_time="$1"
    local end_time="$2"
    local segment_file="$TEMP_DIR/normal_${start_time//:/-}_${end_time//:/-}.mp4"
    
    # Generate normal speed segment with re-encoding
    ffmpeg -fflags +genpts -hwaccel auto -r "$FRAMERATE" -ss "$start_time" -to "$end_time" -i "$INPUT_FILE" -r "$FRAMERATE" -avoid_negative_ts make_zero -fps_mode cfr -c:v libx264 -preset slow -crf 18 -c:a aac -b:a 192k "$segment_file" -y
    if [ $? -ne 0 ]; then
        echo "Error processing normal segment: $start_time to $end_time"
        exit 1
    fi
    
    # Add segment to the list
    echo "file '$segment_file'" >> "$FINAL_CONCAT_LIST"
}

# Function to process sped-up segment
process_sped_up_segment() {
    local start_time="$1"
    local end_time="$2"
    local segment_file="$TEMP_DIR/sped_${start_time//:/-}_${end_time//:/-}.mp4"
    
    # Generate sped-up segment with 20x speed increase and re-encoding
    ffmpeg -fflags +genpts -hwaccel auto -r "$FRAMERATE" -ss "$start_time" -to "$end_time" -i "$INPUT_FILE" -r "$FRAMERATE" -avoid_negative_ts make_zero -fps_mode cfr -filter:v "setpts=0.05*PTS" -c:v libx264 -preset slow -crf 18 -an "$segment_file" -y
    if [ $? -ne 0 ]; then
        echo "Error processing sped-up segment: $start_time to $end_time"
        exit 1
    fi
    
    # Add sped-up segment to the list
    echo "file '$segment_file'" >> "$FINAL_CONCAT_LIST"
}

# Get the duration of the input file
input_duration=$(ffprobe -v error -show_entries format=duration -of default=noprint_wrappers=1:nokey=1 "$INPUT_FILE")
input_duration=$(echo "$input_duration" | awk '{printf "%02d:%02d:%02d\n", $1/3600, ($1%3600)/60, $1%60}')

# Read the ranges file and store in an array
ranges=()
while IFS= read -r line; do
    ranges+=("$line")
done < "$RANGES_FILE"

# Debug: Echo the ranges
echo "Ranges:"
for range in "${ranges[@]}"; do
    echo "$range"
done

# Process each range
previous_end_time="00:00:00"
for range in "${ranges[@]}"; do
    IFS=' ' read -r start_time end_time <<< "$range"
    
    # Process the segment before the current range
    if [ "$previous_end_time" != "$start_time" ]; then
        process_sped_up_segment "$previous_end_time" "$start_time"
    fi
    
    # Process the current range as normal speed
    process_normal_segment "$start_time" "$end_time"
    
    # Update the previous end time
    previous_end_time="$end_time"
done

# Process the segment after the last range
if [ "$previous_end_time" != "$input_duration" ]; then
    process_sped_up_segment "$previous_end_time" "$input_duration"
fi

# Concatenate all segments into the final output file
ffmpeg -f concat -safe 0 -i "$FINAL_CONCAT_LIST" -c copy "$OUTPUT_FILE" -y
if [ $? -ne 0 ]; then
    echo "Error concatenating segments"
    exit 1
fi

# Clean up temporary files
rm -rf "$TEMP_DIR"

echo "Processing complete. Output file: $OUTPUT_FILE"