Resuming ffmpeg HLS Transcoding from an Existing Segment: A Step-by-Step Guide

Are you tired of wasting hours re-transcoding your entire video library every time you want to make a change to your HLS (HTTP Live Streaming) setup? Do you wish there was a way to pick up where you left off and resume transcoding from an existing segment? Well, wish no more! In this article, we’ll take you by the hand and walk you through the process of resuming ffmpeg HLS transcoding from an existing segment.

What is ffmpeg HLS Transcoding?

Before we dive into the meat of the article, let’s take a quick refresher on what ffmpeg HLS transcoding is. ffmpeg is a powerful, open-source command-line tool that can be used for a wide range of video and audio processing tasks. One of its most popular use cases is transcoding video files into different formats, including HLS.

HLS is a streaming protocol developed by Apple that allows for adaptive bitrate streaming, making it ideal for delivering high-quality video content to a wide range of devices. When you transcode a video file using ffmpeg and HLS, you’re essentially converting it into a format that can be played back on devices that support HLS, such as iPhones, iPads, and Apple TVs.

Why Resume ffmpeg HLS Transcoding from an Existing Segment?

There are several reasons why you might want to resume ffmpeg HLS transcoding from an existing segment:

• Time-saving: Transcoding a large video library can take hours, if not days. By resuming from an existing segment, you can save time and avoid re-transcoding the entire library.

• Efficiency: Resuming from an existing segment reduces the computational resources required for transcoding, making it a more efficient process.

• Flexibility: With the ability to resume from an existing segment, you can make changes to your HLS setup and update individual segments without affecting the entire library.

Prerequisites

Before we begin, make sure you have the following:

• ffmpeg installed on your system (latest version recommended)

• A video file or library that you want to transcode using HLS

• A basic understanding of ffmpeg commands and options

Step 1: Prepare Your ffmpeg Command

The first step is to prepare your ffmpeg command for transcoding. This will include specifying the input file, output format, and other options specific to your HLS setup.

ffmpeg -i input.mp4 -c:v libx264 -c:a aac -hls_list_size 0 -hls_segment_filename 'output_%03d.ts' output.m3u8

In this example, we’re specifying:

• -i input.mp4: Input file (replace with your own file name)

• -c:v libx264: Video codec (H.264 in this case)

• -c:a aac: Audio codec (AAC in this case)

• -hls_list_size 0: Disables the default HLS playlist generation

• -hls_segment_filename ‘output_%03d.ts’: Specifies the segment filename format (replace with your own filename)

• output.m3u8: Output playlist file name (replace with your own filename)

Step 2: Identify the Last Transcoded Segment

Next, you need to identify the last transcoded segment. You can do this by checking the timestamp of the last segment file generated by ffmpeg.

ls -l output_*.ts | sort -r | head -1

This command lists all segment files in reverse chronological order and returns the most recent one.

Step 3: Update Your ffmpeg Command

Now, update your ffmpeg command to resume transcoding from the last generated segment.

ffmpeg -i input.mp4 -c:v libx264 -c:a aac -hls_list_size 0 -hls_segment_filename 'output_%03d.ts' -hls_start_number 10 output.m3u8

In this updated command, we’ve added:

• -hls_start_number 10: Specifies the starting segment number (replace with the number of the last generated segment + 1)

Step 4: Run ffmpeg and Resume Transcoding

With your updated command, run ffmpeg and resume transcoding from the last generated segment.

ffmpeg -i input.mp4 -c:v libx264 -c:a aac -hls_list_size 0 -hls_segment_filename 'output_%03d.ts' -hls_start_number 10 output.m3u8

ffmpeg will now resume transcoding from the last generated segment, saving you time and computational resources.

Segment Number	Timestamp	File Name
10	00:00:10	output_010.ts
11	00:00:20	output_011.ts
12	00:00:30	output_012.ts

In this example, ffmpeg will resume transcoding from segment 10 and continue generating new segments up to the end of the input file.

Troubleshooting Common Issues

If you encounter any issues during the transcoding process, here are some common solutions:

• ffmpeg crashes or exits unexpectedly:

  • Check the ffmpeg error log for clues

  • Verify that the input file is not corrupted

  • Try reducing the bitrate or quality settings

• Segment files are not generated correctly:

  • Verify the segment filename format is correct

  • Check the timestamp of the last generated segment matches the expected value

  • Try increasing the verbosity of ffmpeg to debug the issue

Conclusion

With practice and patience, you’ll be able to master the art of resuming ffmpeg HLS transcoding from an existing segment. Happy transcoding!

Bonus Tip: Consider using a wrapper script to automate the transcoding process and make it easier to manage your HLS library.

Did you find this article helpful? Share your experiences and tips in the comments below!

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