Скачать или смотреть Mastering ffmpeg: How to Adjust the Seek Behavior for Accurate Frame Extraction

Learn how to change the seeking behavior in `ffmpeg` to select the closest keyframe after a specified time instead of before. This guide provides a practical solution using `ffprobe` and bash scripting for video editing enthusiasts.
---
This video is based on the question https://stackoverflow.com/q/68464667/ asked by the user 'Kevin' ( https://stackoverflow.com/u/10366799/ ) and on the answer https://stackoverflow.com/a/68480378/ provided by the user 'Kevin' ( https://stackoverflow.com/u/10366799/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: How to change the -ss behavior from closest seek point before position to closest seek point after position

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Mastering ffmpeg: How to Adjust the Seek Behavior for Accurate Frame Extraction

When working with video editing, one often faces various challenges. A common scenario arises when you want to extract a frame at a specific time in a video using ffmpeg. By default, the -ss option will seek to the closest keyframe before the specified time. This raises a question: How can you change this behavior to select the closest keyframe after the specified time? In this guide, we will explore a detailed solution to this dilemma using ffmpeg, ffprobe, and a bit of bash scripting.

Understanding the Default Behavior of -ss

The -ss argument is used in ffmpeg to indicate the starting time for the processing of the video. However, when using -ss, the default behavior is to select the nearest keyframe before the specified time. This can lead to selecting frames that are earlier than desired.

Example Scenario

For instance, if your video file in.mp4 contains keyframes at 98 seconds and 102 seconds:

Using ffmpeg -ss 100 -i in.mp4 -c copy out.mp4 would result in ffmpeg selecting the frame closest to 98 seconds instead of 102 seconds as you might prefer.

The Solution: Combining ffprobe and Bash Scripting

To change this behavior, we can utilize ffprobe to identify keyframes and then implement a bash script. Here’s a step-by-step breakdown of how to achieve this:

Step 1: Get Keyframe Information with ffprobe

We first need to gather keyframe timestamps, particularly those after our desired seek point.

[[See Video to Reveal this Text or Code Snippet]]

-skip_frame nokey: This tells ffprobe to skip non-keyframes.

-select_streams v: Selects the video stream.

-show_entries frame=pkt_pts_time: Displays the packet time for each frame.

-of csv: Outputs the information in CSV format.

-read_intervals: Specifies the interval to read frames for keyframe extraction. Here, T+ 30 ensures we check a bit beyond the desired timestamp.

Step 2: Bash Scripting to Isolate the Desired Keyframe

Now that we have the keyframe timestamps, we need to write a bash script that will process this information and select the closest keyframe after the specified time.

Here’s a simple bash script to get you started:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Explanation of Key Parts of the Script

Input Variable: The script takes $1 as the expected start time you input when running it.

Timestamp Calculation: The script calculates the timestamp by giving a buffer of 20 seconds (adjustable) to ensure all possible keyframes are examined.

Filtering Keyframes: The script checks which keyframe timestamp is the first that exceeds the expected start time, adds 1 to that timestamp, and stores it in $st.

Step 4: Using the New Timestamp in an ffmpeg Command

Once you have the new timestamp ($st), you can now run your ffmpeg command like this:

[[See Video to Reveal this Text or Code Snippet]]

This command will now use the keyframe timestamp after your desired time.

Conclusion

Changing the behavior of ffmpeg to select the closest seek point after a given time may not be straightforward, but with the combination of ffprobe and a simple bash script, it becomes an achievable task. By following the outlined steps, you can now have more precise control over your frame extraction process.

If you have alternative methods or suggestions, feel free to share your thoughts! Happy Editing!