What Does the -g Option Do in FFmpeg?

In FFmpeg, the -g option is a crucial parameter used to define the Group of Pictures (GOP) size, commonly referred to as the keyframe or I-frame interval. This article explains how the -g option works, its impact on video compression, playback, and streaming, and how to use it effectively in your video processing workflows.

Understanding the GOP and Keyframes

To understand the -g option, it helps to understand how video compression works. Videos are made up of three types of frames: * I-frames (Keyframes): Full images containing complete visual data. They do not rely on other frames to decode. * P-frames and B-frames: Delta frames that only store the changes between frames to save space.

The -g option specifies the maximum distance between I-frames. For example, if you set -g 60, FFmpeg will place an I-frame at least once every 60 frames.

How to Use the -g Option

The basic syntax for applying the -g option in an FFmpeg command is as follows:

ffmpeg -i input.mp4 -c:v libx264 -g 60 output.mp4

In this example, if the output video has a framerate of 30 frames per second (fps), a GOP size of 60 means a keyframe will be inserted every 2 seconds (60 frames / 30 fps = 2 seconds).

Why the -g Option Matters

Adjusting the GOP size has a direct impact on video quality, file size, and playback performance.

1. Video Streaming (HLS and DASH)

For modern streaming protocols like HTTP Live Streaming (HLS) or Dynamic Adaptive Streaming over HTTP (DASH), consistent keyframe intervals are critical. Streaming players segment videos into short chunks (usually 2 to 6 seconds long). Every chunk must begin with a keyframe so the player can start playback instantly. If you are streaming a 30 fps video and want 2-second segments, you should set -g 60.

2. Compression and File Size

• Larger GOP (e.g., -g 240): Fewer keyframes are created. Because P and B-frames require much less data than I-frames, a larger GOP size generally results in better compression and smaller file sizes for highly compressible content.
• Smaller GOP (e.g., -g 15): More keyframes are created. This results in larger file sizes but provides better overall picture quality in high-motion scenes.

3. Seeking and Editing

During video playback, when a user drags the progress bar (seeks) to a new timestamp, the player must locate the nearest preceding keyframe to render the image. A smaller GOP size allows for faster, more precise seeking and makes the video easier to cut and edit without re-encoding.