What is GOP (Group-of-Pictures) In Video Compression?

GOP stands for Group of Pictures.

In the field of video compression, a Group of Pictures (GOP) is a fundamental unit that determines the way in which video frames are organized and arranged for efficient video encoding.

Basically, instead of encoding video frames one after the other, multiple frames are grouped together to form a GOP (Group of Pictures) so that they can be compressed and then decompressed more efficiently.

Understanding GOP structure is important for video editing, streaming, or any application, such as Visionular’s where we optimize video compression within H.264/AVC, HEVC, and AV1 video codecs (what is a video codec?).

Structure of a GOP

What is the basic structure of GOP or Group of Pictures? A GOP consists of three types of frames – I, P, and B-frames and each of these have their own special use in the video compression pipeline.

Let’s see how!

I-Frame (Intra-coded frame): In a GOP, the first frame behaves like a complete picture, such a photograph. It is able to be encoded and decoded on its own, everything necessary to display it is inside the I-frame, and it serves as a reference to encode and decode other frames in the video.

P-Frames (Predicted Frames): These frames only save the difference (motion vectors) between themselves and the former frame (usually the I-frame). This significantly reduces file size as most video sequences exhibit significant redundancy between frames due to movement (also known as Temporal Redundancy).

B-frames (Bi-predictive frames): The B-Frames are more densely compressed by taking information not only from the preceding times but also from future times within the GOP. It can achieve the highest compression ratio but, it should be noted that B-frames are also the most dependent on surrounding frames for accurate decoding.

Benefits and Trade-offs of GOPs and their Structures

But, why go through all the trouble of deciding the size and structure of a GOP? What are the benefits of a well-designed GOP structure over something that is chosen in random?

Well, engineers and transcoding scientists spend a lot of time deciding and crafting a GOP’s structure because of the following benefits that it brings to the video compression process (amongst others!).

1) Reduced File Size: A well-designed GOP structure enables considerable video compression by taking advantage of frame-to-frame redundancy, rather than storing each frame autonomously. This compression is critical for economically storing, transmitting, and delivering video.

2) Random Access: Keyframes are a type of I-frame that lets viewers randomly access a video. Keyframes occur at regular intervals, getting rid of the need to decode previous frames if a viewer wants to skip to a certain part of the video. This is useful for video streaming and video editing.

3) Error Resilience: This is yet another major concern in the compression process; specifically, in frame structures. Both P and B-frames can yield extremely high compression, but with a tradeoff. These frames are more vulnerable to transmission errors. If an error occurs and corrupts a reference frame, it will impact the decoding of several subsequent P- or B-frames that depend on that reference frame.

Optimizing the Size and Structure of GOPs

GOP size (optimizing the count and placement of I, P, and B frames) strongly affects video quality, file size and editing flexibility.

And here is why!

Short GOP length: A shorter GOP will generally help improve the video quality, improve random access, error resilience, and make the video easier to edit since it has more frequent I-frames. More I-frames in the video will allow more points for clean cuts and edits without depending on many subsequent frames. However, shorter GOP will use more bits or bitrate to maintain quality.

Long GOP length: A larger GOP can generally achieve better compression (thus smaller file size or low bitrate) at the expense of potentially decreased error resilience and fewer opportunities for user to do edits.

If you are involved in video editing, streaming, or compression, you need to understand GOP structure, as it impacts everything from file size and data rate to quality and editability of the video.

The Future is Here!

For video streaming businesses, AI-driven video compression isn’t just a fad, it’s a strategic advantage. In a competitive market, delivering high-quality videos efficiently is essential. AI compression makes both possible, giving businesses a clear edge.

You can start a no-risk, free trial of Visionular’s AI-driven video compression on the cloud today and experience the power of AI-driven video compression for yourself!