Group of pictures

Key takeaways

Group of pictures belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Group of pictures to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Group of pictures from memory before moving on to harder problems.

Reference excerpt

In video coding, a group of pictures, or GOP structure, specifies the order in which intra- and inter-frames are arranged. The GOP is a collection of successive pictures within a coded video stream. Each coded video stream consists of successive GOPs, from which the visible frames are generated. Encountering a new GOP in a compressed video stream means that the decoder doesn't need any previous frames in order to decode the next ones, and allows fast seeking through the video.

Elements

A GOP can contain the following picture types:

I frame (intra coded picture, also by some sources incorrectly said to always be key frame, but you cannot always start with I frame and decode next frames cleanly) – a picture that is coded independently of all other pictures, each I frame can be decoded fully on its own. Each GOP begins (in decoding order) with this type of frame. IDR frame (Instantaneous Decoder Refresh): I frame with a marking indicating that no subsequent P or B frames have references reaching further back than this I frame. Through the use of these IDR frames, closed GOPs are formed that can’t refer to frames outside the GOP. IDRs are the true keyframes together with clean random access frames (recovery points), CLA. P frame (predictive coded picture) – contains motion-compensated difference information relative to previously decoded pictures. In older designs such as MPEG-1, H.262/MPEG-2 and H.263, each P frame can only reference one picture, and that picture must precede the P frame in display order as well as in decoding order, and the reference must be an I or P frame. These constraints do not apply in the newer standards H.264/MPEG-4 AVC and HEVC. B frame (bipredictive coded picture) – contains motion-compensated difference information relative to previously decoded pictures. In older designs such as MPEG-1 and H.262/MPEG-2, each B frame can only reference two frames, the one which precedes the B frame in display order and the one which follows, and all referenced pictures must be I or P frames. These constraints do not apply in newer standards H.264/MPEG-4 AVC and HEVC. Sometimes, a codec will use unidirectional B-frames. This is a P-frame that, while it does not use data from a future frame, no other frames depend on it. A fundamental property of B-frames is that they can be dropped without affecting the correct decoding of other frames. D frame (DC direct coded picture) – serves as a fast-access representation of a frame for loss robustness or fast-forward. D frames are only used in MPEG-1 video. An I frame indicates the beginning of a GOP. Afterwards, several P and B frames follow. In older designs, the allowed ordering and referencing structure is relatively constrained. The I frames contain the full image and do not require any additional information to reconstruct them. Typically, encoders use GOP structures that cause each I frame to be a "clean random access point," such that decoding can start cleanly on an I frame and any errors within the GOP structure are corrected after processing a correct I frame. In the newer designs found in H.264/MPEG-4 AVC and HEVC, encoders have much more flexibility about referencing structures. They can use the same referencing structures as were previously used in older designs, or they can use more pictures as references and they can use more flexible ordering of the coding order relative to the display order. They are also allowed to use B frames as references when coding other (B or P) frames. This extra flexibility can improve compression efficiency, but it can cause propagation of errors if some data becomes lost or corrupted. One popular structure for use with the newer designs is the use of a hierarchy of B frames. Hierarchical B frames can provide very good compression efficiency and can also limit the propagation of errors, since the hierarchy can ensure that the number of pictures affected by any data corruption problem is strictly limited. Generally, the more I frames the video stream has, the more editable it is. However, having more I frames substantially increases bit rate needed to code the video.

Structure The GOP structure is often referred by two numbers, for example, M=3, N=12. The first number tells the distance between two anchor frames (I or P), also known as the length of a "mini-GOP". The second one tells the distance between two full images (I-frames): it is the GOP size. Instead of the M parameter, the maximal count of B-frames between two consecutive anchor frames can be used; this is the approach used by ffmpeg. Examples:

For M=3, N=12, the GOP structure is IBBPBBPBBPBB. There are 2 B-frames between two consecutive anchor frames. For the sequence IBBBBPBBBBPBBBB, GOP size N=15, anchor-distance M=5. There are 4 B-frames between two consecutive anchor frames. The GOP structure does not need to stay fixed throughout encoding. Varying N to insert an I-frame on scene change is a well-known technique. Newer techniques also vary M based on the amount of motion in the video.

Additional concepts With H.264 and later designs which allow highly flexible reference structures, a B frame in one GOP is able to reference a frame in a different GOP, in particular even before the I frame, which makes I frame non-IDR (not a keyframe). A GOP that contains any such outward-referencing frame is known as an "open GOP". The opposite is a self-contained GOP, known as a "closed GOP". In presentation order GOP can begin with a B-frame, but it cannot end with one. Open GOP starts with a B-frame and it is a little more efficient because starting with an I-frame means that an extra P-frame must be added to the end (a GOP cannot end with a B-frame).

See also Video compression picture types Key frame

References

Worked examples

Example 1 — a first encounter with Group of pictures

Start with the simplest possible case. Write down what Group of pictures claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Group of pictures before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Group of pictures ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Group of pictures

In research: Group of pictures appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Group of pictures in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Group of pictures is common in secondary-school and first-year university syllabi. It links to neighbouring topics MPEG, Video compression, so understanding it makes those chapters shorter.
In everyday life: Look for Group of pictures outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Group of pictures in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Group of pictures means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Group of pictures out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Group of pictures in simple terms?

In video coding, a group of pictures, or GOP structure, specifies the order in which intra- and inter-frames are arranged. The GOP is a collection of successive pictures within a coded video stream.

Why does Group of pictures matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Group of pictures?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Group of pictures.