What’s a GOP?
A major function of MPEG encoding is to identify redundancy not only within a frame, but also among a group of frames. A scene with no movement except a car driving by has a lot of redundancy over time. In this case, a single complete frame could provide the majority of the video; subsequent frames only need to provide for the areas that changed. MPEG uses three frame types to define the video, with a GOP (Group Of Pictures) setting to define the size of the group and its order.An MPEG video stream is composed of Intra (I) frames, Bi-directional (B) frames, and Predicted (P) frames. An I-frame, also known as a reference or “key” frame, contains the complete image of the current frame, without reference to frames that precede or follow it.
P-Frames are encoded from a “predicted” picture based on preceding frames, and B-Frames are encoded based on an interpolation from other frames that come before and after.
Therefore the I-frame is the only type of frame that can stand by itself, without requiring information from other frames in the GOP. Every GOP contains one I-frame.
Chapter points in a DVD Movie can be entered only at GOP boundaries. Otherwise, the picture could not be reproduced when the chapter point is accessed at random. Likewise, MPEG editing functions such as the EyeFrame setting can be done only at GOP boundaries.
There are three aspects to choosing a GOP setting: Length of GOP, the GOP pattern, and whether the GOP is “open” or “closed”.

什麼是GOP?
MPEG壓縮的主要原理是在一組連續畫面的壓縮,參考前後的畫面是否有重複的部份,決定本畫面的壓縮資料量,而不是只針對單一畫面做壓縮.MPEG壓縮後檔案的單位叫做GOP(Group Of Picture - 群組),在一個群組中,畫面會分為三個類型: I-frame (Index), B-frame (Bi-directional)及 P-frame(Predicted).
I-frame是影像群組中最重要的部份,記錄了最完整的資料,不需參考隣近的檔案,就可以壓縮或還原本畫面,每一個影像群組中,只有一個I-frame.
P-frame參考I-frame的資料,相同的部份只記錄預測移動的位置,所以使用的資料量比較少.B-frame分佈上述二種畫面之間,壓縮或還原時,都需要同時參考前後的畫面.使用的資料量最少.
DVD的章節點設定只能設在影像群組的啟始點.MPEG或DVD的編輯工具,也只能處理到影像群組的啟始點,如果設定的點不在影像群組啟始點上,會自動被到最近的上一個或下一個影像群組啟始點上,然後才能抓出縮小圖示,讓您識別.
影像群組的三個最主要的設定參數如下:影像群組長度,群組IBP分配方式,及是為封閉型或開放型群組.

GOP Length
Longer GOP lengths produce efficient encodes, but are less desirable during short duration effects such as quick fades and quick camera pans. DVD defines the maximum lengths to be 18 frames for NTSC or 15 frames for PAL.
影像群組長度
影像群組長度的設定主要看影片內容而言,靜態多的影片,群組長度可以設長一些;經常切換畫面的影片,群組長度可以設短一些.DVD-Video規格中定義,NTSC系統的影像群組最長為18frames (每秒播出29.97frames); PAL系統的影像群組為15frames (每秒播出25frames).

GOP Pattern
Larger GOP patterns produce more efficient encodes, with B- and P-frames being much smaller than I-frames. A typical pattern is IBBP with a length of 15 frames. The actual GOP pattern used in this case would be BBIBBPBBPBBPBBP.
群組IBP分配方式
包含B-,P-frame的群組,可以比只有I-frame的群組提供更有效率的壓縮.DVD最常用的分配方式為IBBP,群組長度為15 frames.

Open or Closed GOPs
An open GOP allows the B-frames from one GOP to look at the reference frame from an adjacent GOP. This is more efficient, but causes issues with multi-angle titles. A closed GOP keeps all references within itself - it stands alone with respect to other GOPs.
開放型或封閉型群組
開放型群組予許B-frame參考到隣近的群組的資料,它更有效率,但是在多重視角切換的影片中,可能會有問題(在DVD播放機,如果切換多重視角時,因為參考資料不足,可能會出現許多小方格狀的影像).封閉型則只會參考本身群組的資料,相容的問題比較少.