图像序列 图像序列是由图像组构成的,是随机存取段落。 sequence_header_code – The sequence_header_code is the bit string ‘000001B3’ in hexadecimal sequence_end_code – The sequence_end_code is the bit string ‘000001B7’ in hexadecimal. MPEG-2为了实现随机访问,在码流中会插入repeat_sequence_header,它可以插在I、P帧或GOP前面;如果是场编码,只能插在第一场前面。 图像组头 图像组(GOP)是为方便随机存取而加的,图像组是随机存取视频单位。一个GOP由一串IBP帧组成,起始为I帧。基准帧(I/P)的重复频率用M表示,它描述为多少帧里出现一次I/P帧。GOP的长度是一个I帧到下一个I 帧的间隔,即多少帧里面出现一次I帧,一般用N表 示,这个长度是可变的,长GOP可以提供高的压缩比,但是会造成随机存取的延迟(必须等到下一个I帧)和误差的积累(P帧的误差传播)。一般是一秒内有两 个I帧,用来作为随机存取的入口。在MPEG2中也没有规定GOP的结构,帧重复方式可以是IP,IB,IBP,IBBP,甚至全部是I帧。 M=1 N=X:IPPP... M=2 N=X:IBPBP... M=3 N=X:IBBPBBP... MPEG-2标准中的GOP语法结构如下:
group_start_code – The group_start_code is the bit string ‘000001B8’ in hexadecimal. closed_gop – This is a one-bit flag whichindicates the nature of the predictions used in the first consecutiveB-pictures (if any) immediately following the first coded I-framefollowing the group of picture header.closed_gop is set to ‘1’ toindicate that these B-pictures have been encoded using only backwardprediction or intracoding. broken_link – This is a one-bit flag which shall be set to ‘0’ during encoding. It is set to ‘1’ to indicate that the first |