H264/AVC视频解码时AVC1和H264的区别
The following media subtypes are defined for H.264 video.
| Subtype | FOURCC | Description |
|---|---|---|
| MEDIASUBTYPE_AVC1 | 'AVC1' | H.264 bitstream without start codes. |
| MEDIASUBTYPE_H264 | 'H264' | H.264 bitstream with start codes. |
| MEDIASUBTYPE_h264 | 'h264' | Equivalent to MEDIASUBTYPE_H264, with a different FOURCC. |
| MEDIASUBTYPE_X264 | 'X264' | Equivalent to MEDIASUBTYPE_H264, with a different FOURCC. |
| MEDIASUBTYPE_x264 | 'x264' | Equivalent to MEDIASUBTYPE_H264, with a different FOURCC. |
The main difference between these media types is the presence of startcodes in the bitstream. If the subtype is MEDIASUBTYPE_AVC1, thebitstream does not contain start codes.
H.264 Bitstream Without Start Codes
The MP4 container format stores H.264 data without start codes.Instead, each NALU is prefixed by a length field, which gives thelength of the NALU in bytes. The size of the length field can vary, butis typically 1, 2, or 4 bytes.
When start codes are not present in the bitstream, the following media type is used.
| Major type | MEDIATYPE_Video |
|---|---|
| Subtype | MEDIASUBTYPE_AVC1 |
| Format type | FORMAT_MPEG2Video |
The format block is an MPEG2VIDEOINFO structure. This structure should be filled in as follows:
- hdr : A VIDEOINFOHEADER2 structure that describes the bitstream. No color table is present after the BITMAPINFOHEADER portion of the structure, and biClrUsed must be zero.
- dwStartTimeCode : Not used. Set to zero.
- cbSequenceHeader : The length of the dwSequenceHeader array in bytes.
- dwProfile : Specifies the H.264 profile.
- dwLevel : Specifies the H.264 level.
- dwFlags : The number of bytes used for the length field that appears before each NALU . The length field indicates the size of the following NALU in bytes. For example, if dwFlags is 4, each NALU is preceded by a 4-byte length field. The valid values are 1, 2, and 4.
- dwSequenceHeader : A byte array that may contain sequence parameter set (SPS) and picture parameter set (PPS) NALUs.
The MP4 container might contain sequence parameter sets (SPS) orpicture parameter sets (PPS) as special NAL units in file headers or ina separate stream (distinct from the video stream). When the format isestablished, the media type can specify SPS and PPS NAL units in thedwSequenceHeader array. If cbSequenceHeader is greater than zero, dwSequenceHeader is the start of a byte array containing SPS and PPS NALUs, delimited by2-byte length fields, all in network byte order (big-endian). It ispossible to have both SPS and PPS, only one of these types, or none.The actual type of each NALU can be determined by examining thenal_unit_type field of the NALU itself.
When this media type is used, each media sample starts at thebeginning of a NALU, and NAL units do not span samples. This enablesthe decoder to recover from data corruption or dropped samples.
FOURCC:AVC1 描述:H.264 bitstream without start codes.
FOURCC:H264 描述:H.264 bitstream with start codes.
H.264 Bitstream with Start Codes
H.264 bitstreams that are transmitted over the air, or contained in MPEG-2 program or transport streams, or recorded on HD-DVD, are formatted as described in Annex B of ITU-T Rec. H.264. According to this specification, the bitstream consists of a sequence of network abstraction layer units (NALUs), each of which is prefixed with a start code equal to 0x000001 or 0x00000001.
这段话的大致意思是：带有开始码的H.264视频一般是用于无线发射、有线广播或者HD-DVD中的。这些数据流的开始都有一个开始码：0x000001 或者 0x00000001.
H.264 Bitstream Without Start Codes
The MP4 container format stores H.264 data without start codes. Instead, each NALU is prefixed by a length field, which gives the length of the NALU in bytes. The size of the length field can vary, but is typically 1, 2, or 4 bytes.
这段话的大致意思是：没有开始码的H.264视频主要是存储在MP4格式的文件中的。它的数据流的开始是1、2或者4个字节表示长度数据。
原文中的"NALU"简单说是H.264格式中的最基本的单元，是一个数据包。