The h.264 Sequence Parameter Set
转债: http://www.cardinalpeak.com/blog/the-h-264-sequence-parameter-set/
View from the Peak
The h.264 Sequence Parameter Set
This is a follow-up to my World’s Smallest h.264 Encoder post. I’ve received several emails asking about precise details of things in two entities in the h.264 bitstream: the Sequence Parameter Set (SPS) and the Picture Parameter Set (PPS). Both entities contain information that an h.264 decoder needs to decode the video data, for example the resolution and frame rate of the video.
Recall that an h.264 bitstream contains a sequence of Network Abstraction Layer (NAL) units. The SPS and PPS are both types of NAL units. The SPS NAL unit contains parameters that apply to a series of consecutive coded video pictures, referred to as a “coded video sequence” in the h.264 standard. The PPS NAL unit contains parameters that apply to the decoding of one or more individual pictures inside a coded video sequence.
In the case of my simple encoder, we emitted a single SPS and PPS at the start of the video data stream, but in the case of a more complex encoder, it would not be uncommon to see them inserted periodically in the data for two reasons—first, often a decoder will need to start decoding mid-stream, and second, because the encoder may wish to vary parameters for different parts of the stream in order to achieve better compression or quality goals.
In my trivial encoder, the h.264 SPS and PPS were hardcoded in hex as:
/* h.264 bitstreams */
const uint8_t sps[] =
{0x00, 0x00, 0x00, 0x01, 0x67, 0x42, 0x00, 0x0a, 0xf8, 0x41, 0xa2};
const uint8_t pps[] =
{0x00, 0x00, 0x00, 0x01, 0x68, 0xce, 0x38, 0x80};
Let’s decode this into something readable from the spec. The first thing I did was to look at section 7 of the h.264 specification. I saw that at a minimum I had to choose how to fill in the SPS parameters in the table below. In the table, as in the standard, the type u(n)
indicates an unsigned integer of n bits, and ue(v)
indicates an unsigned exponential-golomb coded value of a variable number of bits. The spec doesn’t seem to define the maximum number of bits anywhere, but the reference encoder software uses 32. (People wishing to explore the security of decoder software may find it interesting to violate this assumption!)
Parameter Name | Type | Value | Comments |
forbidden_zero_bit | u(1) | 0 | Despite being forbidden, it must be set to 0! |
nal_ref_idc | u(2) | 3 | 3 means it is “important” (this is an SPS) |
nal_unit_type | u(5) | 7 | Indicates this is a sequence parameter set |
profile_idc | u(8) | 66 | Baseline profile |
constraint_set0_flag | u(1) | 0 | We’re not going to honor constraints |
constraint_set1_flag | u(1) | 0 | We’re not going to honor constraints |
constraint_set2_flag | u(1) | 0 | We’re not going to honor constraints |
constraint_set3_flag | u(1) | 0 | We’re not going to honor constraints |
reserved_zero_4bits | u(4) | 0 | Better set them to zero |
level_idc | u(8) | 10 | Level 1, sec A.3.1 |
seq_parameter_set_id | ue(v) | 0 | We’ll just use id 0. |
log2_max_frame_num_minus4 | ue(v) | 0 | Let’s have as few frame numbers as possible |
pic_order_cnt_type | ue(v) | 0 | Keep things simple |
log2_max_pic_order_cnt_lsb_minus4 | ue(v) | 0 | Fewer is better. |
num_ref_frames | ue(v) | 0 | We will only send I slices |
gaps_in_frame_num_value_allowed_flag | u(1) | 0 | We will have no gaps |
pic_width_in_mbs_minus_1 | ue(v) | 7 | SQCIF is 8 macroblocks wide |
pic_height_in_map_units_minus_1 | ue(v) | 5 | SQCIF is 6 macroblocks high |
frame_mbs_only_flag | u(1) | 1 | We will not to field/frame encoding |
direct_8x8_inference_flag | u(1) | 0 | Used for B slices. We will not send B slices |
frame_cropping_flag | u(1) | 0 | We will not do frame cropping |
vui_prameters_present_flag | u(1) | 0 | We will not send VUI data |
rbsp_stop_one_bit | u(1) | 1 | Stop bit. I missed this at first and it caused me much trouble. |
Some key things here are the profile (profile_idc
) and level (level_idc
) that I chose, and the picture width and height. If you encode the above table in hex, you will get the values in the SPS array declared above.
A question I got a couple of times in email was about the width and height parameters—specifically, what to do if the picture width or height is not an integer multiple of macroblock size. Recall that, for the 4:2:0 sampling scheme in my encoder, a macroblock consists of 16×16 luma samples. In this case, you would set the frame_cropping_flag
to 1, and reduce the number of pixels in the horizontal and vertical direction with the frame_crop_left_offset
, frame_crop_right_offset
, frame_crop_top_offset
, and frame_crop_bottom_offset
parameters, which are conditionally present in the bitstream only if the frame_cropping_flag
is set to one.
One interesting problem that we see fairly often with h.264 is when the container format (MP4, MOV, etc.) contains different values for some of these parameters than the SPS and PPS. In this case, we find different video players handle the streams differently.
A handy tool for decoding h.264 bitstreams, including the SPS, is the h264bitstream tool. It comes with a command line program that decodes a bitstream to the parameter names defined in the h.264 specification. Let’s look at its output for a sample mp4 file I downloaded from youtube. First, I extract the h.264 NAL units from the file using ffmpeg:
ffmpeg.exe -i Old Faithful.mp4 -vcodec copy -vbsf h264_mp4toannexb -an of.h264
The NAL units now reside in the file of.h264
. I then run the h264_analyze command from the h264bitstream package to produce the following output:
h264_analyze of.h264
!! Found NAL at offset 4 (0x0004), size 25 (0x0019)
==================== NAL ====================
forbidden_zero_bit : 0
nal_ref_idc : 3
nal_unit_type : 7 ( Sequence parameter set )
======= SPS =======
profile_idc : 100
constraint_set0_flag : 0
constraint_set1_flag : 0
constraint_set2_flag : 0
constraint_set3_flag : 0
reserved_zero_4bits : 0
level_idc : 31
seq_parameter_set_id : 0
chroma_format_idc : 1
residual_colour_transform_flag : 0
bit_depth_luma_minus8 : 0
bit_depth_chroma_minus8 : 0
qpprime_y_zero_transform_bypass_flag : 0
seq_scaling_matrix_present_flag : 0
log2_max_frame_num_minus4 : 3
pic_order_cnt_type : 0
log2_max_pic_order_cnt_lsb_minus4 : 3
delta_pic_order_always_zero_flag : 0
offset_for_non_ref_pic : 0
offset_for_top_to_bottom_field : 0
num_ref_frames_in_pic_order_cnt_cycle : 0
num_ref_frames : 1
gaps_in_frame_num_value_allowed_flag : 0
pic_width_in_mbs_minus1 : 79
pic_height_in_map_units_minus1 : 44
frame_mbs_only_flag : 1
mb_adaptive_frame_field_flag : 0
direct_8x8_inference_flag : 1
frame_cropping_flag : 0
frame_crop_left_offset : 0
frame_crop_right_offset : 0
frame_crop_top_offset : 0
frame_crop_bottom_offset : 0
vui_parameters_present_flag : 1
=== VUI ===
aspect_ratio_info_present_flag : 1
aspect_ratio_idc : 1
sar_width : 0
sar_height : 0
overscan_info_present_flag : 0
overscan_appropriate_flag : 0
video_signal_type_present_flag : 0
video_signal_type_present_flag : 0
video_format : 0
video_full_range_flag : 0
colour_description_present_flag : 0
colour_primaries : 0
transfer_characteristics : 0
matrix_coefficients : 0
chroma_loc_info_present_flag : 0
chroma_sample_loc_type_top_field : 0
chroma_sample_loc_type_bottom_field : 0
timing_info_present_flag : 1
num_units_in_tick : 100
time_scale : 5994
fixed_frame_rate_flag : 1
nal_hrd_parameters_present_flag : 0
vcl_hrd_parameters_present_flag : 0
low_delay_hrd_flag : 0
pic_struct_present_flag : 0
bitstream_restriction_flag : 1
motion_vectors_over_pic_boundaries_flag : 1
max_bytes_per_pic_denom : 0
max_bits_per_mb_denom : 0
log2_max_mv_length_horizontal : 11
log2_max_mv_length_vertical : 11
num_reorder_frames : 0
max_dec_frame_buffering : 1
=== HRD ===
cpb_cnt_minus1 : 0
bit_rate_scale : 0
cpb_size_scale : 0
initial_cpb_removal_delay_length_minus1 : 0
cpb_removal_delay_length_minus1 : 0
dpb_output_delay_length_minus1 : 0
time_offset_length : 0
The only additional thing I’d like to point out here is that this particular SPS also contains information about the frame rate of the video (see timing_info_present_flag
). These parameters must be closely checked when you generate bitstreams to ensure they agree with the container format that the h.264 will eventually be muxed into. Even a small error, such as 29.97 fps in one place and 30 fps in another, can result in severe audio/video synchronization problems.
Next time I will write about the h.264 Picture Parameter Set (PPS).
The h.264 Sequence Parameter Set的更多相关文章
- 获得H.264视频分辨率的方法
转自:http://www.cnblogs.com/likwo/p/3531241.html 在使用ffmpeg解码播放TS流的时候(例如之前写过的UDP组播流),在连接时往往需要耗费大量时间.经过d ...
- iOS VideoToolbox硬编H.265(HEVC)H.264(AVC):2 H264数据写入文件
本文档为iOS VideoToolbox硬编H.265(HEVC)H.264(AVC):1 概述续篇,主要描述: CMSampleBufferRef读取实际数据 序列参数集(Sequence Para ...
- H.264中NALU、RBSP、SODB的关系 (弄清码流结构)
NALU:Coded H.264 data is stored or transmitted as a series of packets known as NetworkAbstraction La ...
- World’s Smallest h.264 Encoder
转载 http://www.cardinalpeak.com/blog/worlds-smallest-h-264-encoder/ View from the Peak World’s Smalle ...
- h.264语法结构分析
NAL Unit Stream Network Abstraction Layer,简称NAL. h.264把原始的yuv文件编码成码流文件,生成的码流文件就是NAL单元流(NAL unit Stre ...
- H.264视频的RTP荷载格式
Status of This Memo This document specifies an Internet standards track protocol for the Internet ...
- 直播一:H.264编码基础知识详解
一.编码基础概念 1.为什么要进行视频编码? 视频是由一帧帧图像组成,就如常见的gif图片,如果打开一张gif图片,可以发现里面是由很多张图片组成.一般视频为了不让观众感觉到卡顿,一秒钟至少需要16帧 ...
- H.264流媒体协议格式中的Annex B格式和AVCC格式深度解析
版权声明:本文为博主原创文章,未经博主允许不得转载. https://blog.csdn.net/Romantic_Energy/article/details/50508332本文需要读者对H.26 ...
- 一步一步解析H.264码流的NALU(SPS,PSS,IDR)获取宽高和帧率
分析H.264码流的工具 CodecVisa,StreamEye以及VM Analyzer NALU是由NALU头和RBSP数据组成,而RBSP可能是SPS,PPS,Slice或SEI 而且SPS位于 ...
随机推荐
- Linux Shell脚本读写XML文件
在Linux下如何用Shell脚本读写XML?现有一个config.xml <?xml version="1.0" encoding="UTF-8"?&g ...
- svn 版本升级的问题
原创文章,转载请注明 svn本地版本由1.6升级到1.7后,再使用时遇到一些问题,这里记录一下以备忘. 升级后,使用任何命令 不能用了,提示的意思大致是本地的workcopy版本太低了(之前用1.6版 ...
- awk输出单引号,双引号
双引号: awk '{print "\""}' #放大:awk '{print " \" "}' 使用“”双引号把一个双引 ...
- Oracle的rownum原理和使用(整理几个达人的帖子)
整理和学习了一下网上高手关于rownum的帖子: 参考资料: http://tech.ddvip.com/2008-10/122490439383296.html 和 http://tenn.jav ...
- 用RSA加密实现Web登录密码加密传输
通常我们做一个Web应用程序的时候都需要登录,登录就要输入用户名和登录密码,并且,用户名和登录密码都是明文传输的,这样就有可能在中途被别人拦截,尤其是在网吧等场合. 这里顺带一个小插曲,我以前有家公司 ...
- 页面多个Jquery版本共存的冲突问题,解决方法!
示例如下: <script type="text/javascript" src="jquery.js"></script> <s ...
- ZOJ 2587 Unique Attack (最小割唯一性)
题意 判断一个无向图的割是否唯一 思路 错误思路:一开始想的是判断割边是否都是关键割边,那既然割边两端点能连通S.T点的边是关键边,那么只要遇到有某个边两端点不连通S or T则这条边就不是关键割边( ...
- OK335xS PMIC(TPS65910A3A1RSL) reset
/*********************************************************************** * OK335xS PMIC(TPS65910A3A1 ...
- apache开源项目-- UIMA
UIMA (Unstructured Information Management applications) 是一个软件系统,用来分析大量的非结构化信息从而发掘中对最终用户有用的知识点,一个最典型的 ...
- 【转】TLB(Translation Lookaside Buffers,TLB)的作用
原文网址:http://sdnydubing.blog.163.com/blog/static/137470570201122810503396/ 从虚拟地址到物理地址的转换过程可知:使用一级页表进行 ...