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H.264从SPS中提取视频宽高

2018-08-05 23:12  jiayayao  阅读(3010)  评论(0编辑  收藏  举报

H.264有两种封装模式:

(1)annexb模式:传统模式,使用start code来分隔NAL, SPS和PPS是在ES流的头部;

(2)mp4模式:没有start code,使用NALU长度(固定字节,通常为4个字节)来分隔NAL。AVCodecContext的extradata内部保存着分隔的字节数,SPS和PPS;

1. 找到SPS

视频的宽高保存在SPS中。那么提取宽高首先要找到SPS。annexb模式直接读取视频数据,根据NAL type找到SPS即可。mp4模式应该从extradata中找到SPS。

mp4格式的extradata格式如下图:

FFMpeg中解析extradata的函数是ff_h264_decode_extradata()。注意第5字节的最后2位,表示的就是NAL size的字节数-1。在AVCC格式中,每个NAL前面都会有NAL size字段。NAL size可能是1字节、2字节或4字节(4字节较常见),解析extradata重要目的就是确认这个值,之后2个字节是SPS长度,长度之后就是SPS。

2. 从SPS中提取宽高

以下代码是网上常见的,但是很明显写的有问题,宽高不一定非要是16的倍数。

        width=(pic_width_in_mbs_minus1+1)*16;
        height=(pic_height_in_map_units_minus1+1)*16;

当宽高不是16整数倍时,frame_cropping_flag值为1,frame_mbs_only_flag为1以下为修正的代码:

#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <math.h>

typedef  unsigned int UINT;
typedef  unsigned char BYTE;
typedef  unsigned long DWORD;

UINT Ue(BYTE *pBuff, UINT nLen, UINT &nStartBit)
{
    //计算0bit的个数
    UINT nZeroNum = 0;
    while (nStartBit < nLen * 8)
    {
        if (pBuff[nStartBit / 8] & (0x80 >> (nStartBit % 8))) //&:按位与,%取余
        {
            break;
        }
        nZeroNum++;
        nStartBit++;
    }
    nStartBit ++;


    //计算结果
    DWORD dwRet = 0;
    for (UINT i=0; i<nZeroNum; i++)
    {
        dwRet <<= 1;
        if (pBuff[nStartBit / 8] & (0x80 >> (nStartBit % 8)))
        {
            dwRet += 1;
        }
        nStartBit++;
    }
    return (1 << nZeroNum) - 1 + dwRet;
}


int Se(BYTE *pBuff, UINT nLen, UINT &nStartBit)
{
    int UeVal=Ue(pBuff,nLen,nStartBit);
    double k=UeVal;
    int nValue=ceil(k/2);//ceil函数:ceil函数的作用是求不小于给定实数的最小整数。ceil(2)=ceil(1.2)=cei(1.5)=2.00
    if (UeVal % 2==0)
        nValue=-nValue;
    return nValue;
}


DWORD u(UINT BitCount,BYTE * buf,UINT &nStartBit)
{
    DWORD dwRet = 0;
    for (UINT i=0; i<BitCount; i++)
    {
        dwRet <<= 1;
        if (buf[nStartBit / 8] & (0x80 >> (nStartBit % 8)))
        {
            dwRet += 1;
        }
        nStartBit++;
    }
    return dwRet;
}

/**
 * H264的NAL起始码防竞争机制 
 *
 * @param buf SPS数据内容
 *
 * @无返回值
 */ 
void de_emulation_prevention(BYTE* buf,unsigned int* buf_size)
{
    int i=0,j=0;
    BYTE* tmp_ptr=NULL;
    unsigned int tmp_buf_size=0;
    int val=0;

    tmp_ptr=buf;
    tmp_buf_size=*buf_size;
    for(i=0;i<(tmp_buf_size-2);i++)
    {
        //check for 0x000003
        val=(tmp_ptr[i]^0x00) +(tmp_ptr[i+1]^0x00)+(tmp_ptr[i+2]^0x03);
        if(val==0)
        {
            //kick out 0x03
            for(j=i+2;j<tmp_buf_size-1;j++)
                tmp_ptr[j]=tmp_ptr[j+1];

            //and so we should devrease bufsize
            (*buf_size)--;
        }
    }

    return;
}

/**
 * 解码SPS,获取视频图像宽、高信息 
 *
 * @param buf SPS数据内容
 * @param nLen SPS数据的长度
 * @param width 图像宽度
 * @param height 图像高度

 * @成功则返回1 , 失败则返回0
 */ 
int h264_decode_sps(BYTE * buf,unsigned int nLen,int &width,int &height,int &fps)
{
    UINT StartBit=0; 
    fps=0;
    de_emulation_prevention(buf,&nLen);

    int forbidden_zero_bit=u(1,buf,StartBit);
    int nal_ref_idc=u(2,buf,StartBit);
    int nal_unit_type=u(5,buf,StartBit);
    if(nal_unit_type==7)
    {
        int profile_idc=u(8,buf,StartBit);
        int constraint_set0_flag=u(1,buf,StartBit);//(buf[1] & 0x80)>>7;
        int constraint_set1_flag=u(1,buf,StartBit);//(buf[1] & 0x40)>>6;
        int constraint_set2_flag=u(1,buf,StartBit);//(buf[1] & 0x20)>>5;
        int constraint_set3_flag=u(1,buf,StartBit);//(buf[1] & 0x10)>>4;
        int reserved_zero_4bits=u(4,buf,StartBit);
        int level_idc=u(8,buf,StartBit);

        int seq_parameter_set_id=Ue(buf,nLen,StartBit);

        if( profile_idc == 100 || profile_idc == 110 ||
            profile_idc == 122 || profile_idc == 144 )
        {
            int chroma_format_idc=Ue(buf,nLen,StartBit);
            if( chroma_format_idc == 3 )
                int residual_colour_transform_flag=u(1,buf,StartBit);
            int bit_depth_luma_minus8=Ue(buf,nLen,StartBit);
            int bit_depth_chroma_minus8=Ue(buf,nLen,StartBit);
            int qpprime_y_zero_transform_bypass_flag=u(1,buf,StartBit);
            int seq_scaling_matrix_present_flag=u(1,buf,StartBit);

            int seq_scaling_list_present_flag[8];
            if( seq_scaling_matrix_present_flag )
            {
                for( int i = 0; i < 8; i++ ) {
                    seq_scaling_list_present_flag[i]=u(1,buf,StartBit);
                }
            }
        }
        int log2_max_frame_num_minus4=Ue(buf,nLen,StartBit);
        int pic_order_cnt_type=Ue(buf,nLen,StartBit);
        if( pic_order_cnt_type == 0 )
            int log2_max_pic_order_cnt_lsb_minus4=Ue(buf,nLen,StartBit);
        else if( pic_order_cnt_type == 1 )
        {
            int delta_pic_order_always_zero_flag=u(1,buf,StartBit);
            int offset_for_non_ref_pic=Se(buf,nLen,StartBit);
            int offset_for_top_to_bottom_field=Se(buf,nLen,StartBit);
            int num_ref_frames_in_pic_order_cnt_cycle=Ue(buf,nLen,StartBit);

            int *offset_for_ref_frame=new int[num_ref_frames_in_pic_order_cnt_cycle];
            for( int i = 0; i < num_ref_frames_in_pic_order_cnt_cycle; i++ )
                offset_for_ref_frame[i]=Se(buf,nLen,StartBit);
            delete [] offset_for_ref_frame;
        }
        int num_ref_frames=Ue(buf,nLen,StartBit);
        int gaps_in_frame_num_value_allowed_flag=u(1,buf,StartBit);
        int pic_width_in_mbs_minus1=Ue(buf,nLen,StartBit);
        int pic_height_in_map_units_minus1=Ue(buf,nLen,StartBit);

        //width=(pic_width_in_mbs_minus1+1)*16;
        //height=(pic_height_in_map_units_minus1+1)*16;

        int frame_mbs_only_flag=u(1,buf,StartBit);
        if(!frame_mbs_only_flag)
            int mb_adaptive_frame_field_flag=u(1,buf,StartBit);

        int direct_8x8_inference_flag=u(1,buf,StartBit);
        int frame_cropping_flag=u(1,buf,StartBit);
        int frame_crop_left_offset=0;
        int frame_crop_right_offset=0;
        int frame_crop_top_offset=0;
        int frame_crop_bottom_offset=0;
        if(frame_cropping_flag)
        {
            frame_crop_left_offset=Ue(buf,nLen,StartBit);
            frame_crop_right_offset=Ue(buf,nLen,StartBit);
            frame_crop_top_offset=Ue(buf,nLen,StartBit);
            frame_crop_bottom_offset=Ue(buf,nLen,StartBit);
        }

        width = ((pic_width_in_mbs_minus1 +1)*16) - frame_crop_left_offset*2 - frame_crop_right_offset*2;
        height= ((2 - frame_mbs_only_flag)* (pic_height_in_map_units_minus1 +1) * 16) - (frame_crop_top_offset * 2) - (frame_crop_bottom_offset * 2);

        int vui_parameter_present_flag=u(1,buf,StartBit);
        if(vui_parameter_present_flag)
        {
            int aspect_ratio_info_present_flag=u(1,buf,StartBit);              
            if(aspect_ratio_info_present_flag)
            {
                int aspect_ratio_idc=u(8,buf,StartBit);   
                if(aspect_ratio_idc==255)
                {
                    int sar_width=u(16,buf,StartBit);                                  
                    int sar_height=u(16,buf,StartBit);                                      
                }
            }
            int overscan_info_present_flag=u(1,buf,StartBit); 
            if(overscan_info_present_flag)
                int overscan_appropriate_flagu=u(1,buf,StartBit);                   
            int video_signal_type_present_flag=u(1,buf,StartBit); 
            if(video_signal_type_present_flag)
            {
                int video_format=u(3,buf,StartBit);                         
                int video_full_range_flag=u(1,buf,StartBit);                       
                int colour_description_present_flag=u(1,buf,StartBit);
                if(colour_description_present_flag)
                {
                    int colour_primaries=u(8,buf,StartBit);              
                    int transfer_characteristics=u(8,buf,StartBit);                     
                    int matrix_coefficients=u(8,buf,StartBit);                          
                }
            }
            int chroma_loc_info_present_flag=u(1,buf,StartBit);  
            if(chroma_loc_info_present_flag)
            {
                int chroma_sample_loc_type_top_field=Ue(buf,nLen,StartBit);             
                int chroma_sample_loc_type_bottom_field=Ue(buf,nLen,StartBit);       
            }
            int timing_info_present_flag=u(1,buf,StartBit);        
            if(timing_info_present_flag)
            {
                int num_units_in_tick=u(32,buf,StartBit);
                int time_scale=u(32,buf,StartBit);    
                fps=time_scale/(2*num_units_in_tick);
            }
        }
        return true;
    }
    else
        return false;
}

参考资料:

1. http://www.latelee.org/my-study/get-width-height-framerate-from-bitstream.html

2. https://blog.csdn.net/yue_huang/article/details/75126155