h264裸码流,根据nalu_header可以知道类型,例如该帧是I帧,P帧/B帧。

  例如,常见的0x65代表I帧,0x41代表非关键帧,即P帧或B帧,但是只根据nalu_header是无法区分P帧和B帧的,还需进入到RBSP内部根据语法含义来做判断。

1. JM实现的判别

 1 int FirstPartOfSliceHeader()
 2 {
 3   Slice *currSlice = img->currentSlice;
 4   int dP_nr = assignSE2partition[currSlice->dp_mode][SE_HEADER];
 5   DataPartition *partition = &(currSlice->partArr[dP_nr]);
 6   Bitstream *currStream = partition->bitstream;
 7   int tmp;
 8 
 9   UsedBits= partition->bitstream->frame_bitoffset; // was hardcoded to 31 for previous start-code. This is better.
10 
11   // Get first_mb_in_slice
12   currSlice->start_mb_nr = ue_v ("SH: first_mb_in_slice", currStream);
13 
14   tmp = ue_v ("SH: slice_type", currStream);
15   
16   if (tmp>4) tmp -=5;
17 
18   img->type = currSlice->picture_type = (SliceType) tmp;
19 
20   currSlice->pic_parameter_set_id = ue_v ("SH: pic_parameter_set_id", currStream);
21   
22   return UsedBits;
23 }
View Code

  如上代码,第二次的ue(无符号哥伦布编码)得出了帧类型,但是还需注意到做差:"if (tmp>4) tmp -=5;",才能得到真正的SliceType:

1 typedef enum {
2     P_SLICE = 0,
3     B_SLICE,
4     I_SLICE,
5     SP_SLICE,
6     SI_SLICE
7 } SliceType;

  例如,一段nalu数据为:0x00 00 01 65 88 80 06 64

  那么,从0x65后的字节0x88开始算起,

  第一个ue得到start_mb_nr,即其值为(2^0 - 1 + 0)= 0,0x88使用了最高位,当前bit_offset=1

  第二个ue得到tmp值,即其值为(2^3 - 1 + 000b)= 7,使用了7bit来计算,当前bit_offset=8,再将tmp-5得到2,那么picture_type为I帧。

  第三个ue得到pps_id值,从0x80的最高位算起,值为(2^0 - 1 + 0)= 0

2. ffmpeg的实现

  实现类似,参考文件h264_slice.c,关键代码如下:

 1 static int h264_slice_header_parse(const H264Context *h, H264SliceContext *sl,
 2                                    const H2645NAL *nal)
 3 {
 4     const SPS *sps;
 5     const PPS *pps;
 6     int ret;
 7     unsigned int slice_type, tmp, i;
 8     int field_pic_flag, bottom_field_flag;
 9     int first_slice = sl == h->slice_ctx && !h->current_slice;
10     int picture_structure;
11 
12     if (first_slice)
13         av_assert0(!h->setup_finished);
14 
15     sl->first_mb_addr = get_ue_golomb_long(&sl->gb);
16 
17     slice_type = get_ue_golomb_31(&sl->gb);
18     if (slice_type > 9) {
19         av_log(h->avctx, AV_LOG_ERROR,
20                "slice type %d too large at %d\n",
21                slice_type, sl->first_mb_addr);
22         return AVERROR_INVALIDDATA;
23     }
24     if (slice_type > 4) {
25         slice_type -= 5;
26         sl->slice_type_fixed = 1;
27     } else
28         sl->slice_type_fixed = 0;
29 
30     slice_type         = ff_h264_golomb_to_pict_type[slice_type];
31     sl->slice_type     = slice_type;
32     sl->slice_type_nos = slice_type & 3;
33 
34     if (nal->type  == H264_NAL_IDR_SLICE &&
35         sl->slice_type_nos != AV_PICTURE_TYPE_I) {
36         av_log(h->avctx, AV_LOG_ERROR, "A non-intra slice in an IDR NAL unit.\n");
37         return AVERROR_INVALIDDATA;
38     }
39 
40     sl->pps_id = get_ue_golomb(&sl->gb);
41 ...
42 }
View Code

  同样也是超过4时,再减去5即得到了帧类型。

3. 小工具实现

  本人曾做了一个小工具,参考核心代码如下:

 1 static int GetFrameType(NALU_t * nal)
 2 {
 3     bs_t s;
 4     int frame_type = 0;
 5 
 6     bs_init(&s, nal->buf+1, nal->len - 1);
 7 
 8     if (nal->nal_unit_type == NALU_TYPE_SLICE || nal->nal_unit_type ==  NALU_TYPE_IDR)
 9     {
10         /* i_first_mb */
11         bs_read_ue(&s);
12         /* picture type */
13         frame_type = bs_read_ue(&s);
14         switch(frame_type)
15         {
16         case 0: case 5: /* P */
17             nal->frame_type = FRAME_TYPE_P;
18             break;
19         case 1: case 6: /* B */
20             nal->frame_type = FRAME_TYPE_B;
21             break;
22         case 3: case 8: /* SP */
23             nal->frame_type = FRAME_TYPE_P;
24             break;
25         case 2: case 7: /* I */
26             nal->frame_type = FRAME_TYPE_I;
27             break;
28         case 4: case 9: /* SI */
29             nal->frame_type = FRAME_TYPE_I;
30             break;
31         default:
32             printf("unknown frame type! nalu_data[%#x,%#x,%#x,%#x]\n", nal->buf[0], nal->buf[1], nal->buf[2], nal->buf[3]);
33             break;
34         }
35     }
36     else
37     {
38         nal->frame_type = nal->nal_unit_type;
39     }
40 
41     return 0;
42 }