1. H.264 code tracing
Group 徐君潔

2. Trace 關係圖 encode_one_slice write_one_macroblock terminate_macroblock
proceed2nextMacroblock
terminate_slice
writeMBLayer
writeMB_typeInfo
writeMB_skip_flagInfo_CABAC
writeB8_typeInfo
writeMotionInfo2NAL
writeCBPandLumaCoeff

3. write_one_macroblock()
Target
passes the chosen syntax elements to the NAL
Input
eos_bit (end of slice)
/*0:write the Top MB data to the bitstream
1:write the Bottom MB data to the bitstream */
Output
none
In which file
macroblock.c
Caller
encode_one_slice()

4. UseConstrainedIntraPred
Flow chart(1)
Macroblock* currMB =
&img->mb_data[img-current_mb_nr];
int* bitCount = currMB->bitcounter;
img->SumFrameQP += currMB->qp;
UseConstrainedIntraPred:
0: Inter MB are allowed for intra prediction
1: Not allowed
#define IS_INTRA(MB)
((MB)->mb_type==I4MB ||
(MB)->mb_type==I16MB ||
(MB)->mb_type==I8MB ||
(MB)->mb_type==IPCM
Initial
Accumulate FrameQP
UseConstrainedIntraPred
& (P_SLICE || B_SLICE)
yes
intra_block[]=
IS_INTRA()

5. write_terminating_bit()
Flow chart(2)
Intras:
record intra block numbers
eos_bit = 1: write the bottom MB to
bitstream
write_terminating_bit () :
write flag = 1
biari_encode_symbol_final():
Arithmetic encoding for last symbol
before termination
current_mb_nr = 0
yes
Intras = 0
IS_INTRA()
yes
Intras++
CABAC&!first MB&eos_bit
yes
write_terminating_bit()

6. bitCount[BITS_TOTAL_MB]
Flow chart(3)
cabac_encoding: use in Arithmatic coding
writeMBLayer(0,&i):Codes macroblock header
set_last_dquant():
Set global last_dquant according to
macroblock delta qp
Rate Control:
img->NumberofMBHeaderBits
img->NumberofMBTextureBits
generic_RC->NumberofMBHeaderBits
generic_RC->NumberofMBTextureBits
if(img->BasicUnit < img->FrameSizeInMbs)
generic_RC->NumberofBasicUnitHeaderBits
generic_RC->NumberofBasicUnitTextureBits
BasicUnit:picture, slice,macroblock raw…
cabac_encoding = 1
writeMBLayer()
set_last_dquant()
Set total bit counter
bitCount[BITS_TOTAL_MB]
Rate Control
RCEnable
record total # of MBs
cabac_encoding = 0

7. terminate_macroblock()
Target
terminates processing of the current
macroblock depending on the chosen
slice mode
Input
end_of_slice
/* returns true for last macroblock of a slice, otherwise false*/
recode_macroblock
/* returns true if max. slice size is exceeded an macroblock must be recoded in next slice*/
Output
none
In which file
macroblock.c
Caller
encode_one_slice()

8. SyntaxElement(SE) se.value1 = img->cod_counter; se.value2 = 0;
typedef struct syntaxelement {
int type; //!< type of syntax element for data part.
int value1; //!< numerical value of syntax element
int value2; //!< for blocked symbols, e.g. run/level
int len; //!< length of code
int inf; //!< info part of UVLC code
unsigned int bitpattern; //!< UVLC bitpattern
int context; //!< CABAC context
} SyntaxElement;
Ex:
SyntaxElement se;
se.value1 = img->cod_counter;
se.value2 = 0;
se.type = SE_MBTYPE;
writeSE_UVLC(&se, dataPart);

9. Flow chart(1) Slice *currSlice = img->currentSlice;
Macroblock *currMB =
&img->mb_data[img->current_mb_nr];
int use_bitstream_backing =
(input->slice_mode == FIXED_RATE ||
input->slice_mode == CALL_BACK);
if ( (img->current_mb_nr==0) ||
(FmoGetPreviousMBNr(img->current_mb_nr)<0) )
else if( img->mb_data[FmoGetPreviousMBNr
(img->current_mb_nr)].slice_nr !=
img->current_slice_nr )
Initial
New slice
yes
new_slice = 1

10. Flow chart(2) slice_mode (1) NO_SLICES num_mb++ *recode_MB = FALSE
typedef enum
{ NO_SLICES, // 0=off
FIXED_MB, // 1=fixed # mb in slice
FIXED_RATE, // 2=fixed # bytes in slice
CALL_BACK, // 3=use callback
} SliceMode;
slice_mode
(1)
NO_SLICES
num_mb++
*recode_MB = FALSE
num_mb++
*recode_MB = FALSE
(2)
max num of MBs||
end of slice group
end of slice group||
max # of MBs in slice
FIXED_MB
yes
yes
*end_of_slice = TRUE
*end_of_slice = TRUE

11. Flow chart(3) (3) FIXED_RATE cod_counter cod_counter : skipped MBs
currStream->bits_to_go
currStream->byte_pos
currStream->byte_buf
yes
(UVLC)
write out cod_counter
writeSE_UVLC()
record rlc_bits
save bitstream &
restore bitstream
slice_too_big(rlc_bits) ||
(!recode_MB&last MB)
!new_slice&
slice_too_big(rlc_bits)
yes
yes
*end_of_slice =TRUE
*recode_MB =TRUE

12. Flow chart(4) mb_nr>0& !new_slice& slice_too_big (4) CALL_BACK
yes
*recode_MB =TRUE
*end_of_slice =TRUE
!recode_macroblock
&end of slice
default
yes
*end_of_slice =TRUE
error msg

13. Flow chart(5) end_of_slice& *recode_macroblock cod_counter
yes
yes
restore bitstream
writeSE_UVLC
UVLC
yes
handle skip MBs
writeSE_UVLC()

14. proceed2nextMacroblock()
Target
updates StatParameters for the next macroblock
Input
none
Output
In which file
macroblock.c
Caller
encode_one_slice()

15. StatParameters (4) (2) (3) (1) typedef struct {
int quant0; //!< quant for the first frame
int quant1; //!< average quant for the remaining frames
int b8_mode_0_use[NUM_PIC_TYPE][2];
int mode_use_transform_8x8[NUM_PIC_TYPE][MAXMODE];
int mode_use_transform_4x4[NUM_PIC_TYPE][MAXMODE];
int intra_chroma_mode[4];
int64 mode_use[NUM_PIC_TYPE][MAXMODE];
//!< Macroblock mode usage for Intra frames
int64 bit_use_mode[NUM_PIC_TYPE][MAXMODE];
//!< statistics of bit usage
int64 bit_use_mb_type[NUM_PIC_TYPE];
int64 tmp_bit_use_cbp[NUM_PIC_TYPE];
int64 bit_use_coeffY[NUM_PIC_TYPE];
int64 bit_use_coeffC[NUM_PIC_TYPE];
int64 bit_use_delta_quant[NUM_PIC_TYPE];
} StatParameters;
StatParameters stats;
(4)
(2)
(3)
(1)

16. mode_use&b8_mode_0_use
Flow chart
!I_SLICE&P8x8
bitCount >
max_bitCount
yes
update
mode_use&b8_mode_0_use
mode_use_transform
yes
error msg
mb_type&(cbp&15)
update(1)
bitcounter
yes
update
mode_use_transform
IS_INTRA
update(3)
P_SLICE ||
SP_SLICE
yes
update(2)
mode
yes
update(4)

17. terminate_slice() Target terminates a slice (but doesn't write it out)
Input
lastslice
Output
0:if OK
1: in case of error
In which file
slice.c
Caller
encode_one_slice()

18. 名詞解釋 SODP String of data bits (original data) RBSP
Raw byte sequence packet
end with 1 and trailing 0 for
alignment
EBSP
Encapsulate byte sequence
payload in order to avoid start
code conflict in NALU, add 0x03
after two 0x00

19. write_terminating_bit()
Flow chart(1)
static int MbWidthC [4]= { 0, 8, 8, 16};
static int MbHeightC [4]= { 0, 8, 16, 16};
//calculate RawBits
Slice *currSlice = img->currentSlice;
Initial
CABAC
yes
write_terminating_bit()

20. Flow chart(2) SODBtoRBSP():converts String Of Data Bits (SODB)
to Raw Byte Sequence
Packet (RBSP)
RBSPtoEBSP():coverts a RBSP
payload to to an EBSP
payload return size of
streamBuffer after stuffing
for this slice’s each partition
UVLC
UVLC
yes
SODBtoRBSP()
RBSPtoEBSP() no
record stuffing bits

21. arienco_done_encoding()
Flow chart(3)
CABAC
arienco_done_encoding()
arienco_done_encoding():Terminates
the arithmetic codeword, writes stop
bit and stuffing bytes
RawMbBits = 256 * img->bitdepth_luma +2
* MbWidthC[active_sps->chroma_format_idc]
* MbHeightC[active_sps->chroma_format_idc]
* img->bitdepth_chroma;
min_num_bytes = ((96 * get_pic_bin_count()–
(RawMbBits * (int)img->PicSizeInMbs *3)+
1023) / 1024;
RBSPtoEBSP():add stuffing_bytes
lastslice&
max_part_nr-1
yes
calclulate min_num_bytes
min_num_bytes
>bytes_in_pic
yes
calculate stuffing_bytes
RBSPtoEBSP()

22. free_ref_pic_list_recodering_buffer()
Flow chart(4)
store_contexts():
store motion coding and texture
coding context
free_ref_pic_list_reordering_buffer():
Free memory for buffering of
reference picture reordering
commands
CABAC
yes
store_context()
!I_SLICE||
!SI_SLICE
yes
free_ref_pic_list_recodering_buffer()

23. write_MBLayer() Target Codes macroblock header Input rdopt coeff_rate
/* true for calls during RD-optimization */
coeff_rate
/* bitrate of Luma and Chroma coeff */
Output
no_bits
/* number of bits for header coding */
In which file
macroblock.c
Caller
write_one_macroblock()

24. Flow chart(1) int mb_nr = img->current_mb_nr; int prev_mb_nr =
FmoGetPreviousMBNr(img->current_mb_nr);
Macroblock* currMB = &img->mb_data[mb_nr];
Macroblock* prevMB = mb_nr ?
(&img-mb_data[prev_mb_nr]) : NULL;
int* bitCount = currMB->bitcounter;
Slice* currSlice = img->currentSlice;
int skip = currMB->mb_type ?
0:((img->type == B_SLICE) ? !currMB->cbp:1);
/* FmoGetNextMBBr: Returns the MB-Nr (in scan order) of
the next MB in the (FMO) Slice, -1 if the SliceGroup is
finished */
Initial

25. Flow chart(2) MbaffFrameFlag top MB was skipped mb_nr is even
one field cannot use the macroblocks
in the other field of the same frame as a
Reference for motion prediction in order to
preserve as much spatial consistency as possible
MbaffFrameFlag
yes
top MB was skipped
mb_nr is even no
yes
yes
WriteFrameFieldMBInHeader = 1
prevMbSkipped = 0
WriteFrameFieldMBInHeader = 1
topMB=&img->mb_data[prev_mb_nr]
prevMbSkipped=topMB->skip_flag

26. Flow chart(3) (1) I_SLICE write mb_aff writeFieldModeInfo() I_SLICE
yes
write mb_aff
writeFieldModeInfo()
I_SLICE
write mb_type
writeMB_typeInfo() no
(2) not I_SLICE but CABAC
write mb_skip_flag
writeMB_skip_flagInfo_CABAC()
CABAC
yes
write mb_aff
writeFieldModeInfo() no
write mb_type
writeMB_typeInfo()

27. last MB &cod_counter>0
Flow chart(4)
(3) VLC not intra
mb_type!=0 ||
(B_SLICE&&cbp!=0)
//UVLC
yes
writeSE_UVLC()
(4) skipped MB no
write mb_aff
writeSE_Flag()
last MB &cod_counter>0
yes
write mb_type
writeMB_typeInfo()
writeSE_UVLC()

28. Flow chart(5) IPCM arienco_bits_written() arienco_done_encoding()
returns the number of currently
written bits
arienco_done_encoding():
terminates the arithmetic
codeword, writes stop bit and
stuffing bytes
arienco_start_encoding():
Initializes the
EncodingEnvironment for the
arithmetic coder
reset_pic_bin_count():
pic_bin_count = 0;
IPCM
arienco_bits_written()
arienco_done_encoding()
arienco_start_encoding()
reset_pic_bin_count()
yes
CABAC
yes no
write mb_type
writeSE_Fix()
CAVLC
yes

29. Flow chart(6) writeIntraModes() encode luma writeChromaIntraPredMode()
encode chroma
mb_type
writeMotionInfo2NAL()
IS_P8x8
writeB8_typeInfo()
writeMotionInfo2NAL()
mb_type||
(B_SLICE&cbp)
INTRA_4x4
INTRA_8x8
writeCBPandLumaCoeff()
writeChromaCoeff()