Re: [Linphone-developers] H.263 bitstream syntax error ; an FFMPEG problem?

[Simon Morlat]

Hello,

I had a look at ffmpeg code and here's what I saw.

When UFEP=1, ffmpeg considers that there is an optional PLUS PTYPE bitfield.

Here is an extract of the h263_decode_pict_header():

/* H.263v2 */

s->h263_plus = 1;

ufep = get_bits(&s->gb, 3); /* Update Full Extended PTYPE */

/* ufep other than 0 and 1 are reserved */

if (ufep == 1) {

/* OPPTYPE */

format = get_bits(&s->gb, 3);

dprintf("ufep=1, format: %d\n", format);

s->custom_pcf= get_bits1(&s->gb);

s->umvplus = get_bits(&s->gb, 1); /* Unrestricted Motion Vector */

if (get_bits1(&s->gb) != 0) {

av_log(s->avctx, AV_LOG_ERROR, "Syntax-based Arithmetic Coding (SAC) not supported\n");

}

s->obmc= get_bits1(&s->gb); /* Advanced prediction mode */

s->h263_aic = get_bits1(&s->gb); /* Advanced Intra Coding (AIC) */

s->loop_filter= get_bits1(&s->gb);

s->unrestricted_mv = s->umvplus || s->obmc || s->loop_filter;

s->h263_slice_structured= get_bits1(&s->gb);

if (get_bits1(&s->gb) != 0) {

av_log(s->avctx, AV_LOG_ERROR, "Reference Picture Selection not supported\n");

}

if (get_bits1(&s->gb) != 0) {

av_log(s->avctx, AV_LOG_ERROR, "Independent Segment Decoding not supported\n");

}

s->alt_inter_vlc= get_bits1(&s->gb);

s->modified_quant= get_bits1(&s->gb);

if(s->modified_quant)

s->chroma_qscale_table= ff_h263_chroma_qscale_table;

skip_bits(&s->gb, 1); /* Prevent start code emulation */

skip_bits(&s->gb, 3); /* Reserved */

} else if (ufep != 0) {

av_log(s->avctx, AV_LOG_ERROR, "Bad UFEP type (%d)\n", ufep);

return -1;

}

/* MPPTYPE */

s->pict_type = get_bits(&s->gb, 3);

switch(s->pict_type){

case 0: s->pict_type= I_TYPE;break;

case 1: s->pict_type= P_TYPE;break;

case 3: s->pict_type= B_TYPE;break;

case 7: s->pict_type= I_TYPE;break; //ZYGO

default:

return -1;

}

skip_bits(&s->gb, 2);

s->no_rounding = get_bits1(&s->gb);

skip_bits(&s->gb, 4);

...

Maybe UFEP=1 is not in H283+ baseline ?

The H263 encoding function behaves like this :

int ufep=1;

/* H.263v2 */

/* H.263 Plus PTYPE */

put_bits(&s->pb, 3, 7);

put_bits(&s->pb,3,ufep); /* Update Full Extended PTYPE */

if (format == 7)

put_bits(&s->pb,3,6); /* Custom Source Format */

else

put_bits(&s->pb, 3, format);

put_bits(&s->pb,1, s->custom_pcf);

put_bits(&s->pb,1, s->umvplus); /* Unrestricted Motion Vector */

put_bits(&s->pb,1,0); /* SAC: off */

put_bits(&s->pb,1,s->obmc); /* Advanced Prediction Mode */

put_bits(&s->pb,1,s->h263_aic); /* Advanced Intra Coding */

put_bits(&s->pb,1,s->loop_filter); /* Deblocking Filter */

put_bits(&s->pb,1,s->h263_slice_structured); /* Slice Structured */

put_bits(&s->pb,1,0); /* Reference Picture Selection: off */

put_bits(&s->pb,1,0); /* Independent Segment Decoding: off */

put_bits(&s->pb,1,s->alt_inter_vlc); /* Alternative Inter VLC */

put_bits(&s->pb,1,s->modified_quant); /* Modified Quantization: */

put_bits(&s->pb,1,1); /* "1" to prevent start code emulation */

put_bits(&s->pb,3,0); /* Reserved */

put_bits(&s->pb, 3, s->pict_type == P_TYPE);

put_bits(&s->pb,1,0); /* Reference Picture Resampling: off */

put_bits(&s->pb,1,0); /* Reduced-Resolution Update: off */

put_bits(&s->pb,1,s->no_rounding); /* Rounding Type */

put_bits(&s->pb,2,0); /* Reserved */

put_bits(&s->pb,1,1); /* "1" to prevent start code emulation */

One thing is sure: what you sees as PPTYPE is not a PPTYPE but an "OPPTYPE", the PPTYPE is 18 bits "later".

Simon

Le Vendredi 24 Février 2006 15:29, Francois-Xavier KOWALSKI a écrit :

> Folks,

>

> FYI, the H.263 video bitstream generated by FFMPEG is an H.263-1998 with

> a syntax error in the picture header. This error is located in the

> mandatory PTYPE bitfield. It may cause H.263 decoder to abort the stream.

>

> ----------H.263+ : MANDATORY PLUS PTYPE----------

> pict_type: 000(0)

> reference_picture_resampling_mode: 0(0)

> reduced_resolution_update_mode: 0(0)

> rounding_type: 0(0)

> rtype: 0(0)

> reserve, reserve, spare: *101*(5) <= should be 001(1)

>

> The H.263 specs (I checked from H.263v1 to H.263v3) states the following:

>

> 5.1.4.3 The mandatory part of PLUSPTYPE when PLUSPTYPE present

> (MPPTYPE) (9 bits)

> Regardless of the value of UFEP, the following 9 bits are also

> present in PLUSPTYPE:

> – Bits 1-3 Picture Type Code:

> "000" I-picture (INTRA),

> "001" P-picture (INTER),

> "010" Improved PB-frame (see Annex M),

> "011" B-picture (see Annex O),

> "100" EI-picture (see Annex O),

> "101" EP-picture (see Annex O),

> "110" Reserved,

> "111" Reserved;

> – Bit 4 Optional Reference Picture Resampling (RPR) mode (see Annex

> P), "0" off,

> "1" on;

> – Bit 5 Optional Reduced-Resolution Update (RRU) mode (see Annex Q),

> "0" off, "1"

> on;

> – Bit 6 Rounding Type (RTYPE) (see 6.1.2);

> *– Bit 7 Reserved, shall be equal to "0";

> – Bit 8 Reserved, shall be equal to "0";

> – Bit 9 Equal to "1" to prevent start code emulation.*