[M4IF Technotes] question about intra_dc_vlc_thr2

[Gaurav Aggarwal]

Looking for a clarification on an old post:
For an Intra block with Intra AC VLC turned on, suppose there was
only 1 DCT coefficient, then is it correct to say that the decode
of the first VLC DCT coefficient will give out a LAST==1? 
Or lets say that Intra AC VLC is turned on and cbpy[] for an intra
block is 0. Then it means that there is no DCT coefficient at all
for this block. Neither DC nor AC.
And to understand this completely, if the Intra AC VLC is turned 
off for the block and there is only 1 DCT coefficient, it means
that only DC coefficient is present. Hence, the cbpy[] for this
block will be '0' since there are no AC coefficients. Correct?
Gaurav
> -----Original Message-----
> From: technotes-admin   lists.m4if.org
> [mailto:technotes-admin   lists.m4if.org]On Behalf Of Kumar, Gautam
> Sent: Monday, February 24, 2024 8:29 AM
> To: tetsuo kosuge
> Cc: Technotes   lists.m4if.org
> Subject: RE: [M4IF Technotes] question about intra_dc_vlc_thr2
> 
> 
> Kosuge-San,
> 
> > What does it(mcbpc and cbpy in subclause 6.3.6 are changed 
> >accordingly)  represents ? It is  that (CBP==1) -> (CBP==0)
> > or (CBP==1) -> (CBP==0),or (Intra MB) -> (Inter MB) ? 
> 
> generally:
> 1. for inter mb we consider all 64 values(including dc value) in 
> a block to
> calculate cbp ((cbpy<<2) | (mcbpc & 3)).
> 2. for intra mb we consider only 63 values(excluding dc value) in 
> a block to
> calculate cbp ((cbpy<<2) | (mcbpc & 3)).
> 
> When the intra AC VLC is turned on, case 1 should be applicable for intra
> mb.
> 
> 
> Thanks and Regards,
> Gautam
> 
> 
> >-----Original Message-----
> >From: tetsuo kosuge [mailto:kosuge   td.idc.lsi.sanyo.co.jp]
> >Sent: Monday, February 24, 2024 6:39 AM
> >To: Kumar, Gautam
> >Cc: Technotes   lists.m4if.org
> >Subject: Re: [M4IF Technotes] question about intra_dc_vlc_thr2
> >
> >
> >Dear Mr.Gautam and other MPEG-4 Experts :
> >
> >Thanks to answer my question again, Mr.Gautam
> >
> >Your thouhgt is reasonable,I think so.
> >If (Qp >= intra_dc_vlc_thr && pattern_code==0),usually think 
> >dc_coeff exist.
> >
> >But I checked this sequence in microsft_vfdis_v10-990812 
> >codec,seem the bitstream doesn't obey that sequence.
> >In this bitstream, <pattern_code == 0> has priority over Intra_MB.
> >When (Qp >= intra_dc_vlc_thr && pattern_code==0), the block 
> >doesn't have dc_coeff(say has 0 value in all coeff).
> >The other bitstream which does't make microsft codec obeys the 
> >same sequence.
> >
> >And I read 14496-2:2001,this represents as follows:
> >
> >	7.4.1.4 Intra dc coefficient decoding for the case of 
> >switched encoding
> >	
> >	At the VOP layer,using quantizer value as the 
> >threshold,a 3 bit code(intra_dc_vlc_thr) allows switching 
> >	between 2VLCs(DC Intra VLC and AC Intra VLC)when 
> >decoding DC coefficients of Intra macroblocks,see Table 
> >	6-21.
> >
> >	NOTE	When the intra AC VLC is turned on,IntraDC 
> >coefficients are not handled separately any more,but 
> >	treated the same as all other coefficients.That means 
> >that a zero Intra DC coefficient will not be coded 
> >	but will simply increase the run for the following AC 
> >coefficients.The definitions of mcbpc and cbpy in 
> >	subclause 6.3.6 are changed accordingly.
> >
> >
> > What does it(mcbpc and cbpy in subclause 6.3.6 are changed 
> >accordingly)  represents ? It is  that (CBP==1) -> (CBP==0)
> > or (CBP==1) -> (CBP==0),or (Intra MB) -> (Inter MB) ? I can't 
> >judge correct sequence(for my lack of English kowledge? 
> >(TT) ).
> > Which is sequence correct?
> >
> >Bye, Tetsuo Kosuge
> >	 
> >

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