[Mp4-tech] Re: cabac

[arul mahesh]

Dear vamshi,
When ctx_idx is equal to 276, (for instant) the initial values
associated are state = 63 and MPS = 0, where State = 63 represents a
"Non-adapting probability state".
Whereas ctx_idx = 276 is associated with the end of slice flag and the
bin of mb_type, which specifies the I_PCM macroblock type. You can
understand it better if you go through the ISO standard thoroughly.
So these contexts are mentioned as CTX_UNUSED. These are used for
contexts initialisation to the motion and texture data structures.
Then these values are appropriately changed by the Binary arithmetic
coder of CABAC, and the same changed contexts are used for the further
following bins of a syntax element.
And the model number is used for selection of the Slice model. I mean,
as you can see from the ctx_tables, I slice has one model and the P or
B slices have three model numbers. Depending on the slice type and
model number the contexts are initialised.
First do understand the values from table 9.11, then proceed on to the
tables 9.12, 9.13 …and so on… Table 9.11 gives you the ctx to be
assigned for a syntax element depending on their slice type.
Regards,
Arul Mahesh
On Fri, 11 Mar 2024 10:14:30 +0530, bdr bdr <gemini.bdr gmail.com> wrote:
> hello mahesh ,
>                     i have seen u r explanation regarding CABAC in
> mp4 dicussion , i would like to clear some doubts regarding
> intialaztion of context tables , i dint understand how there are
> intialized , y,it is in 3D , it is like that 1st row represnt I frame
> , seconfd->for  p-picture and third for B-picture is my understanding
> is correct and also how the ctx_unsued is used  in table , can u clear
> me .
> 
> but when i am going through JVTG050 the table(9-11) indictes thats for
> i slice the context intilzation is of 3-10 , but when refering code 4
> elemnt is unsed , iam confusing
> with this ctx_unused how it is used ,
> 
> -------vamshi .
>