[Mp4-tech] H.264 CABAC decode on mb_type

[EricChuang via.com.tw]

Dear experts,
I had a trivial question about the CABAC decoding on H.264 macroblock
type field.
According the spec., the ctxIdx of mb_type of I Slice should be between
3~10 from Table 9-11, but in reference decoder function
readMB_typeInfo_CABAC(), as below shows,
  if(img->type == I_SLICE)  // INTRA-frame
  {
    if (currMB->mb_available_up == NULL)
      b = 0;
    else 
      b = (((currMB->mb_available_up)->mb_type != I4MB &&
currMB->mb_available_up->mb_type != I8MB) ? 1 : 0 );
    if (currMB->mb_available_left == NULL)
      a = 0;
    else 
      a = (((currMB->mb_available_left)->mb_type != I4MB &&
currMB->mb_available_left->mb_type != I8MB) ? 1 : 0 );
    act_ctx = a + b;
    act_sym = biari_decode_symbol(dep_dp, ctx->mb_type_contexts[0] +
act_ctx);
    se->context = act_ctx; // store context
    if (act_sym==0) // 4x4 Intra
    {
      curr_mb_type = act_sym;
    }
    else // 16x16 Intra
    {
      mode_sym = biari_decode_final(dep_dp);
           .....
It seems the ctxIdx for binIdx 0 only depends on nebouring mb type only,
and there is no sigifiant place to restrict ctxIdx between 3~10.
Do I missed something ? or my understanding on the spec. is oncorrect ?
Thanks,
Eric
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