[Mp4-tech] H.264--Questions about POC

[Zhao Ran]

Hi, all
I have two questions about poc. Is there anybody could help me make them clear.
As we know, in the bitstream created by jm refenrence encoder, the poc increments 2 every complete frame, just like 0, 2, 4, 6,.. 
Then, is standard permit such a bitstream which has poc incremented by 1 for every complete frame, just like 0, 1, 2, 3,..
Another problem confused me is that is a negative poc allowed. I suppose that for IDR picture, the poc should be equal to 0, and followed pictures should have poc in ascending order. In other words, poc should be positive or zero.
However, for pic_order_cnt_type==0, poc is derived from pic_order_cnt_lsb and delta_pic_order_cnt_bottom. The former is in the range of 0 to MaxPicOrderCntLsb-1 and the latter is range of -231 to 231-1. There might be a bitstream with a bottom field picture having pic_order_cnt_lsb==2 and delta_pic_order_cnt_bottom==-4, well then the poc would be negative. Is such a bitstream valid?
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