h264 syntax (P frames syntax details)

Question

I'm parsing h264 NAL Units. In my sequence - there are NAL Units with picture parameters set, sequence parameters set, I frames (they are also IDR) and P frames (I and P frames consits of single slice). (No B frames at all)

So I have the stream of NAL Units looks like:

[SPS] [PPS] [I(IDR)] [P] [P] [P] ... [P] [P] [SPS] [PPS] [I(IDR)] [P] [P] [P] ....

Each I frame in my stream is also IDR frame, so its frame_num is 0 (acording to h.264 standart).

Also each P frame has frame_num per unit greater than the previous frame.

But, I'm confused about pic_order_cnt_lsb.

What does pic_order_cnt_lsb represents?

In my sequence:

• if P frame has frame_num==2 its pic_order_cnt_lsb==4
• if P frame has frame_num==3 its pic_order_cnt_lsb==6
• if P frame has frame_num==4 its pic_order_cnt_lsb==8
• etc... (in my situation pic_order_cnt_lsb == 2*frame_num)

And why there is such correlation between frame_num and pic_order_cnt_lsb?

Thanks,

Answer 1

It looks like you have interlaced data. With interlaced data, one frame is composed of two fields. The other field of each frame will have the odd values for pic_order_cnt_lsb.

pic_order_cnt_lsb is just the least significant bits of a counter tracking the order of the pictures. If (as in typical interlaced data) you have two pictures per frame, then it'll increment by two for every change in the frame_num. If your stream was progressive instead of interlaced, the two would increment together.