Hi Hans,
On Thu, 2019-09-26 at 15:23 +0200, Hans Verkuil wrote:
> On 9/5/19 3:12 PM, Philipp Zabel wrote:
> > Since pic_order_cnt_bit_size is not a syntax element itself, explicitly
> > state that it is the total size in bits of the pic_order_cnt_lsb,
> > delta_pic_order_cnt_bottom, delta_pic_order_cnt[0], and
> > delta_pic_order_cnt[1] syntax elements contained in the slice.
> >
> > Signed-off-by: Philipp Zabel <[email protected]>
> > ---
> > Documentation/media/uapi/v4l/ext-ctrls-codec.rst | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
> > b/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
> > index c281bc7ed1b3..08b49b2bbfa8 100644
> > --- a/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
> > +++ b/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
> > @@ -1799,7 +1799,8 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
> > - Size in bits of the dec_ref_pic_marking() syntax element.
> > * - __u32
> > - ``pic_order_cnt_bit_size``
> > - -
> > + - Size in bits of the pic_order_cnt_lsb, delta_pic_order_cnt_bottom,
> > + delta_pic_order_cnt[0], and delta_pic_order_cnt[1] syntax elements.
>
> It's a little bit ambiguous: this field contains the size in bits of all these
> fields combined, right? Not the size in bits of each field separately.
Yes.
> I.e. if these fields are all 8 bits, then pic_order_cnt_bit_size is 4*8 and
> not just 8.
The size of pic_order_cnt_lsb is determined by another field's value
(log2_max_pic_order_cnt_lsb_minus4), and the other three are
exponential-Golomb coded, so each can be of different length (including
zero).
> I think this should be rephrased.
Ok, how about:
"Combined size in bits of the picture order count related syntax
elements: pic_order_cnt_lsb, delta_pic_order_cnt_bottom,
delta_pic_order_cnt[0], and delta_pic_order_cnt[1]."
Actually, there's either pic_order_cnt_lsb + (optionally)
delta_pic_order_cnt_bottom present, or delta_pic_order_cnt[0] +
(optionally) delta_pic_order_cnt[1], never all four. Describing that in
the uapi docs seemed overly verbose, though. The relevant part in the
slice_header() syntax spec looks like this:
if (pic_order_cnt_type == 0) {
pic_order_cnt_lsb
if (bottom_field_pic_order_in_frame_present_flag && !field_pic_flag)
delta_pic_order_cnt_bottom
}
if (pic_order_cnt_type == 1 && !delta_pic_order_always_zero_flag) {
delta_pic_order_cnt[0]
if (bottom_field_pic_order_in_frame_present_flag && !field_pic_flag)
delta_pic_order_cnt[1]
}
pic_order_cnt_bit_size is supposed to be the size in bits of this whole
block. pic_order_cnt_type and log2_max_pic_order_cnt_lsb_minus4 are from
the SPS header, bottom_field_pic_order_in_frame_present_flag is from the
PPS header, and field_pic_flag is contained earlier in the slice header.
regards
Philipp
