[Mp4-tech] [H.264] Macroblock Skip at the border of the picture

[Tourapis, Alexis]

Dear Luca,
Unfortunately at the left and top slice boundaries the condition that a
skip mode has the same motion vector value as the predictor mv is
switched of (i.e., it is forced to 0). In your case, the predictor seems
to be (-16,0).
Best regards,
Alexis
-----Original Message-----
From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Luca
Sent: Tuesday, April 29, 2024 11:57 AM
To: mp4-tech lists.mpegif.org
Subject: [Mp4-tech] [H.264] Macroblock Skip at the border of the picture
Dear Experts,
I would like to ask you whether some of you can explain me this behavior
of the JM. When encoding some static sequences, it looks like that the
MB in the first row and in the first column of the picture are not
skipped increasing the mb_skip_run counter but rather encoded in the
following way:
*********** Pic: 1 (I/P) MB: 2 Slice: 0 **********
@1499 mb_skip_run
       1 (  0)
@1500 mb_type (P_SLICE) ( 2, 0) =   1                                  1
(  0)
@1501 mvd_l0 (0) =   0  (org_mv -16 pred_mv -16)                   1 (
0)
@1502 mvd_l0 (1) =   0  (org_mv   0 pred_mv   0)                    1 (
0)
@1503 CBP ( 2, 0) =   0
        1 (  0)
I would like to ask you if this is equivalent to skip the macroblock
and, in this case, why the encoder not decide to directly skip it by
means of mb_skip_run.
Thank you,
Luca.
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