[Mp4-tech] [H.264] Is slices based on only a picture or on whole picture sequences?

[Gary Sullivan]

A slice can basically be reconstructed independently of other slices in the picture.  However, you are correct to notice that the deblocking filter can have an effect across slice boundaries.  The encoder can turn off the effect of the deblocking filter across slice boundaries if it wishes to do so.  However, if the encoder does not turn it off, then slices cannot quite be decoded exactly correctly when nearby slices have been lost.
Actually, the standard does not specify how a decoder will respond when some slices are lost.  The design is intended to be reasonably robust to such losses, but there is no requirement in the standard governing what to do in such cases.
Applying the deblocking filter after performing some reasonable loss concealment method (such as copying the corresponding region of a temporally-nearby picture) is a reasonable response to loss of a slice.
To answer your additional numbered questions:
1. Not perfectly (depending on what value the encoder sends in the syntax element "disable_deblocking_filter_idc").
2. Yes. (The encoder can voluntarily choose not to send such motion vectors, but that is a choice made by the encoder.)
3. No.
Best Regards,
Gary Sullivan
+> -----Original Message-----
+> From: mp4-tech-bounces lists.mpegif.org
+> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of lurui
+> Sent: Thursday, October 25, 2023 7:35 PM
+> To: mp4-tech lists.mpegif.org
+> Subject: [Mp4-tech] [H.264] Is slices based on only a
+> picture or on whole picture sequences?
+>
+> Dear expert:
+>     I know slice is part of a picture, it means a slice can
+> be reconstructed independently of other slices within a
+> picture level.
+>     The question is:
+>     1.If a slice of a picture lost, can the deblocking
+> procedure correctly do without the lost slice?
+>     2.Can motion search across the slice boundary of the
+> slice of the reference picture? For example, if the previous
+> reference picture is composed by two slices--the upper slice
+> and the bottom slice,can a MB belongs to the bottom slice of
+> the current picture search it motion vector that points to
+> the upper slice of it previous reference picture or even
+> points cross the boundary of the slices?
+>     3.Does the slice number and size and start-end postion
+> must be same for every picture under a picture sequences to encoded?
+>
+> lurui
+>
+>
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