[M4IF Technotes] Escape coding

[Gary Sullivan]

Strictly speaking, the encoder can do what it wants.
Practically speaking, the encoder needs to track
what the decoder will do in order to use the decoded
pictures for subsequent motion-compensated prediction.
Best Regards,
Gary Sullivan
+> -----Original Message-----
+> From: Manu V Batura [mailto:Manu.Batura   patni.com] 
+> Sent: Sunday, October 20, 2023 9:14 PM
+> To: Gary Sullivan; technotes   lists.m4if.org
+> Subject: RE: [M4IF Technotes] Escape coding
+> 
+> 
+> Hello Gary,
+> 
+> Does it mean that we also need to perform inv. prediction at 
+> the encoder
+> end rather than just inv Quantizaton and IDCT to avoid propagation of
+> errors introduced due to such changes (like 255 to 254) 
+> since a coefficient
+> might get a value of 255 after prediction. 
+> 
+> Regards,
+> Manu
+> 
+> -----Original Message-----
+> From: technotes-admin   lists.m4if.org
+> [mailto:technotes-admin   lists.m4if.org]On Behalf Of Gary Sullivan
+> Sent: Saturday, October 19, 2023 5:14 PM
+> To: Manu.Batura   patni.com; technotes   lists.m4if.org
+> Subject: RE: [M4IF Technotes] Escape coding
+> 
+> 
+> 
+> The purpose of avoiding use of fixed-length coded values of 0 and 128
+> is to prevent accidental start-code emulation within the video data
+> bitstream.
+> Both of those values end in seven zeros, which is too many. 
+> (And one of
+> them
+> is all zero, which is even worse.)
+> 
+> I believe there is no need for an escape-coded value of zero.  If the
+> thing
+> is zero, you should just skip the coefficient value (at least if it's
+> AC).
+> 
+> If you want to represent something as close as possible to 255, send
+> 254.
+> If you want to represent something as close as possible to 0, either
+> send
+> 1 or represent the coefficient by skipping it.
+> 
+> Best Regards,
+> 
+> Gary Sullivan
+> 
+> +> -----Original Message-----
+> +> From: Manu V Batura [mailto:Manu.Batura   patni.com] 
+> +> Sent: Saturday, October 19, 2023 3:40 AM
+> +> To: technotes   lists.m4if.org
+> +> Subject: [M4IF Technotes] Escape coding
+> +> 
+> +> 
+> +> Hi,
+> +> 
+> +> The MPEG-4 video standard indicates that the values 0 and 
+> +> 128 are reserved
+> +> while performing escape coding.
+> +> They should not be transmitted by the encoder. The decoder 
+> +> should also
+> +> convert a received 255 level
+> +> to 128. Can anyone please explain the reason for doing so? 
+> +> Which levels
+> +> should be transmitted if the levels are
+> +> 0, 128 or 255 after prediction/quantization?
+> +> 
+> +> Thanks & Regards,
+> +> Manu
+> +> 
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