[Live-devel] RFC 2435 compliance

[Guy.Bonneau at miranda.com]

There is no issue with the RFC 2435 Appendix A neither with the live555 
procedure to scale a quantization table with a Q factor.  Both are 
correct.. If you read the header of Appendix A it clearly states the 
reason of the example code :   "The following code can be used to create a 
quantization table from a Q factor..." Now when you recompute the 
quantization table from a selected Q factor your only scale the 64 
quantization values by the same factor. Doing the procedure on zigzag 
order on doing it on "standard" non-zigzag order doesn't matter. It will 
yield the same "scaled quantization factor". 

Jpeg encoding/decoding of spectral component of the picture operate on 
block of 64 values. Those value must be processed in zigzag order. If you 
use a zigzaged table then the encoding/decoding only increment the index 
of the quantization value in the table to get the good quantization value. 
However if you use a non-zigzag table then the encoding/decoding must find 
and pick-up the good quantization inside the table. This is up to the 
implementation of the code.

Guy Bonneau 




From:   Alexandr Němec <a.nemec at atlas.cz>
To:     LIVE555 Streaming Media - development & use 
<live-devel at ns.live555.com>, 
Date:   2013-11-25 07:06
Subject:        Re: [Live-devel] RFC 2435 compliance
Sent by:        live-devel-bounces at ns.live555.com



> This same question (I think) came up last January.  Here is the answer 
that I gave at that time:
 
Thanks for your quick reply. Sorry, I googled through the archives, but 
did not find this one. The "zigzag" idea is the correct answer, as this 
order is required for the JPEG DQT segment. So Live555 is absolutely 
correct here, but it turns out, that the code in Appendix A of RFC 2435 
omits to use the reordered tables when creating jpeg headers, ie. it 
contains a bug. I just could not believe that a RFC code example can 
contain a bug.
 
Thanks.
 
Best regards
 
Alex
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