<font size=2 face="sans-serif">I didn't looked at appendix B. But given
that at page 7 of the RFC it is stated: "...Each table is an array
of 64 values given in zig-zag order, identical to the format used in a
JFIF DQT marker segment..." then I agree that it couldn't be possible
to use all the code of the RFC specification. The fact that the quantization
table is not zig-zag ordered lead to confusion.</font>
<br>
<br><font size=2 face="sans-serif">Guy</font>
<br>
<br>
<br>
<br><font size=1 color=#5f5f5f face="sans-serif">From:
</font><font size=1 face="sans-serif">Alexandr Němec <a.nemec@atlas.cz></font>
<br><font size=1 color=#5f5f5f face="sans-serif">To:
</font><font size=1 face="sans-serif">LIVE555 Streaming Media
- development & use <live-devel@ns.live555.com>, </font>
<br><font size=1 color=#5f5f5f face="sans-serif">Date:
</font><font size=1 face="sans-serif">2013-12-01 18:42</font>
<br><font size=1 color=#5f5f5f face="sans-serif">Subject:
</font><font size=1 face="sans-serif">Re: [Live-devel]
RFC 2435 compliance</font>
<br><font size=1 color=#5f5f5f face="sans-serif">Sent by:
</font><font size=1 face="sans-serif">live-devel-bounces@ns.live555.com</font>
<br>
<hr noshade>
<br>
<br>
<br><font size=3>Hi Guy,</font>
<br><font size=3> </font>
<br><font size=3>thanks for your posting. I understand your point, but
I still think there is a problem with the RFC. Yes, appendix A says, that
you can use the example code to compute a quantization table only (zigzag
does not really matter), nothing wrong here. But later, in Appendix B it
says (comment before MakeHeaders) that you can use the MakeTables routine
from Appendix A to generate the quantization tables lqt and cqt as input
for the MakeHeaders routine. Then, in MakeHeaders (MakeQuantHeader) "memcpy"
is used to insert quantization table data into the JPEG header. And because
Appendix B also says that MakeHeaders can be used to "generate a JPEG
copressed image in interchange format", following these rules and
instructions will lead to a corrupted JPEG image.</font>
<br><font size=3> </font>
<br><font size=3>So what I want to say, it is just not possible to take
all the RFC example code "as is" for a working implementation.</font>
<br><font size=3> </font>
<br><font size=3>Best regards</font>
<br><font size=3> </font>
<br><font size=3>Alex</font>
<br><font size=3>______________________________________________________________<br>
> Od: <Guy.Bonneau@miranda.com><br>
> Komu: "LIVE555 Streaming Media - development & use"
<live-devel@ns.live555.com><br>
> Datum: 29.11.2013 20:35<br>
> Předmět: Re: [Live-devel] RFC 2435 compliance<br>
></font>
<br><font size=2 face="sans-serif">There is no issue with the RFC 2435
AppendixA neither with the live555 procedure to scale a quantization table
witha Q factor. Both are correct.. If you read the header of AppendixA
it clearly states the reason of the example code : </font><tt><font size=2> "The
following code can be used to create a quantization table froma Q factor..."
Now when you recompute the quantization table froma selected Q factor your
only scale the 64 quantization values by the samefactor. Doing the procedure
on zigzag order on doing it on "standard"non-zigzag order doesn't
matter. It will yield the same "scaled quantizationfactor". </font></tt><font size=3><br>
</font><tt><font size=2><br>
Jpeg encoding/decoding of spectral component of thepicture operate on block
of 64 values. Those value must be processed inzigzag order. If you use
a zigzaged table then the encoding/decoding onlyincrement the index of
the quantization value in the table to get the goodquantization value.
However if you use a non-zigzag table then the encoding/decodingmust find
and pick-up the good quantization inside the table. This is upto the implementation
of the code.</font></tt><font size=3><br>
</font><tt><font size=2><br>
Guy Bonneau_______________________________________________<br>
live-devel mailing list<br>
live-devel@lists.live555.com<br>
</font></tt><a href="http://lists.live555.com/mailman/listinfo/live-devel"><tt><font size=2>http://lists.live555.com/mailman/listinfo/live-devel</font></tt></a><tt><font size=2><br>
</font></tt>
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