[sldev] Meaning of RequestImage DiscardLevel field?

[John Hurliman]

I've ran some tests and I think I'm getting a better handle on how the image
request system works. However, I encountered a discrepancy between what the
LL servers think makes up a DiscardLevel and what OpenJPEG thinks. I've
posted the findings here: http://jira.secondlife.com/browse/SVC-3146

John

On Sun, Sep 28, 2008 at 10:45 PM, Carsten Juttner <carjay at gmx.net> wrote:

> Teravus Ovares wrote:
>
>> The client asks for discard level 5...      We send it bytes 0-1000.
>> The client asks for discard level 3 of the same image..    we know it
>> has already received the discard level 5 bytes..     so we send bytes
>> 1001-5000.
>>
>>
>
> Interesting, that would indicate the discard level in the protocol is also
> used as a kind of bookkeeping on the server. Which makes no sense really
> since the client is the only one knowing for sure how many bytes it has
> already received.
>
> The only reason to state the discard levels in this way would be to make
> sure the client receives the necessary amount of data without having to
> parse the packets.
>
> Which leads to another interesting question: how do they determine the byte
> offsets?
> Are they guessed (there is a kind of 1/8th rule of thumb built into the
> client so it could be the same for the server) or based on the real packet
> boundaries in the codestream?
>
>
>  But!, it would be nice to get a LL Developer's view on what Discard
>> Level actually represents.
>>
>>
>
> Yes, indeed. We've been asking about this for months and all the Lindens
> we've met so far tried there best to help (e.g. Qarl applied some client
> patches to improve the situation with OpenJPEG) but they were not familiar
> with the server code and didn't have the time to dig into it which is
> understandable but so we're still left with some open questions.
>
>
> Carsten
>
>
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