[sldev] Meaning of RequestImage DiscardLevel field?

[Carsten Juttner]

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