Subject: Re: [linux-audio-dev] [RFC] Lite OSC API
From: rd_AT_alphalink.com.au
Date: Mon Jan 26 2004 - 15:59:30 EET
> OK, interesting - it wouldn't be a huge task, but why is this a good
idea?
> It just seems inconvienient for the recipient and I dont know of any
other
> RPC mechanisms where its allowed (or wanted). In the case of
typetag-less
> messages it would be difficult to handle, you'd have to try to discover
> the type of the argument by inspection. Do you have a link to a
rationale
> for this?
Firstly, from the horses mouth, "non-type-tagged messages [are] totally
deprecated". See
<http://www.create.ucsb.edu/pipermail/osc_dev/2003-November/00032
9.html> and surrounding posts. I don't think you need to even try to think
about possibly considering perhaps partially supporting such things!
Secondly, it is actually no work for the receiver. The receiver says that is
needs a signed 32bit integer and it gets such an integer. It is a little
work for the library. On the other hand it can be a lot of work for the
sender. Domain specific languages often do not implement full C like
numerical towers, a signal processing language could easily only
implement floating point numbers, Lua famously has only double
precision floating point numbers, and lots of people use Lua.
Since I think this is really important I will give an example, and from the
other end of a continuum. I use scheme for most of my music work.
Scheme has a very sophisticated numerical tower. I might write (->
"/n_set" 1001 "freq" 440) to set the frequency of an instrument at SC3.
The OSC encoder encodes values based on their lisp type, 1001 is an
integer and gets encoded as 'i'. Lets assume that to the receiver the
frequency argument is a float, here I lose because 440 is an integer. I
argue that a receiver implemented like that is just wrong, that an integer
440 frequency is completely valid, and SC3 thankfully agrees and plays
the right note.
However if you disagree and say that I should write the literal as a float
then that makes scheme a hopeless language to work in, 440.0 is still
an exact integer, I need to write #i440 to get an inexact integer. But the
situation is far worse, if I write (* m 1.5) this seems like it should make
an inexact value regardless where m arrives from, however scheme
knows that (* 0 1.5) is an exact zero and I lose again, it may even know
that (* 2 1.5) == (* 2 3/2) == 3, an exact integer . Forcing a user to _very_
carefully annotate _all_ code that might get sent as an OSC packet
_only because_ the OSC receiver cannot accept exact integers is not
going to work, I just won't be able to work with the process. SC3 gets
this very right, as usual, and in fact allows:
(-> "/n_set" #i1000 (exact->inexact 0) 440)
The node and index values at SC3 are syntactically integers (the index
can also be a string name that maps to an integer), and the argument is
syntactically a float, but it accepts floating point encodings of integers
and integer encodings of real values. SC3 does it like this, QED :)
> I agree that combining the verb and subject seems a little odd, but that
> is the sitatuion in the web services world too - when neccesary they
get
> round it by adding a verb argument eg ("/my/path", "verb", ...), which is
> a useful cnvention but its not neccesary to set it in stone.
I think the real question is, does anyone really want to do pattern
matching on the operator? I have never seen a convincing example. It
would introduce all sorts of really strange sequencing issues to start
with... The pattern matching people seem to like is usually of the form
("/instr[1-4]/freq" 440), not ("/instr1/{freq,ampl}" 330). If you restrict the
matching to the subject, make the subject an argument, and make the
operator the OSC address then the OSC dispatch mechanism can be
reasonably efficient, which matters in RT synthesis environments, and
people who want RE descriptors for subjects can do that without
imposing an onerous burden on the core dispatch mechanism. And it
looks better! In fact it looks comfortingly like lisp, ("/freq" "/instr[1-4]" 550)
:-\
> > * not allocate any memory or call any other non-RT safe procedure
> This one is inherantly not possible, youre doing i/o operations.
Well actually it is only doing the byte encoding. SC3, being dragged out
for yet another example, allows users to link in the synthesis engine and
push OSC packets directly onto a queue, thus avoiding any IPC
operations but still using an identical communication protocol. For the
liblo case you are of course correct, but the temporal behaviour of the
encoder/decoder can be pretty predictable.
> I've always been more of a strongly-typed guy :)
Me too, we schemers all are, we just think that an exact integer is a real
number :)
Regards,
Rohan
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