Multicast Directshow Graph Bridging, COW Rustling, & the Use of Portable Holes in Cartoon Physics.
- From: Jamie Faye Fenton <jamiefaye@xxxxxxxxx>
- Date: Sun, 18 May 2008 16:34:20 -0700 (PDT)
Hello Boys and Girls. I am a mad scientist on the loose who wants to
hack DirectShow and its progeny in new and even more unusual ways! I
have already done a number of strange things, most recently a
"Buffered-data switching matrix w/ plug in media processors." Beyond
its potential as an acronym, this DS Filter collection uses mapped-
memory to share buffer pools across processes and works pretty well.
You get two basic Direct Show filters, a $ink and a $ource. (or Sink
and Source if you are likely to hate it when people substitute $ for
S). There are also specializations that act more like renderers or
capture filters, including one that combines 4 channels into a 2x2
quad.
A sink or source can be commanded to "tune into" a virtual channel
group and either copy buffers into or out of the ring so selected. You
can have any number of channels and any number of groups, the mapping
can be changed in a few microseconds, and video formats and sizes are
either renegotiated or converted on the spot to what each receiver
requires. Fan-out is not limited. Buffering can be set to whatever,
typically 4-6 frames works out well for the relatively low bandwidth
stuff web-cam applications demand.
But wait, there is more! Each source filter can be sent an XML
description of a synthesizer patch to run during the time it transfers
the buffer into the destinations world. Each output stream has its won
video-oriented virtual machine with a frame stack and a list of plug
ins to call in sequence (I use the Freeframe open standard for this,
which gives us 300 or so effects, and will likely add the Japaneze
EffectTV package and whatever else I can scrounge).
So we now have a Swiss Army Direct Show filter that you can put into
your old, inflexible, DS Filtergraph of your choice and have it re-
emerge in other places and in other roles, with add-on eye candy. You
can share web cams indiscriminately, glue video applications together,
and do a wild variety of other things people always ask about on this
newsgroup but get frustrated doing.
I may even ship the darned thing. I am hesitant, mostly because doing
it the first time gave me insight on how to do it better. What will
likely emerge is a scheme based on what I already have with a
transitional road-map to get where I want it to go.
So what do I want to do that I can't do now? A big one is to copy
frames less. While I am pretty careful about that topic, there are a
few places where I can squeeze better performance out of things. I
also want to keep the semantic noise down - by doing DirectShow stuff
using DirectShow compatible APIs, etc., to the maximal extent without
buying into some of the policy choices made way-back-then. I also want
to add YUV formats, audio, and add a lot more to the DirectX side of
things.
So here a few reseach directions.
Hacking the GMFBridge Toolkit - In particular I want to add "multicast
eavesdropping." This means we have the GMFBridge work like it already
does, except that we graft on our own allocator that does some
trickery. The sink and source filters in GMFbridge work as they
always did, but it will also be possible to "Eavesdrop" on the signal
flow, much the same way my Money$hot source filters peer into a ring
buffer. An ideal scheme for this would have the grafted-on allocator
issue each GMF-source a virtual memory alias with COW (Copy on Write)
semantics. Unlike the infinite Tee, each GMF-Source would gets its
buffer right-away, and only the prime-recipient (or non-eavesdropper)
would be able to influence the quality-control loop, etc.
Since the way COW works in Windows is that each view gets its own
version of whatever it changes, the original buffer is unmolested. (We
can allow the non-sneaky sink to be an exception if that matters).
Aside from the fact that a lot of MediaSamples already do shared-
memory stuff (which may make some of the sneaky stuff I have in mind
easier), are there any show-stoppers that any of you know about? (I
expect to handle the renderer./ video memory problem by either copying
or mapping when I can. DRM I figure will go away pretty soon, or the
angry mobs will burn their villages to the ground.
Thinking about virtual memory hacking has made me consider this
"Willey Coyote" scenario: I know there is a buffer somewhere, perhaps
in another process, that has data I want to have a copy of. I want to
put a portable hole under that roadrunner. What I want to do is in a
"close to atomic" operation declare myself to be one of the viewers of
that buffer as a shared-memory or memory mapped file, and let the
other guy view it the same way. I write to that zone, I go COW, he
writes to his, he goes COW. What I got was a temporal snap shot that I
can work with at my leisure. Where this gets tricky is when the
address range is already mapped to some other shared-memory scheme and
we don't want to trouble our victim in any way...
Another wacko idea is to hang out where he derefs his Media Samples
and returns them to his allocator pool. Then I cut a deal with the
allocator where he gives me non-zero pages full of sample-stuff and I
give him nice fresh zeros in return. (Note this isn't a hack for
electronic espionage, I just want to copy my samples a few times less
then necessary if I can get away with it). Here the big issue is that
everybody links to a different version of the base classes, so placing
intercepts could be a nightmare.
I already do tight windowing on my video buffers, and hence create
plenty of page-faults. It would be nice to have fewer faults per
frame. Perhaps kernel-mode driver hacking will be my next-big-thing.
As for Money$hot, I will hopefully be going beta pretty soon and I
will announce that when its ready to go. I am planning to make it an
open-source/free ware kind of thing. I also have to get it done and
find some work that pays, since Money$hot is a good description of my
financial affairs. So if anyone out there needs a Direct Show expert
from the lunatic fringe, I am available.
As for the technical issues and implications of some of the ideas
mentioned before, your insights are vital. It is so easy to blunder
around in the dark with this API and I have multiple bruises and
scars. Still I think the creation of a flexible and elegant multimedia
API is possible and know that our group here knows how.
.
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