You could in theory use a QTPixelBufferContext (rather than the
QTOpenGLTextureContext) and request RGB frames from it, and then pass
the CVPixelBufferRef to QC as well, but I think that will be less
optimized speed because QTOpenGLTexture Contexts (I believe) can
optimize with YUV frames and YUV texture formats for lower bandwidth
(and figure out what is 'right' per movie), and you may have some
conversion overhead to your requested pixel buffer format. But that
should work, and you can grab the CVPixelBufferRep and do whatever you
want to it in serial/parallel to QCs processing. Just make sure to use
the CVBufferRetain/Release stuff on it.
I also think there is a way to request both a CVPixelBufferRef and a
CVOpenGLTextureRef. Another thing to be aware of is dealing with the
working and output space, set those keys on your contexts when you
make them, to make sure your color looks right in QC.
Hope this helps.
On Sep 22, 2009, at 11:43 AM, Chris Wood wrote:
2009/9/22 vade <[email protected]>:
I would highly highly highly suggest using a QTOpenGLTextureContext
and
insert frames with a CVOpenGLTextureRef, using
QTCaptureVideoPreviewOutput,
and using setVisualContext on it. NSImage is really not the way to
go if you
are looking to avoid latency.
Ah, and CVOpenGLTextureRef is one of the supported types for a QC
input. Excellent! One question though: what's the best way of getting
the image from quicktime before it's set up as a texture? I'm going to
need to do some processing on the images on the CPU, in parallel with
the QC stuff, so it makes sense to grab it early and not waste
bandwidth on the bus.
Thanks!
Chris
On Sep 22, 2009, at 10:14 AM, Christopher Wright wrote:
Ah, that does sound promising. I'll try throwing a pre-loaded
image at
the input port when the capture session is stopped, see what
happens.
Good idea -- let us know how that goes.
Is adding the CVImageRep to an NSImage then passing that to the
QCRenderer the most efficient way to handle it btw?
You're in tradeoff land here, if QTKit is in fact reclaiming the
texture
(still speculative) -- the CVImageRep may be the most efficient in
terms of
code, memory usage, and execution speed, but it's also non-
functional with
this use case (again, speculatively). I'm _guessing_ you can keep
doing
this, but have some method of cloning the texture when you stop
the capture
session so that it doesn't get freed. This should keep stuff
running
smoothly without requiring you to change up your pipeline. I
don't know how
easy/impossible it is to copy CVImageReps...
Backtrace below (it does appear to crash when setting a port value
within QC - see thread 1).
(note: please include headers when posting crash logs -- a few
weeks ago
someone posted a crash log to the QT-dev mailing list, and the
clue was
actually in the header (they were using NSOperations on a 10.5.6
system, and
NSOperation had a subtle race condition on x86 processors prior to
10.5.7
that could cause the app to crash with an unhandled exception --
the actual
backtraces weren't useful in that case))
Looks like a standard message-send-to-a-freed-object crash, so
that's
promising.
unrelated: looks like you're building from a flash drive; how
has that
worked out for you? are builds noticeably faster/has drive
reliability
dropped any? (I do dozens to hundreds of builds a day, so if it
saved a few
seconds, and didn't toast the flash drive, I might consider that for
myself... I always hear about how flash can sustain a bazillion
writes now,
but I still have my doubts...)
--
[ christopher wright ]
[email protected]
http://kineme.net/
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