Re: Alpha Channel in real time
Re: Alpha Channel in real time
- Subject: Re: Alpha Channel in real time
- From: Dirk van Oosterbosch <email@hidden>
- Date: Fri, 17 Sep 2004 18:36:13 +0200
on 9/16/04 4:39 PM, Lorenzo at email@hidden wrote:
Hi,
I have a basic question.
I have just built an application which makes real-time graphics for
TV.
Now I would like to know how my graphics could be get out and put
over a
real time TV signal using the alpha channel. I mean, my software
should be
used in a TV direction room, and my graphic should go over the current
signal on air, in real time. Do I need a special video-card, or
special
harware?
My hope is that I can use a background window color with alpha = 1 so
I can
see only the animation and not the background. Then, I don't know
anything
else. Any help?
Best Regards
Lorenzo,
I suggest you take a look at three pieces of the QuickTime
documentation:
Sequence Grabbers, Component Output, and Video Effects.
Have fun...
- douglas
Hi Lorenzo,
as I understand your question, you have a similar situation I ran into.
I didn't solve mine yet, but not to let you dispair. Here's what I've
learned about realtime compositing alpha-containing animation of live
footage:
[ sorry, list, this is getting a bit off-Cocoa-topic ]
- First off all, it's too bad we all have mac's and not commodore
amiga's ;-) for these kind of tricks were already done on those systems
in the early 90-ties. But I won't be suggesting here that you should go
looking for an old amiga and video toaster package. :-)
- The standard way to get computer image onto a live tv signal (PAL or
NTSC) is to use a scan converter (like CORIOgen eclipse for instance).
It takes a XGA input and a composite video and will output a composite
video. The problem is that the XGA output of your computer won't be
holding an alpha, only RGB. So you can choose for standard (=limiting)
method, selecting one key color as alpha. So all blue is replaced by
video-image (like a blue screen). You can also take black or white for
that, but in the end you always loose one color (or worse range of
colors) you can use in your animation. You can also get this effect on
some of the more proffesional video-mixers (Panasonic MX-50 for
instance).
- There is a trick I thought of, but never tested though. You *could*
maybe also get your alpha channel in the video mixer, if you have dual
screen on your computer. If you have two outputs you can use one for
the RGB and use the other for the A, by only showing the alpha in black
and white. Those more complex video-mixers can use one channel as alpha
to mask out another channel over a third. (Tricky: you need to get a
real good video-mixer to be handling three channels at the same time
and maybe also two scan converters to get the *whole* computer image
over the screen and loose the black border)
- The last solution, I am also tending towards, is indeed to do it all
with QuickTime *inside* your computer, like Douglas is suggesting. You
still will need an analog-digital convertor, I think, to get the video
signal into your mac by Firewire. And I suggest you better get your
self a monster machine (does I hear dual G5?), for it will be quite
computational intensive to render your animation over the video layer,
maintaining 25 or 30 frames per second and the full PAL or NTSC
resolution. Also you have to keep in mind that you will always have a
delay (between the original video signal and the post-composite
computer-out signal). The digitizing process is just not as fast as
analogue video processing, ... and probably will never be (or is that
too pessimistic?)
Anyway, this last solution is certainly the most fun and promising (you
can also manipulate the video layer itself!). That is, if you can
afford it, otherwise I would just try the 2nd solution. It's not the
full deal, but gets you reasonably close.
- I also read something about a matrox video card (RTMac) for G4's with
which you could do realtime video, but supposedly that's only working
with FinalCutPro... (and I read they will not continue this product :(
.
Good luck, sounds like fun, indeed,
Dirk
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