#ALPHA CHANNEL
32 messages in this thread
HELP!
I'm having alpha anxiety, I have used 3DS for some time now (R1, R2, and R3)
and have always output to BetaCAM SP. We have always just keyed the signal
over other video by doing a simple chroma-key. I have never taken the time to
figure out if I can take the image directly out of my Matrox (ILLum-Pro) frame
buffer and us the alpha channel to key with. Is this possible, the manual
doesn't go in to the video side very much.
Thanks in advance!
Jimbo
That's not the way you want to do it. To make an alpha key channel, turn
Render Alpha ON, and turn Alpha Split ON. (read up on Alpha Split in the
manual for more info) This'll give you a seperate animation of just the matte
channel.
– G
Gary, Thanks for the info. But how does the alpha information get to tape any
differently than just the RGB signal? Is it encoded with the component output
and is there a way to do this without using a "matte channel"? My editor seems
to think that this was the way it was done long ago. Thanks for your help, the
NBC affiliate job I have been working on is about ready to air and I want to
make sure it is perfect.
Thanks
JImbo…AMI
I do not believe there is ANY way to get alpha information on to tape. Alpha
in 3DS is strictly computer stuff. It is a greyscale map of transparency
information, like a sort of high contrast black and white version of your
rendering. I imagine someone could write a routine that could take the 8bit,
seperate "alpha split" file and turn it into video alpha info, but I'm no
programmer, and I'm not even really clear on the use of alpha signals in live
video.
John Tissavary (LUNA cie)
John,
My matrox Illuminator pro is outputing an alpha signal. This signal goes to
the "ISO" input in the edit suite which should give an alpha matte on the
switcher when keying to the ISO…..Or so I'm told! If it isn't possible to us
an alpha channel or "negative black" signal to key an animation over video, how
am I supposed to get a clean key? Currently we do a chroma-key, but that
doesn't work if you have and dark shadows. Do you know of anyone that is
successfully keying (very clean) over video without chroma-keying? Any other
method?
Any help would be appreciated.
Jimbo
Jimbo:
>> Do you know of anyone that is successfully keying (very clean) over video
>> without chroma-keying?
The BEST method of doing this is with an A/B/C roll system, where A is the live
video background, B is the animation, and C is the alpha channel.
Just render with alpha ON and alpha split ON, and take the resulting 8 bit
files out to tape for your C roll.
We do all our broadcast titles this way, as well as many other types of
animation where we must do live compositing.
Greg Pyros
>> The BEST method of doing this is with an A/B/C roll system, A is the live
video background, B is the animation, and C is the alpha channel. <<
Sorry but this is incorrect. The best way is to digitally composite your
animations with a 4:4:4:4 system like Digital F/X. The next best solution with
a typical A/B/C roll system is the technique I suggested.
Just to keep the facts straight. <g>
-JE
John:
>> I do not believe there is ANY way to get alpha information on to tape.
See my message to Jim about how to get the alpha info out!
Greg Pyros
I figured A/B/C roll was how to do it. But James Beyer was asking if he could
get Alpha from a 3DS .tga file going out of his Illuminator Pro board (which
has alpha out), which is, if fact, impossible without writing some sort of
special routine that could take an alpha split and load it into the second
buffer as the rgb image was being loaded, and I don't even know if that would
work 'cause I'm not sure of programming limitations of the Ill Pro.
John Tissavary (LUNA cie)
John and Greg,
My original intent when starting this thread was to find out more about 3ds's
use of the alpha channel and how my Matrox Illum Pro board treats the signal.
It would seem to me that the alpha out of the board should output the alpha
information from the rendered .tga file (whether it is being rendered at that
time or just loaded in the buffer, as I do when doing frame by frame out to
tape).
The solution Greg suggested worked: we rendered a matte version of the
animation and the animation itself. We then took the signal from one betacam
deck and connected it to the ISO input of the switcher. This signal was then
keyed with ADO which can generate it's own "negative" black. This entire
signal was then recorded on 1" tape for a second generation version of the
final animation which can be keyed over anything quite cleanly.
My original quest still stands: Since the alpha channel from my matrox card is
connected directly into our Ampex switcher ISO inputs, why can't the alpha
information (that my matrox board outputs every time a frame is rendered) be
used just like any other ISO input and cut the matte for the rest of the
animation/frame? This in turn is mixed with my ADO's negative black to get a
first generation, keyable recording.
Maybe I just don't get it, but it seems to me that if a board is going to
output an alpha channel on a separate cable it should be useful. Unless the
alpha channel signal out of the board isn't loaded at the same time an image is
loaded in the buffer, this would explain it.
Thanks to everyone for their responses to this thread. It has made us stop and
look at our capabilities and how to integrate new ways of doing things. Thanks
for the help….Just wait til my PAR gets here 🙂
Jimbo, "Too high-tech for his own good in Des Moines", Beyer
Jimbo:
>> The solution Greg suggested worked
Aaaaaarrrrright! <g>
The problem with the separate alpha is that you still need to record it on
something for frame-by-frame animation. Unless you have two synchronized VTRs
(interesting concept!) That will both record a single frame simultaneously,
one the image and one the matte, your matt only works as a still.
But glad the idea helped!
Greg Pyros
James,
By using Alpha Split you get two images. One is the "matte" which is a white
"fill" of the objects in your animation. The other is the rendered image. By
laying the two seperately to tape, you have a "key" signal (alpha matte) and
the "fill" your graphic image. Then using a true key, you key into the alpha
matte which is digitally composited into your video. The matte cuts a video
"hole" into which your animation sequence is inserted. That's the way its done
except for some high end propietary systems which are able to split the
channels. You do have a key signal out which can be used for graphic stills
with 32 bit to cut a "hole" in your video and fill it with your graphic still.
But imagine what has to happen to do it the way your are thinking about.
First you would need to have an image you wanted to key into, then you need the
key signal and finally a frame of animation. Now, if this isn't happening in
real time, then you need a system which will display a frame insert the key
signal and composite in the animation frame. This would have to be done "frame
by frame".
Were you thinking of running the animation once created through the Illuminator
Pro in real time with say a tape deck, and have the Illuminator Pro do the
keying? That is possible but where does the Illuminator Pro get its key signal
when playing a tape through it, as tape decks don't support an alpha channel?
This would have to be an internal luminance key. Both luminance keys and chroma
keys have their drawbacks. A luminance key is unsatifactory if you have any
dark areas in your image. A chroma key is unsatisfactory if you have any colors
which match your key color. So the best way is to create a "flying matte" which
will cut a hole into the video signal into which you can insert your animation.
Neither tape nor DDR's at this time support Alpha except on high end
proprietary systems. It would be nice if the PAR or the MAX provided a seperate
key signal which it could read from alpha or a seperate matte like 3DS creates.
This would allow you to play back animations in real time and provide a true
key signal.
So the long and short of it is, until tape decks support a key channel or DDR's
support a key channel, you need to create an "alpha" matte seperate from your
animation and run them on two seperate decks, one as the "key" and one as the
"fill" and key insert them into your video.
I hope this clears up your questions.
-JE
Thank you John. Excellent description of the process.
– G
3DS does make it a lot easier being able to render a "flying matte"
simultaneously. I used to have to do a "matte in" after I rendered to get the
same result. I guess its pretty common now but it wasn't before the release of
3DS.
-JE
When we first put that alpha split stuff in, barely anyone knew what a
downstream keyer did in an ABC-roll system. Now everybody does. <g>
– G
Pretty amazing, isn't it.
-JE
Wow, that one's getting filed away for future reference. I was thinking along
those lines, but do not know enough about video and video equipment to be able
to have come up with a detailed description like that. I was thinking the
split alpha could be used to key through the Illuminator Pro, which does have
an alpha channel cable out. I'm not sure how it would come together, though.
Maybe a proprietary routine that loaded one of the two buffers with rgb, and
the other with the split alpha image… hmm… but would it work?
John Tissavary (LUNA cie)
John,
Because the Illuminator Pro has a "key" signal, in theory the Max could support
it. It would require that the Max maintain an alpha channel of an image which
could then be sent out the Ill Pro's key channel during playback. This would
enable you to do real time output with a true "key" signal.
I have not heard of any plans to do this but it would be a first in the PC
domain. You might be able to contact them and ask if they have plans to do this
or what would be involved in making this possible.
If they do support the key signal on the Ill Pro with the Max, you should be
able to run live video into the Illuminator Pro and do a clean key with your
animations in real time. Now that really would be something.
-JE
I'm quite sure the folks at Drastic are pretty open minded, and I'm going to be
in touch with them about all sorts of little things, this will be very high on
my list. I think features like this will show the real strength of the system
as a videographics/animation setup.
John Tissavary (LUNA cie)
John,
>> I think features like this will show the real strength of the system as a
videographics/animation setup. <<
I agree and I don't think anyone has come closer to a fully integrated system
as Matrox has. They have been working on many advanced features and have
without a doubt made the most progress in bringing high-end video to the
desktop PC. I'm glad you have their ear as I think you can make a significant
and positive difference in the way they approach the animation market.
-Je
I'm going to be reviewing the MAX, and will be in touch with Drastic
Technologies (who wrote the software). I've been told they're a pretty cool
bunch. Hopefully they'll be responsive to suggestions such as yours.
John Tissavary (LUNA cie)
John,
I've got to correct my terminology before Don calls me on it. <g>
I've been calling a "linear" key a "true" key. I use the term "true" key as
there is a distinct key signal as opposed to a key which uses luminance or
chrominance off of which to key.
A "linear" key is a "true" key.
-JE
All that stuff is lost on me <g>. I hope Don is listening in so he can get the
benefit of the correct syntax <g>.
John Tissavary (LUNA cie)
Isn't this possible with a Targa+ and TIPS? I thought I did this when I first
got my targa as an experiment. I was able to key any graphic or computer image
onto the the live video. This is more like a chroma KEY but the effect was
there. Not the same though as using the alpha info to feed a switcher. I've
never did that here, either. Must try it some time. How would you suggest I
utilize the alpha split matte into the key input of the switcher? ie I think I
know how to wire it but how would I sync the animation of the alpha with the
real thing? Would I first record the alpha matte to tape and play back this
to the key input then set up the PAR with the GPI Trigger to start at the same
time feeding its signal to the B-roll with the live video to the A-roll. The
result would be instead of a dissolve there would be a key of the PAR animation
that would match the corresponding matte in the key input. This sounds like it
could be useful <G>
Having the ability to do this split within the Ill Pro sounds like one of those
reasons one might want the MAX over the PAR. You wouldn't have to do the setup
like I described above but just feed to alpha output to the switcher and then
just roll the MAX animation. It would be like the PAR would be playing two
simultaneous animations, alpha split and 24 bit animation. This is a great
thread! I'm learning new tools to try.
Don,
I started to answer your question and realised you'd answered if for yourself.
<g> I think you got it exactly, record the alpha matte to tape and use the GPI
to trigger the Preroll or however you have it set up. You could use the PAR to
record the alpha matte and play it back in realtime at what ever point you
specify. Should work very nicely, and is very useful because you don't limit
yourself to high contrast "flying logos" etc, and you don't have to worry about
"super black".
Always nice to see you excited about a new use for your tools. <g>
-JE
James (who asked the question) and John, Greg, John and Don:
Thanks for an EXTREMELY interesting and useful thread. You can bet this one is
getting filed away for future use. I just luminance-keyed an animation over
some live footage and the results were only so-so. Even taking advantage of
3DS super-black didn't provide for perfect keying, since my Hi-8 tape was a bit
noisy. Now I know the RIGHT way for next time!
>> Thanks for an extremely interesting and useful thread. <<
You're very welcome. It certainly does make a big difference when integrating
animation and live video. You can really take advantage of all those intrecate
subtle nuances that 3DS is capable of producing. (To the limits video will
allow. <g>)
-JE
I'm pretty psyched about this thread, myself. Now I've got a juicy request for
Drastic Tech, and they'll be able to make the MAX more useful than I originally
suspected (they're probably working on that already, but you never know).
John Tissavary (LUNA cie)
John, I have been in contact with Drastic about new software for My animation
comander board for EVO suport at the time everyone was working on the MAX
stuff. the last time I spoke with them they said all the MAX software was done
and the software I need should soon follow. This was about 2 weeks ago, I don't
have the Comander card in my computer anymore and won't install it untill the
new software is out.
Yes, I know Drastic has been working very hard to get the MAX ready for
shipping, since it was due out last fall, and is incredibly late to market.
John Tissavary (LUNA cie)
They told me the software was done for the MAX and now everyone was back
working on stuff for the Animation Commander.
Thanks John, I think I just might have a use for it on a project coming up
real soon.