Bob,

Thanks for the answer.

In a tape linear format you normally have a tape which has the fill video and tape which has the matte (alpha) video. You roll them in sync and do a matte/fill key on the switcher. Black does not work unless you use negative black, but this has its own limitations. Can't use a stored image because I need to do traveling mattes.

This is digibeta format by the way. Thanks again.

Dave T2

Before you read further, I don't know how to do it. I'm just trying to set up the problem.

I'd think that you could generate it out of Vegas if you knew what the requirements of the linear system were.

In general, there are two types of transparency used for alpha:

- A single color value as Bob is describing. This is a pretty primitive way of doing it but you'd use it if you wanted to embed the transparency in a 24-bit file (3 8-bit channels) that didn't have another 8-bit channel for alpha.

- A range of values. For example, (and I know you know this), an uncompressed AVI can have 4 8bit channels, each channel having 256 values. Three channels are used for color values and the fourth has 256 alpha values. If you were to look at just one of any of those channels it'd be a grayscale image.

So task one is to figure out just what the linear system needs in a matte - a simple black/white (on/off) matte, a grayscale matte, or something else. Task 2 is to figure out how to get it out of Vegas and onto a tape. One way that occurs to me is to use Satishs frameserver to export the project as a sequence of images with alpha channels and the run a batch action in Photoshop to turn off all three color channels. (This is a fanciful solution, I haven't done it, and it'd be more elegant to do the whole job in Vegas). Another possibility might be that Virtual Dub could do the job

In the end, you wouldn't need to print anything to tape that actually had an alpha channel in it - you'd be printing a matte image that represents an alpha channel. The trick is to turn everything else off so that the alpha renders as a grayscale image.

Rob

Rob,

Thanks. You have pretty much summarized the final desired output. Those of you who have never edited on tape in a linear suite would be amazed at what could be done with tape, mattes, negative black and such. And in this case linear editing will be more profitable and time resourceful for this project.

I think I have figured out that After Effects can output an alpha channel file only. I just need to explore this and would appreciate anymore feedback or ideas.

Dave T2

We've actually done this, very crudely and stupidly. Only because I wasn't there at the tiem to save someone wasting a lot of time.

He recorded a static bug printed on card with a camera for an hour and then using two VCRs and a MX50's downstream keyer to comp both the source tape and the "bug" tape which was then recorded to a third VCR. It worked but the MX50 isn't the best of vision mixers so the results were not spectacular but good enough for the purpose.

Aside from that anecdote I can't help too much. We've sold off our DVW 500As and I can't find a full spec sheet for them at the moment. Checking photos of the back panel I don't see and separate SDI output for the alpha channel so I'm pretty confident in saying that Digital Betacam does not support an alpha channel.

However as Rob has hinted above one solution is to supply two tapes. One with the mask and another with the footage. If you checkout some of the Artbeat clips that contain fire etc that are for compositing you'll see they come as two files, one being the matte. There's no reason I can think off why the same thing cannot be printed to DB tape using Vegas. Certainly you'd want to be working in 10bit YUV / 32 bit. Vegas has a matte generator and the FXs that create alpha channels such as the keyer let you output just the mask.

So prepare two 10bit YUV files and print to tape. You need a Decklink card (or similar that works with Vegas) and a DVW 500.

If this is going into a linear editing system it will have to have some form of keyer though and you'll need three playback decks and you'd better have everything locked to TC! As far as I know SD SDI doesn't support an alpha channel, the mixer has to generate one internally.

If you can tell use which linear editing system this will be used in I can poll some old hands to perhaps get you specifics on how it's done.

Bob.

See below. Vegas CAN output an alpha channel only file, an uncompressed AVI file will give you this.
Now you're saying your linear system can do mattes!
It'd have to be a pretty sad system if it couldn't.
That changes everything, see below.

Bob.

If I've read what you're saying above correctly, that your linear system can use "negative" black to create a matte then the task could be very simple. Vegas will all too easily create negative (super) black and you can print this is to lowly DV tape.
If you're only needing to comp animated bugs or supers doing this should be trivial and not tie up too much expensive kit.
On the other hand if you're trying to comp fire or explosions in your linear system maybe not so easy.

Give us some more info, this could be very simple.

Bob.

Bob,

Thanks for your help. All you have told me is things I already know. I have been linear editing for 28 years using multiple format tape machines. Our editor is an Axial edit controller that can control up to 8 tape machines with complete house sync/timecode.

I have used negative black, and it is sufficient in some cases but is not flexible enough in this situation. The graphics we are creating will vary from a bug, to an animated logo with soft edges. A traveling matte is the best solution. Most graphic design computers were built to output the matte/alpha channel seperately from the fill/beauty channel. Unfortunately, many of the NLE systems are not. They are designed to create files with an embedded alpha channel. There are several workarounds, but here again, not sufficient for our task.

I was curious if Vegas might have this capability of creating an alpha/matte output. No linear tape, afaik, allows for a seperate matte output. We have always used seperate matte/fill tapes. Our facility still has one linear suite, two Final Cut suites , and Avid Adrenaline and a Quantel Henry suite. From what I can tell, Final Cut does not allow this either, but it looks like with a combination of either Vegas or FC and After Effects, I might be able to create the elements I need.

People may say, why edit in linear. These are long form programs that for the most part are already edited. They just need some fine tuning and graphics added along with opens and closes. It is much more time efficient to edit linearly, which can almost be real time, as opposed to capturing 2 hours, editing and outputting two hours. Doubling the time needed to do the project. Linear editing is still a very valid form of editing, and in this situation is the best.

Dave T2

I fully understand why you'd want to do this in a linear suite.

As I said above Vegas can output a b&w / greyscale mask from its alpha channel. If that's not all you need sorry but I'm not seeing what else you could print to tape that'd satisfy your needs.
What you'd end up with is one tape with your super or whatever, another with the mask for it. From these two tapes your Axial system could generate the complete vision with alpa to then comp onto the program.

Bob.

Bob,


"Vegas can output a b&w / greyscale mask from its alpha channel. If that's not all you need sorry but I'm not seeing what else you could print to tape that'd satisfy your needs."


That is exactly what I need, but from what I can tell, the alpha channel is embedded in the video stream. It cannot be output seperately, or am I missing something. You are helping so don't let me discourage you. I see the option to create an alpha in an uncompressed format video, but I don't see a way to make it its own stream. Is there a setting for this that I am missing?

Thanks

Dave T2

You have to fiddle it a bit. Most of the FXs that generate alpha data will also output just the mask. There is also a Mask Generator that'll do it as well. Certainly out of the box Vegas will not just output alpha as greyscale. I've certainly used Vegas to create greyscale mask files that I've then bought back into Vegas to use as um, masks.

Without knowing exactly what your Vegas project looks like it's hard to give you specifics however play around a bit, I think you'll soon see what I'm talking about. For example if you had a track with just alpha then adding a track of black below it should convert alpha to greyscale.

I think it'd also be fairly easy to render out an uncomp AVI with alpha, PTT that. Then bring that file back into Vegas to create a greyscale file from the apha data and then PTT that.

I just rang my mate whose another editor from the linear days. Mostly with Grass Valley system but did use an "Axis" system as well. He said to try a Linear Key. Sorry if I'm telling you how to suck eggs.

One other tip if I may. If you're using DV into a linear suite get a DSR45 or similar deck and use the component outputs. Those VCRs do chroma smoothing which will avoid the jaggies from the DV.

Bob.

Okay Bob, I am a bit dense and thinking in old ways, but I think I am beginning to understand what you are saying. I shouldn't expect an alpha/matte stream, but can create one from the content by playing with greyscale and thresholds. I think I am gettiing in my head. Sometimes I can't see the forest for the trees.

This will all be D1 digibeta, so no need to worry about DV jaggies.

Okay, I have to wrap my head around this and connect some new synapses. Thanks for your patience and help.

Dave T2