Photoshop Luma Matte

[Zulema Estabrooks]

Alphamattetransitions are made up of opaque (or a solid color) and transparent(clear) area of the clip. The idea is similar to luma matte, but in this caseone clip is masked to the solid area and the other is masked to the transparentarea.

In Adobe Premiere Pro we will need to place our two video clips (that we want to transition) on two separate video tracks. (Such as V1 and V2.) Then we need to place our luma matte transition above them, on another video track. Think of this as a timeline sandwich: your second clip between the first clip and the luma matte transition clip. Make sure you reference the image below if you are not sure! Both clips need to overlap the same length of the luma matte transition.

You should now see the luma matte transition joining both clips! The same method works for alpha matte transitions. Just set the Composite Using option on the Track Matte Key effect to Matte Alpha instead of Matte Luma.

I have two files rendered out of 3D Studio max. One .AVI which is the RGB data, one .AVI which is the Alpha. The RGB data is already premultiplied as is proper, but AE has no way to use this properly.Using the alpha as "Track Matte" doesnt work, you get dark fringe edges due to it AGAIN multiplying the already premultiplied RGB data with Alpha. I've tried "Set Channel" and "Set Matte" and god knows what else..... but I can't get it to work.Can someone please tell me how on earth you do this?It's as if I have the RGB data and the Alpha data of a premultiplied TGA file, only in SEPARATE FILES and I simply want to do the simple simple simple excercise of applying the RGB from one and ALPHA from the other. Can't be done./Z

I have my reasons. Besides, straight alpha are the spawn of the devil. And besides again, it's beside the point. Sure there is "another way" to get the data out of my 3D program. But I have the data. The data is proper. I simply cannot get AE to interpret it correctly.You can't set alpha interpretation options for a clip that doesn't have alpha. As soon as you set alpha, AE destroys your RGB channels by yet-again-multiplying them with alpha, in spite of my incessant trying to stop it from doing it.I dunno how many times I have to say that AE's alpha handling is fundamentally flawed before it sinks in on this BB :) :) *grin giggle*Just tell me how to do it. Anyone? PLEASE!/Z

This yeilds the correct composit, so it is "possible" and the data is "right". Yet doing it that way is useless because I want to treat it as one unit, be able to blur, mask, etc. it. Can't do that with two layers.All I want is to get this into one layer. And it's no fancy compositing, it's just plain old normal stuff. It's just a heavily motion blurred obeject and the black fringing in the blurred area is readily apparent.With the workaround, it works correctly. I just repeatedly pray for AE to get this simple, trivial, 1985-ish technology thing *right*./Z

What you are describing though is final compositing. What if, (as is the case in Photoshop) I wanted to create a final product that had an alpha completely unrelated to the RGB? In Photoshop, I can take a black and white image or selection and save a new alpha channel. Imagine the mayhem if whenever I created a selection in Photoshop, the RGB automatically got chopped up or or otherwise changed as is the case with AE!Even with multi layers you could create a circle in the alpha and a large overlapping square in the RGB as a final rendered quicktime movie from AE? If you can, please describe!

:)

Mike

The background:

The RGB data, properly premultiplied:

The alpha:

This is what AE gives me:

This is what I want to see:

The "problem" lies in the fact that AE does not understand premultiplied data unless it is embedded in ONE SINGLE file, a .tga or .avi/.mov with alpha channel.There is NO WAY to get AE to understand premultiplied when it comes from separate files.Furthermore, when we say that AE changes the RGB data (which you claims it does not) we are talking about the OUTPUT rgb of the layer. In a premultiplied system, input RGB equals output RGB of the layer, and what we put in is exactly what we want out, and alpha only governs the scale factor for the background ONLY, not the foreground. Alpha should NOT have an impact on foreground in premultiplied mode!!!!!!/Z

Exactly; That is precicely the proper appearance of a premultiplied file. Remember, in a premultiplied file, the RGB data already "looks correct".One of the hundreds major advantages of premultiplied vs. straight, is that the RGB data is directly viewable and will look completely correct as the final image upon a black background.This is all completely correct, and the flaw in handling this completely correct image lies within AE.

The alpha channel properly sets values for all pixels, but the white ones

you want to add to the background are not white, but various shades of

gray (and colors for the handle) because they have been mixed with the

background layer

Exacly - because it's *premultiplied*, Rick. Stop thinking in terms of Straight alpha.In a straight alpha system, 50% transparent white (i.e. a color which in the final image ends up 50% gray and 50% of the background) is, in RGBA data (ranges 0-1) "1, 1, 1, 0.5", but in a premultiplied system, 50% transparent white is "0.5, 0.5, 0.5, 0.5".You see ANOTHER of the hundreds of advantages of prem.alpha is that "all channels are created equal" and an operation you do to one, you do to them all. So to to make the pixel 50% transparent you simply divide ALL components (not just the alpha) by two.It's Premultiplied! REAL alpha. The way God meant it to work :-) (Okay, just kidding, but the way Jim Blinn, Ed Catmull and Alvy Ray Smith, the people who actually invented the damned thing, meant it to work, and yes, I asked myself)

It happens to be that, yes. Sorry for all my examples *incidentally* happen to be around lightsabers :-) pure coincidence actually.... don't go all elite on me just because I happen to be doing lightsabres.... but.... Rick... you [b]don't have 3D studio?[/b] I thought you said you did!Here's how to get these results out of 3D studio:1. make a really motion blurred object which is 100% self-illuminated.2. Render it to a .JPG file or ANY other pure "RGB" format. *not* any format that inherently includes alpha.3. In "Render Elements" in the Render dialog, render Alpha separately. Surely you done this some day? (I use render elements all the time to get my stuff in layers for compositing later.)You will get files precicely as these./Z

Why?- the math is more complicated

- data in the RGB channels when alpha is zero is unused and wasted

- etc.Please motivate this statement. Why is it better? And why is all the founding fathers of Computer Graphics not agreeing with you?

You didn't actually try this, did you?Try this. Then add a mask to the rgb layer. Nothign happens. Try adding a quick blur to the object layer. Instant mess.For the record, though - If I do *pre compose* these layers and apply masks and blurs to the precomposite, it DOES work, however.But isn't this an awful lot of hoops to jump through to get a simple thing right?Wouldn't a simple checkbox for, say, "Set Channel" or "Track Matte" or whichever of them you choose that says "interpret rgb data as premultiplied" be a lot easier on everyones hair?!?!?!/Z

I especially like the phrasing about "combating the unnecessary multiply with an equally unnecessary divide", which is precicely the hoops one is left to jump through to get this stuff right.Straight alphas makes sense ONLY when the originating data comes from a real world image out of which you want to *extract* something.... straight alphas does not make sense when you already have a proper image with proper RGB data in it, i.e. when it comes from a computer rendering software. There only "premultiplied" makes sense. Making the file 'straight' by imposing the 'needless divide' is an UGLY KLUDGE FIX stinking of bad workaround slime so bad that one never, as a rendering software developer, should have to contemplate doing, yet have been forced to do for decades due to bad compositing software coders only thinking about 'real world' originating imagery!!!/Z

I don't know if this might solve your single layer problem. Steven Walker has a plugin called Composite. It's $20. It has alpha tools and and all of AE's transfer modes in an effect. Which means that you can apply the Lumenisent Premultiply and effects to the same layer. I tried it out on some the sample images you've posted and it seems to do the job. You should check it out.

I thought you were looking for a method to do it in a comp with only two layers a foreground and background. Using you're graphics. I tried with the files in this thread and seemed to have gotten the result you are looking for, and the plugin is only $20.I thought that was the point you didn't want to have 3 layers or to have to apply the alpha to the background.As for your assertion that this should be a bread and butter function of AE, why is that so many here have had to have the problem pointed out to them? Would it not have been readily apparent to them?(Sigh) I don't know, I guess you've got a point about AE working internally only as straight alpha. All I can say is that this has never been an issue for me. Yes I do composite 3D graphics into AE. But again I've never rendered 3D and it's alpha separatly. The file had the alpha with it.Jim

The only probelm that exists using this transfer mode is a render pipeline issue. Don't expect Adobe to sit on their render pipeline issues forever.(basic text for example) I expect that it will be solved in the near future.

Now add the "Remove Color Matting" effect to the precomposed layer, and select black.This will "un-premultiply" your 3D output, the same thing that would happen if you had the alpha embedded in a movie and could select that in the interpolation settings for the footage.

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