View Full Version : alpha channels

06-27-2003, 07:31 AM
Hi folks...been at LW for about a month, having some fun with the Stage by oh boy the modeler! Anyway, as a video person, I'm using LW objects as part of video scenes (in Final Cut Pro and AfterEffects). I find that if keying these, I'm losing lots of resolution and clarity and would prefer to have them come in with alpha, but can't figure out how to do it. Any tips?

06-27-2003, 07:41 AM
You need to save 32-bit to have an included alpha. Pict32, BMP32, Tiff32, etc. all have an embedded alpha channel. The other option is to render a seperate alpha. You can set this up in your render options, before you render the final image. This will produce two images, one is just the alpha, the other the full-color image.

Darth Mole
06-27-2003, 08:35 AM
Also, f you're saving out QuickTimes, the option is to save as Millions of Colours + (the + being the alpha channel).

As a sidenote, you need to render with nothing in the background - just empty space - and it doesn't matter what colour the backdrop is either. You can also use a gradient backdrop or fog and you still get a solid alpha channel.

When importing into After Effects, if you're using a plain black backdrop, you'll need to change the footage interpretation to premultiplied with black to get a clean matte edge.

06-27-2003, 09:28 AM
Thanks for your swift replies. As it happens I am working with animations so the suggestion for "millions of colors+" may work. I am also working with black backgrounds. I'll report back on the success of your suggestions.

06-27-2003, 03:31 PM
To echo what I have been a strong proponent of on this board, I STRONGLY encourage you to use image sequences over quicktime. If you are bringing them into After Effects it works just fine, and in general LW behaves much more friendly with image sequences than with QT movies. :D

Darth Mole
06-27-2003, 03:42 PM
Never had any problem with QTs. I got hundreds of MBs of 'em sitting here on my desktop.

But hey, each to their own.

06-27-2003, 06:05 PM
Originally posted by jdavidbakr
I STRONGLY encourage you to use image sequences over quicktime.
Yeah, but there's a place for Quicktime also. Not everything needs network rendering. Not everything is a long render. Sometimes QT is a fast way to move video into LW from a compositing or video app. Quick previews are also better in QT. File sizes are smaller.

In another recent thread, Luxology's Arnie Cachelin informed us that the previous QT framerate bug has been fixed in LW7.5c.

06-27-2003, 06:11 PM
The language that software makers use to describe alpha channels causes confusion.

The term "millions of colors" harks back to the days when Apple wanted everyone to know that the Mac could display more colors than Windows could.

The term "millions of colors +" to describe RGB + alpha is even sillier. Now that there are 16bpc Quicktimes, they are calling that "trillions of colors +".

Also silly is the term 24-bit to describe RGB, and 32-bit to describe RGB + alpha. It is causing mass confusion because people are mixing it up with bits-per-channel.

I would like to see software makers use the terms RGB, and RGBA to describe whether an image format has an alpha channel. Everyone would understand this, and know it has nothing to do with bits-per-channel.

06-27-2003, 06:34 PM
I'll second the notion that important footage be in sequences instead of QT's - more flexibility compatibility and safety.

"And it doesn't matter what colour the backdrop is either"

I'm still learning about compositing and haven't been able to get any info on this: the background color gets anti-aliased into your objects, so if the color isn't close (or a good color to key out ), your compositing is pretty obvious.

Is there something I'm missing?

Electric Image somehow figured out how to AA straight to transparency instead of the background color - if the program wasn't so lame I'd use it for rendering.

06-27-2003, 06:55 PM
I didn't say that QT should always be used instead of image sequences. When you have a 1-computer render that needs live video footage QT is an easy solution for inputting video. If you've got a 2 day render going over a network it's better to output image sequences.

Regarding colors in alphas, you can hit the "fader alpha" button in the render settings to avoid this (this button should be renamed "straight alpha" like the rest of the industry). Only do this if you have an external compositing application.

06-27-2003, 07:39 PM
hmmm...hopefully I'm doing something wrong - I've rendered a black sphere against a white bg, and the one with "fader alpha" turned on actually looks worse.

I changed the alpha "interpretation" 3 different ways, did the test twice to make sure I didn't switch them: the one on the left is with fader alpha turned on and both pre-multplied with white matte in AE

Darth Mole
06-28-2003, 02:11 AM
Don't know what you've done there - all I did was render a solid black model of Beethoven (from Worley's plug-ins) against white, as you did.

The image on the left is the 32-bit TIFF imported as a straight alpha; the one on the right I simply altered the Interpretation (Apple-F) to Premultiplied with solid white, the same as the background - the matte line disappears.

Why are you using Fader Alpha? This mode is designed for video faders, not digital compositors. :confused:

06-28-2003, 02:36 AM
that's what someone recommended a few posts earlier -

I get good results premultiplying with color, but not when there are 2 bg colors to blend into.

If, for example, you composite Beetoven onto a white table with a black bg, you'd either see the matte line over the table or over the background, depending on the color you used.

Can you render in such a way that the bg color does not get AAed into the objects color?

06-28-2003, 06:14 AM
If you select "Fader Alpha" in Lightwave, select "Straight Alpha" in your compositing app. They are the same thing, just different manufacturer's terminology.

If you leave the Fader Alpha button unchecked in Lightwave, select "Premultiplied Alpha" in your compositing app.

Both these alternatives should work if you have a compositing app. If you use Lightwave's internal compositer it only works with a premultiplied alpha...that is, no-fader-alpha.

Once again:

Fader Alpha ON = Straight Alpha
Fader Alpha OFF= Premultiplied Alpha

06-28-2003, 09:59 AM
Wow! you all are terrific. I used the "millions+" setting for Animation and the alpha worked in FCP. The confusing thing is that I am choosing "Quicktime" in the Render Options, but then choosing "Animation" in the Options. I've always been adverse to "Animation" for video from AE for instance, as it doesn't render appropriate DV specs. So I guess what I'm doing is rendering a sequence of individual frames (images) in LW at 30 frames with no field rendering which are translating into Quicktime for FCP, yes? I haven't yet tried to import Quicktime into LW, but will see how that works soon. Is that done through the texture editor?

06-28-2003, 12:59 PM
"Fader Alpha ON = Straight Alpha
Fader Alpha OFF= Premultiplied Alpha"

I did use Straight Alpha. The Fader alpha always looks worse. I can't get rid of the matte line at all.

06-29-2003, 01:49 AM
If you're using a single computer for your work then Quicktime will work OK. For network rendering QT is not as efficient.

If you want to try loading a Quicktime into Lighwave, go to the Image Editor and hit the load button.

Then, you can load it into the Surface Editor if you want to map it to the surface of an object. (Choose image map option)

Alternatively, load up Lightwave's Compositor (under the Scene tab) and choose the QT as a backdrop. The movie will then be playing in the background behind your models.