Buffer Export with Alphas

#1

Why is it that no matter what image format I choose in the Render Buffer Export filter, I can’t get my render buffers to render with an alpha channel? I’ve been sluthing on the internet all day, including these very forums trying to find an answer to that very question.

Here’s a step-by-step which illustrates what I’m talking about. I’m using LW9.3 and AFX7:

I have in image I’ve rendered in LightWave. It’s just an object with nothing in the background, so the background should be transparent as long as I export a 32-bit image.

If I save it out, as is, from the preview window in LightWave, then bring that into AFX (I’m using a 32-bit PNG), sure enough I get what I would expect: transparency, but with out goodies like specularity and reflections.

So in order to get those goodies, I know I need to do multi-pass rendering. So I use the Render Buffer Export filter to export, say a Shaded Diffuse, Shaded Specular, and a Transparency passes. I hit F9, it spits out my passes, I bring the diffuse pass in to AFX and what I get is the inability to knock out the background. Apparently my passes aren’t coming with an alpha channel, no matter what format I use.

I’m either doing something wrong in LightWave, or I’m doing something wrong in After Effects. I’ve tried every blending mode in After Effects and I can’t get that diffuse pass to look normal AND have a knocked out background. The same goes for my other passes. They comp fine with eachother, but I can’t get rid of their backgrounds.

Any help would be greatly apprecaited.

Thanks,
Dustin

#2

why not export an alpha channel?

#3

I’m not sure I understand what you’re suggesting. Like I said, the image format I’m using (32-bit PNG) SHOULD have an alpha channel built right in. But even if I bring my buffer frame in to Photoshop I can see that it has RGB channels, but no alpha.

#4

Instead of using shaded specular/diffuse, use diffuse shading and specular shading.

This will out put greyscale mattes for you, which, in my experience, offer far more flexibility. Also, in AE it is also beneficial to keep 2 copies of the beauty pass, with and without embedded alpha, respectively, so that you don’t lose any information due to transparency.

#5

I meant to export an alpha pass, a single greyscale alpha image sequence. You can then use that on any other pass. I usually do that because I assume that it takes up less HD space and memory.

Imagine 5x 32bit passes (embedded alpha)…essentially 20 channels

Then 5x 24bit passes (just RGB) plus a single alpha…16 channels total (5xRGB, 1xalpha)

Does that make sense? I just find it easier to have all my passes seperate including the alpha.

#6

Thanks for the suggestions, guys.

ares,
The only difference I see is that…
diffuseShading/diffuseSpecular = greyscale images
shadedDiffuse/shadedSpecular = RGB images
Neither has a built in alpha channel. Do you know of a way to include an alpha channel in these images?

biliousfrog,
It’s worth a try. Do you happen to know how I can “link” an external alpha in After Effects like you’re suggesting?

Thanks,
Dustin

#7

As an example, take the beauty pass and the alpha pass.

place the alpha pass above the beauty on the layer stack
Select the “track Matte” on the beauty layer and try each to see what you need, there are four options, it’s usually the third one I think, whatever that is (I can never remember).

The track matte is automatically the layer above the one you’re selecting. You can even add another layer above the alpha and use that as a track matte for the alpha which in turn is a track matte for the beauty pass…sounds more complicated than it is.

If you had RGB, Diffuse, Shadow and Reflection passes plus the alpha pass the layer’s would probably look like this:

Alpha
Shadow + trkmt
Alpha
Reflection + trkmt
Alpha
Diffuse + trkmt
Alpha
RGB + trkmt

Another way is to pre-compose all of the passes then bring that into the final comp with a single Alpha to cut out all of it.

You’ll soon see why node based compositors are a neater way of working with passes because you can just link to the same alpha without actually loading it several times…but I find AE quite a friendly app to use.

There might even be a neater way within AE but off the top of my head, that’s how I usually tackle it.

#8

Have you tried using the PSD export fuunction in the processing window? alternatively using the layer with transparency as a mask in AfterEffects by selecting layer/blending options/stencil alpha or stencil luma?
works a treat and i use it often when compositing

#9

biliousfrog - Thank you very much for taking the time to explain that. It was very helpful. The third Track Matte option down is called Luma Matte. I’m torn between using Luma Matte and Alpha Matte. Luma Matte is nice because it will knock out the black background from the alpha. The problem I’m finding with Luma Matte is that it really waters down the effect of the layer. For example, when I apply it to my reflection pass, there is hardly any reflection showing, or it’s very dark and I have to really crank up the curves on that layer to bring it back to a respecable visibility. That’s where Alpha Matte comes in. It’s looks exactly how I want it to look, only it doesn’t knock out the back background from the alpha.

Or at least that’s what I’ve found so far. I might be missing something.

Bonzodogboy - Yeah I played with PSD Export briefly, but didn’t invest too much time in it simply because it’s a LightWave-specific feature and I don’t want to become too dependent on it, as I don’t use LightWave exclusively.

Thanks again, guys!

  • Dustin
#10

You can of course change the blending mode of the reflection layer to make is sit better, the usuals are multiply, overlay and screen. I’ll usually add the reflection layer and then an occlusion pass to stop reflections from appearing in creases…they’re all things that you can experiment with depending on the scene. That’s the nice thing about AE, it’s like Photoshop but with animation controls.

#11

yeah the alpha matte can be a bit tricky, what i have found is that you shift+apple+c (pre-compose) the alpha stnecil layer with the layer you want to stencil… the black dissapears.
i know that is not how i think it should work but it does.
give it a shot :slight_smile:

#12

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