View Full Version : How can I save reflection to an alpha channel?

03-29-2004, 04:35 PM
Hey, emergency help needed :/ Looming deadline...

I'm compositing some 3D elements over video, and as part of that, I have an 100% transparent polygon that Im catching reflections on (where water is in the video)...the RGB portion looks great, but how can I export the where the reflection is to the alpha channel so when I bring it into after effects it sees the main object and its reflection...is there some special buffer or export?

please help :/

03-29-2004, 04:59 PM
The problem was that the poly that was catching the reflection was 100% transparent (even though it would still render the reflection it wouldnt save it to the buffer.

03-29-2004, 06:40 PM
Gah,...i thought i had it figured out. These buffer exports are totally useless witout 32Bit support...i need my reflection WITH AN ALPHA :/ This seems impossible

03-29-2004, 07:59 PM
If you use the Screen blend mode for the reflection pass you shouldn't need an alpha - but if you still do, render the reflective objects separately

03-29-2004, 08:00 PM
I'm not sure what you mean,
you probably know that already:
when you render the reflection pass (using buffer viewer or
buffer export) take this reflection pass and "screen" it on top
of you main RGB image in photoshop or aftereffect or any other
programs like that.
(reflection=screen, shadow=multiply,etc...)
the mirror buffer will give you an alpha of all the reflected

that helped?

03-29-2004, 08:01 PM
funny, 1 minute later

03-31-2004, 12:41 AM
did that work for you mankeroo -

03-31-2004, 08:21 AM
Unfortuneatly I was looking to have them all separated by alpha because i'm using different blending modes than screen or add...so the black background comes into play. As far as rendering rendering the reflection separately, how can you do that?

All that I really want, is to have where my object is reflecting show up as white on the alpha channel.

Due to time contsraints, I got rid of the reflection and im just flipping a duplicate vertically in after effects....its a hack and not always physically accurate, but what are ya gonna do :)

Thanks for your help though!

PS: Off topic, but when does lightwave 8 ship? I've got a brand new G5 sitting here but its got panther loaded on it and I can't use it because i hear 7.5c crashes on os x 10.3.x

03-31-2004, 10:17 AM

Here you can watch the multipass rendering psd vid. It works great in dxf and I know it's supposed to work well in AFX 6. It can work in AFX 4 and 5 but it 's not as automated.

03-31-2004, 09:39 PM
really curious as to why you're using different blending modes than Screen - (I'm no compositor)

Do you want it to be a solid alpha wherever there's a reflective surface, or just where there is a reflected image?

can you duplicate the reflection buffer images, make them greyscale and use that as an alpha? or how about keying out the black?

Rendering the reflections separately just involves removing everything that's not involved -

btw, OSX now works with LW 7.5c, with version 10.3.3: upgrade and go for it! My G5 comes in this week :D

We don't know when LW 8 comes out - 'any day now', for the past two weeks -

04-01-2004, 08:37 AM
I really just wanted where the object was reflecting (not just a solid alpha) but what I did was made the object GREEN for a render pass, and keyed it out that way and made a mask on the render that has everything integrated into the scene...it worked.

I find soft light worked better for the reflections on water for my particular project :)

>> btw, OSX now works with LW 7.5c, with
>> version 10.3.3: upgrade and go for it! My G5 >> comes in this week

All those graph editor and crashes are gone? So, the crashing was only on versions 10.3.2? and the 10.3.3 from apple fixed them all? I just wnat to be sure before I switch computers mid project :)


04-01-2004, 08:57 AM
I don't have personnal experience yet but here's where the news broke - everybody seems to be satisfied - but you probably shouldn't switch mid-project anyway