View Full Version : Ray Trace Transparency and Alpha Channel

08-15-2003, 09:41 AM
Does anyone have any ideas why the alpha channel for a semi transparent object becomes totally solid (white) when you turn on Ray Trace Transparency?


08-15-2003, 01:59 PM
I ran into this too, and wasn't able to solve it. This must be possible right?

08-15-2003, 10:50 PM
you could always render a separate alpha with no raytracing, until they fix it -

08-16-2003, 04:51 AM
Was just told from a former work colleague that this is the way Ray Trace Transparency works. And for this particular scene I need it turned on which means there's no fix for my problem . :mad:

08-16-2003, 05:21 AM
1. are you sure you have to use ray-trace transparency instead of plain-old tranparency

2. if you do, compositing is a simple solution - assuming you own a compositing program

08-16-2003, 05:41 AM
Well, I've been provided with high-poly hair object at work which takes too long to render with "plain old" transparency. For some reason I managed to cut down rendertimes by a fifth when I switched on Ray Trace Transparency. Don't ask me why it renders much faster with Ray Tracing but it does.... :confused:

And yes, I need to composite the hair, but without a proper alpha channel its not going work.

Anyway, thanks for trying to help but I guess I'll just have to make sure I submit the hair pass as soon as possible to the renderfarm in order to get it finished on time. :)

08-16-2003, 05:53 AM
that's odd that it goes faster -

what I meant was to render a separate pass to use as an alpha. For this you could render the transparent surface by itself, nothing else in the scene, and it should go pretty fast, then apply it in after effects or whatever you use. That's what the movie studios do - actually they wouldn't render the transparency together with the hair anyway - does the hair give you an alpha? I know it's a post process, but that's about all I know

08-16-2003, 06:09 AM
Right, lets see if I can explain in a bit more detail:

I have a bald character which I'm rendering on a separate pass.
Next I want to render the hair separately which is in a scene on its own.

If I render with "plain-old" transparency everything's fine. The hair alpha is looking nice and it comps well in After Effects, but the render time goes through the roof. :(

What I found out is that the hair rendered much faster with Ray Trace Transparency turned on and low ray recursion limit (once again I don't know why) but then the alpha channel become solid and I couldn't comp it properly in AE without getting ugly edges around the hair. :(

08-16-2003, 06:19 AM
so you have a hair object, and the hair itself has the transparency, is that it? I thought you were using Sasquatch and it was behind something transparent -

Is there perhaps a way to render white hair on a black background without transparency?

08-16-2003, 06:33 AM
Yeah, sorry forgot to mention that.

The hair strands themselves are polygons with a transparency map which gives them the look of fine hair.

The thought of trying to make a fake RGB which looks like the alpha occured to me, but I haven't had any greater luck of making it exactly identical to the alpha...