View Full Version : Alpha Channel Compositng

08-22-2004, 02:06 PM
What I have:
A yacht and water.

What I want:
Render out only the yacht (for compositing purposes). However, I still want to have the water raytraced on my yacht. Also, I want to have the alpha channel from the yacht (black).

Question: How can I get only the yacht rendered with the water deducted from my yacht alpha channel?




Triple G
08-22-2004, 02:51 PM
Go into your render options for the object, and make the water Unseen by Camera but SEEN by Rays.

08-22-2004, 04:13 PM
Thanks for your reply.

That doesn't work... As soon as you make an object "unseen by camera", it will indeed calculate raytracing but not alpha channels.

Other suggestions?


08-22-2004, 05:03 PM
Set the alpha channel in your surface editor/advaned panel to constant value 0, for your ocean surface.

Triple G
08-22-2004, 05:06 PM
Ahhh....I misunderstood what you were trying to do. Trevor's suggestion should work. :)

08-22-2004, 05:28 PM
Thanks Tripple G and Trevor!

I understand how I can deduct the alpha channel of the water from the yacht. However, the water will render in that case (higher render time)... I only want to render the yacht.

Sorry for not being clear...

Triple G
08-22-2004, 06:07 PM
Well, you could turn the water into a matte object (black). Technically, it would still be rendering, but it'll almost certainly be a lot quicker than a full-on render with all your surfacing applied. I think you'll still need to set your alpha value to 0 for the water surface(s). This should work for you. :)

08-22-2004, 11:27 PM
Great idea Tripple G!

I thank you.

The render time is now indeed sub-marginal.

Great thinking.

08-22-2004, 11:28 PM
Speaking of which... Do you have any experience with rendering out in PSD exporter plugin and use color id and and Z-depth?

Triple G
08-23-2004, 09:52 AM
Glad that worked out for you.

I tend to not use the PSD exporter much, since the layers it produces are stacked in the wrong order, with improper transfer modes. Also, since I use After Effects for most of my compositing work, the fact that AE doesn't support layered PSD sequences makes this kind of useless for me.

I do use the extended RLA/RPF exporters every now and then. They work well for some things, not so well for others...it's just a matter of playing with them and seeing what they can do for you.