Hello,

Is there a way to specify which objects get alpha channels and which do not? For example: Let's say I have a blue sphere and a red square in a scene. I want to render the blue sphere by itself with an alpha, but still want to have the reflection from the red square present in the sphere's reflection. How can I hide the square from rendering, while still allowing it's color to affect the look of the sphere?

The reason for all of this is I want to do some post compositing on my film in After Effects to have more control over the look of a scene as far as levels, motion blur and depth of field.

Any suggestions? Thanks.

Chuck

Here's a couple of ways that I've pulled this off: #1. Try enlarging the cube but move it out of the field of view. That way, the reflection will have the same proportions but eliminate the Alpha issue. #2. Make a texture map of the cube and multiply it into a value shader in the reflection channel. (this can easily be done using the spherical camera function while hiding the sphere - this map will also have the other characteristics of your scene like lighting etc.) There is not a way I'm aware of to hide an object from rendering while retaining it's reflections in other objects, though. Hope this helps. Mark

Thanks for the suggestions Mark! Chuck

Chuck,
A tedious way of doing this is to do 2 renders.

The first is the full scene and will include all reflections...

The second is the alpha render and will include only those objects that you want to have an alpha channel...

Take the alpha from the second and transpose to the first image (overwrite or delete the old alpha).

This should get you what you want. It IS tedious, but it will work.
Edit: I just noticed the full comment in the post. You have to move more than just the alpha, but if you have (and know how to use) PS or PSP you should have no problem with the compositing.
Ken

Message edited on: 06/28/2004 11:12

And create a batch to run your sequential images through PS to do the alpha replacements. Shouldn't take to long to run it.

Ken: When you say more than just the alpha, are you refering to multiple frames in an animated sequence instead of just a single still image? bluetone: Are you setting up this batch in Carrara or Photoshop? I assume Photoshop with an "Action?" Either way, how do you set that up to replace the alpha? Thanks guys! Chuck

Both. C can run a batch to create the 2 versions of the animation. If you create the animation as sequencial TIFs, (or the still image that supports alphas of your choice,) then run the second animation of seq. stills, then in PS figure out what you need to replace the alphas, create the action, and let it fly. If you practice on a single frame, you should be able to figure out the steps needed. From each pic, copy the alpha channel to the clipboard, then remove the alpha from the other clip and paste in the new one. Close each file, saving each to a new folder. Creates a lot of pics, but it should work fine. Or, if you use rendered animations, then in your video editer program, tell it to use the alpha from a one rendered animation to control the other. I'm sure it works in Premeier. I can do it in FinalCutPro. Good luck!

I'm following you now... just wanted to make sure I was clear on what you were saying. Thanks! I'll probably go the route of applying a rendered animation's alpha to control the other in Final Cut Express or AE. Thanks again... Chuck

Sorry for the late response, but ebot seems to have missed me on the thread.... Chuck, What I mean is that you can use the alpha to cut the image of the reflected sphere into the on with the rectangle not rendered... but bluetone's suggestion with Premiere works as well... and yes, blue, premiere can do that as well. Ken

Thanks again guys... I did a test shot tonight and it worked great. BTW, it works in Final Cut Express too using the "Travel Matte."

Great! Glad to hear that. Hopefully you can post a link to the final animation when your done? I would love to see it in action.

Attached Link: http://www.littlemanproductions.com/AIRU/progress

http://www.littlemanproductions.com/AIRU/progress

Hey, Chuck -- I just checked out your AIRU site; it looks very cool. Since you're posting here, I take it that you're producing this stuff in Carrara, but didn't see that on your site. It's pretty inspirational to see what folks like Taylor or you are doing with off-the-shelf software. I've bookmarked your site and will check it out periodically. Keep us posted here, and keep up the good work. - Dex

Hi Dex, Thanks for the great feedback... now I know at least one person is watching my progress! :-) To answer your question, yes, I am producing this film with Carrara. Of course texture maps were done in Photoshop, and some of the final compositing will be done in AE and the entire thing will edited in Final Cut Express, but 99% of the work is coming straight out of Carrara. Thanks again for the feedback... it keeps me moving forward! Feel free to e-mail me with questions - my address is on the AIRU site. Chuck

I appreciate your giving us the link. Great work. Inspiring. Keep it up and I'm looking forward to watching at a Loews theatre near me soon! ;)

I can't resist: "Thank you for coming to Lowe's. Sit back and relax; enjoy the show!" (Imagine that I'm dancing wildly to the snappy beat.) - Dex