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View Full Version : um... help? composite w/ lots o' stuff going on



jeromeOlivier
05-07-2003, 07:57 AM
Hello!

If someone can help me out with this, I would really really really appreciate it!

The project:

I have a multi-screen animation project where acrobatic airplanes are flying around 4 large screens in a convention area. The video images on each screen is a composite of 4 elements. The four elements, going from back to foreground, are:

1. sky (done in sky tracer, saved as a jpeg and brought back into the scene as a background image using the compositing tab)

2. corp logo (photoshop image with alpha chanel applied to the surface of a single polygon)

3. smoke trails (sprite hypervoxel)

4. airplanes (simple model following a motion path)

I need to render the smoke and airplanes separately because there is a generous amount of motion blur on the airplanes (requiring multipass rendering) and none on the smoke.

The problem:

When I turned on ray trace transparency from the render set-up, the alpha chanel on the single polygon surface suddenly "punches through" the composite background image and reveals the project's background color.

What could be the problem?

I know I need to turn on ray trace transparency in order to see the hypervoxels behind the transparent surface, but why does it affect the composite background images as well?

If someone could help me with this problem, I would be infinitely grateful!

Thank you!

Red_Oddity
05-07-2003, 09:01 AM
I had some funky effects with Raytrace Transparancies aswell...
Is the Alpha on the image put on a different alpha Layer in the Surface editor, or are you using just one Layer with the photoshop image using it's own embedded alpha?

Lightwolf
05-07-2003, 09:23 AM
If your logo is the one posted, I'd to the following:
replace the logo plane with a white plane and clip map the alpha of your PSD. It will render faster, and you won't have the alpha problem.

Lightwolf
05-07-2003, 09:28 AM
I just re-read your post, and I see what the problem is:
You applied the sky as a bg image. The bg image can not be seen by raytraced rays (since it has no real position in 3D space).
You could:
a) leave out the bg image, and comp it in later (wait, this is not going to work either, because the alpha of your single plane logo will be screwed up due to the raytracing as well...)
b) map it onto a single plane polygon

b) is your only option.

Btw, imho you shouldn't have saved it as a .jpg, it will always use the same amount of memory when loaded by lightwave (height * width * 3 bytes for an RGB image).

Cheers,
Mike

jeromeOlivier
05-07-2003, 08:45 PM
Red_Oddity, I tried using photoshop's alpha channel, but was getting a blank white poly. I instead opted to add a b&w image and use that on the transparency channel.

Lightwolf, if I'm reading your (b) solution correctly, I should apply the sky image to a single plane polygon (like a cube skywarp).

Yes, I thought of that. But if I apply it to a polygon, I will lose my rendered alpha channel (I need this channel in order to comp everything together in after-effect afterwards). The reason I need to apply the background as I did is simply because semi-transparent areas (like the smoke and motion blurs) retain some of the color info of the background. Without the background image, the composite in after effects wont be as clean. :(

I'm wondering if there's a way of telling Lightweave to apply an alpha to a polygon surface during the render process (so that it will effectively knock out a background polygon). I'll see what happens today.

Another poissibility is simply to use the eps importer and use it to create an object, then apply a glow to it during the render (though I don't like this option since it will increase render times). :mad:

hcoat
05-08-2003, 12:57 AM
I rendered smoke on an all black background and placed it in after effects and then use it again as the alpha channel and it let the color come through just fine and it was simple. You may have to tweak it alittle but it should work.

jeromeOlivier
05-08-2003, 02:00 AM
hcoat,

Thank you for the reply. Yes, I tried that too, but I got the result you see below. If there's something I'm doing wrong, please let me know. I thought I'd mastered AE, but it looks like I might've been fooling myself. :cool:

PS
Gave up on using a single sided polygon as it was also mucking up the alpha chanel for the smoke (it would render the color channels, but not the alpha). Using an object instead resolves the issue.

Lightwolf
05-08-2003, 02:49 AM
Originally posted by jeromeOlivier
I'm wondering if there's a way of telling Lightweave to apply an alpha to a polygon surface during the render process (so that it will effectively knock out a background polygon). I'll see what happens today.
Yup, Surfaces->Advanced->Alpha Channel->Unaffected by Surface
:D

Lightwolf
05-08-2003, 02:50 AM
BTW,
your black background should work if you use an additive comping method. Or if you can tell AE that the image is pre-multiplied with black.
(Sorry, I'm a DF user, but that should be possible in AE as well).
Cheers,
Mike

Red_Oddity
05-08-2003, 02:59 AM
Yup select the corresponding file in your Project Source window.
Right click or CTRL-click (Mac) and select Interprete Footage -> Main (or hit CTRL-F or Applebutton-F (Mac))
And then set Alpha to Premultiplied - Matted With Color (choose color here, usually black)

jeromeOlivier
05-08-2003, 07:06 AM
Excellent! I learned something new with AE & alpha channels. Thank you! :)

Well, I think I arrived at a stalemate regarding my original post.

1. I can't use a single sided poly with a photoshop layer in the color & transparency channels for two reasons:

a) the poly will block the scene's alpha channel

b) if I set the alpha value manually, the poly with the ray traced hypervoxels will be erased when I import the image file into AE

2. I can't use an object for the logo because the smoke trails continue in front of and behind the logo. Since I have to render the airplane separately from the smoke, I would have to cut out tiny little masks in AE for each scene when the airplane passes behind the logo (too time consuming).

The best way for me to do this scene would be to save each render pass in the deluxeRLA format and save the depth info, but since I only have the standard version of AE, 3D channels are out. :(

I guess it'll have to look like this:

Lightwolf
05-08-2003, 07:25 AM
Hi there,
have you tried the clip map approach?
Cheers,
Mike

jeromeOlivier
05-09-2003, 10:54 PM
Sorry for not getting back to you sooner, Lightwolf, but both my PCs have been rendering non-stop for the past two days as I rush to finish this job.

No, I haven't tried the clip map approach. I'll do some tests once this job is done. Right now, I need to focus on meeting the deadline (next Friday!) & still have a few minutes of final CG to render out.

Thanks for the help! I hope we can more exchanges in the future.

See you soon. :)

Lightwolf
05-10-2003, 09:54 AM
Originally posted by jeromeOlivier
Sorry for not getting back to you sooner, Lightwolf, but both my PCs have been rendering non-stop for the past two days as I rush to finish this job.
No problems, I have two deadlines on monday, and one the week after.


Thanks for the help! I hope we can more exchanges in the future.

See you soon. :)
I'm sure we will.
Cheers,
Mike