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Matthias
07-30-2003, 07:35 AM
I have a nice glass-textured object and want to compose it in after effects. It renders perfect, but the alpha channel for the object is pure white. I want it to have some shades of grey, depending on the transparency of the object.
I read something about this problem on this site, so i tried to turn of raytrace transperancy. The alpha is then completely black, the rendered image doesn`t look as good as before.

What is going on here?
How can i solve that problem in LW?

Matthias

marvin
07-30-2003, 11:15 AM
What you have to do is (I know it sounds stupid) but make sure you have some transparency on it. It is glass, so it should be fine. If Ray Traced Transparency is ON, it will NOT work. Without Ray Traced Transparency, and go into the advanced tab for surfaces and on the surface for the glass, make sure Alpha Channel is set to Surface Opacity.

That's all it took for me. Another side note is I have my Ray Recursion Limit set to 16. It should still work with a lower setting though.

marvin

Matthias
07-31-2003, 02:07 AM
Thanks for your answer,
but it didn`t help.
There`s transparency, i checked that with a second object, turned off raytrace transparency and the alpha channel is set to surface.
The alpha channel is still pure white for the object.

marvin
08-01-2003, 05:53 PM
Here is a test I did. I'm showing my surface settings also. My render settings were having raytrace shadows checked and ray recursion limit to 16. I also have the alpha channel setting under the advanced tab for surfaces set to "surface opacity." This means that if I had the surface opacity set at 100%, then the alpha channel would be completely black. The alpha channel is flat because there is a flat transparency set on the object.

At first I had a gradient for incidence angle on opacity, but I wanted to keep it simple so I got rid of it.

marvin

toby
08-01-2003, 08:07 PM
In case you didn't notice, Marvin has no refraction in the glass surface. If you have refraction and are ray-tracing refraction, it will ruin your alpha channel. Use the rgb from the nice ray-traced render, but use the Alpha from the non-raytraced render.

Matthias
08-05-2003, 11:03 AM
Thanks again for your help. The Gradient is a good idea, it just doesn`t really solve my problem.
What I am looking for is something where the highlights/reflections are also taken into account in the alpha channel.
The attached jpg is a Cinema4D render. The RGB Image doesn`t look by far as good as my Lightwave rendering, but at least i can use the alpha for compositing in AE.
Can it be possible that something like this is not possible in LW. Is Raytracing useless in LW?

toby
08-05-2003, 02:05 PM
??? there's actual shading on that alpha channel... that's not really what it's for, the alpha should have nothing to do with the lights in the scene - but if you want to duplicate that, render a separate spec pass and combine it -

geoff3dnz
08-05-2003, 07:48 PM
Originally posted by toby
In case you didn't notice, Marvin has no refraction in the glass surface. If you have refraction and are ray-tracing refraction, it will ruin your alpha channel. Use the rgb from the nice ray-traced render, but use the Alpha from the non-raytraced render. This is true, but it used not to be the case - attached is an example showing the difference in alpha between versions 6.5 and 7.5...
http://www.geoff3d.com/images/LW/6.5_alpha_refraction.jpg
http://www.geoff3d.com/images/LW/7.5_alpha_refraction.jpg
Deuce said he submitted this to bugs, but nothing ever came of it in either 7.5b, or 7.5c :-/

UnCommonGrafx
08-05-2003, 08:57 PM
Bump!

I want this one to be fixed really bad.

wacom
08-05-2003, 09:28 PM
I just checked it out too...ditto here...if I try to render out with refraction I get the crappy B&W alpha...no refraction and I get the alpha. There isn't a real good work around for this if you've got stuff inside of the glass or liquid etc. as the alpha from the non-refraction image will not match up with the refraction enabled image.

This is a real pain- I'm supposed to do a whole mad scientist lab for a guy who is then going to re-comp it in PhotoShop with other elements- so he and I MUST HAVE THE ALPHA. Those image are going to look lame'ah then hell if I can't get my refraction turned on w/alpha! GRRRRRRRRRRRR.... This is SUCH a fundamental flaw...and just when they give us DFX...ha!:mad: :mad:

BTW I'm using LW 7.5c:mad:

UnCommonGrafx
08-05-2003, 10:12 PM
But Wacom,
If you have DFX, there is actually a no-brain fix:
Render a 24 bit image for color
Save your alphas without refraction, or other raytracing choices as a separate entity. Probably a 24 bit render for this will suffice, as well.

Merge them with the Alpha cutting out the Color image for the effect you are after.

You can probably then send back out tgas or rtvs with alpha if ya got a VT.

But it still sucks that it's like this.

wacom
08-05-2003, 10:40 PM
Originally posted by UnCommonGrafx
But Wacom,
If you have DFX, there is actually a no-brain fix:
Render a 24 bit image for color
Save your alphas without refraction, or other raytracing choices as a separate entity. Probably a 24 bit render for this will suffice, as well.

Merge them with the Alpha cutting out the Color image for the effect you are after.

You can probably then send back out tgas or rtvs with alpha if ya got a VT.

But it still sucks that it's like this.

This is true if you don't have anything inside of or behind the object that is using refraction...if you do then your Alpha image will not match up with you're color image...aka the surface of your liquid will have the alpha of the NON refracted render.

You see I have to do a "Monster Lab" scene that has a guy floating in a huge vat/tank with a ton of liquid and thick glass. The client wants to put stuff behind this tank...and the alpha channel for the guy floating in the tank is going to be way diffrent from the one I'd get not using refraction.

If you leave refraction on and go to a surface that uses it and change its Additive transparency to anything above 0% you'll get an alpha channel...BUT its still the wrong alpha!

I've included a quick/poor render...but it illustrates my point