View Full Version : Solid Apha problem when using refraction

08-05-2003, 09:46 PM

Read this thread started above by another LW'er. I ran a test as well and was shocked to find it true...and a work around SHOULD NOT be needed for something like this. How is DFX+ going to help my 3D when this isn't working...

Anway...let me get to the point. When you turn on refraction for a render it screws up your alpha channel. For glass and liquid you end up getting a solid B&W alpha- not good. I have a mad scientist lab project that uses glass and liquids all over the place- and refraction is a key player in this. The files I render need to be tweaked by someone in photoshop...and we're depending on the alpha to help.

This wasn't broken in 6.5 ... why did it break? I need a fix before 8 and this isn't a feature request- this is a bug!:mad:


08-05-2003, 10:53 PM
I've done a few more tests...and can get an alpha from the image when I turn on the additive tranparency in the advanced tab to above 0% (though it slows down the rendering a bit). But this has led me to another problem that maybe someone can explain. Can the Ligthwave render engine acount for renfraction when adding an alpha channel? I've included an image to illustrate my point (this is from the same render using refraction...but I get a Alpha for a non-refraction enabled surface...hmm). Normally this wouldn't be a problem for some people...but when you have objects floating in a liquid...or just liquid in a thick glass this can be a problem...

08-06-2003, 09:29 AM
First, lets put refraction on the side for a second.

A Transparent surface is a Color-layer superimposed over an image. The amount of opacity=Alpha Channel.

Refraction involves distortion of the scene behind the transparent surface. Its impossible to involve Refraction into Alpha Channel, (nor Z-Buffer btw)

For your project you need 2 different Masks.
1. The Shape of the transparent surfaces (solid Alpha)
2. The Level of transparency (ordinary Alpha)

The simplest way to retrieve both, is to render your scene twice as follows :

One of the two is the ordinary render, that creates ordinary Alpha.
The second, you can replace your objects with non-transparent ones, and render without shadows etc, just for creating the matte.

Then in your compositing app, load your ordinary rendering and use the normal alpha to adjust your transparent surfaces.
Save this result as a separate sequence.

Then load your normal render, your processed one, and the solid alpha and blend accordingly.

Good luck