View Full Version : Problems Rendering transparencies

12-15-2003, 03:02 PM

Everytime I render a transparent object, the alpha is not transparent. This is a critical issue. Is this another mac bug? THANKS!.


12-15-2003, 08:43 PM
Yeah, i've run into this many times too. I don't know if it is a mac bug, wouldn't be suprized though. I've found that with raytrace transparency off it will render into the alpha, but with raytrace transparency on it does not affect the alpha channel. Stupid, i know.

12-15-2003, 10:58 PM
Your probly using an image format that supports alphas and there for must go in to the edit image panel and set the alpha to disabled

Julian Johnson
12-15-2003, 11:40 PM
It happens on the PC, too. When you turn on ray trace transparency you're effectively telling LW to perform the exact same tracing functions it performs when you have refraction on - only with no distortion. As with refraction, LW is unable to calculate the net transparency of all the objects it sees behind the traced object, for some reason, and just makes the alpha 100% white for the transparent area.


12-16-2003, 10:25 AM
As a workaround i guess you could render a second pass with all raytracing options off and just use the alpha channel from that with the RGB channels from your first render.

12-19-2003, 06:20 AM
Have you tried rendering the scene with "Foreground Fader Alpha" selected in the render options panel? BTW - any sequence generated using this will need to be comped in a 3rd party package like after effects as the RGB portion is not premultiplied against the background colour. If using aftereffects, make sure footage interpretation has it set as "Alpha - Straight (unmatted)"

12-19-2003, 08:52 AM
Thank you for the suggestion, but I'm having a hard time finding "Foreground Fader Alpha" in the render option. Please provide screenshot if possible.


12-19-2003, 10:13 AM
The fader alpha option should be a tickbox directly under the Alpha output filepath dialogue box.

IIRC it's right at the bottom of the render options/output window - BTW you can only turn it on if LW is set to render an Alpha.

12-19-2003, 10:17 AM
Another possible workaround is to set the value of the alpha for that surface manually (obviously this'll only work if the object you want to render transparent has consistent transparency all over).

You can set it in the Surface Editor / Advanced / Alpha Value

12-19-2003, 04:35 PM
The problem with setting it manually is that it then overrides any surfaces behind it. So, say you have a box sitting on a ground plane, and only the top have of it should be transparent, and you set it to 50% transparent. Rather than only the top have of the box being 50% gray in the alpha channel, it will actually make the whole surface of the box 50% gray in the alpha, ignoring the solid ground plane behind it. It's a huge pain.

12-26-2003, 12:33 AM
I wonder if there is a noticable loss in quality when not raytracing transpanrency ?

Since disabling raytracing seems now the only (simple) way to allow for compositing onto a fixed background (video etc.) and allowing for transparent objects?...

With raytracing on (trans), it seems that only the background is excluded from alpha channels produced by objects (that transparent object portion seems to be rendered against a black background), and that any objects behind do show trough transparent objects...

(In my case I've used as background a microsoft DV AVI file.)