View Full Version : Opacity / Fog / 3ds Max Problems

07-07-2003, 04:06 PM
Here is the deal -

I'm using an opacity box in front of my wall as a decal to add some snow -

The problem is, I can really see the edge of the box when I render, and the box becomes more and more visible the farther I place the camera from the wall.

My opacity map is the typical black and white map... White as opaque... Black as transparent... Bottom is white and half the top of the map is black...

Object is tagged as cast no shadows, receive no shadows...

Any ideas why the opacity object is still somewhat visible ?

07-07-2003, 10:26 PM
Try changing Specular Level spinner to 0.

07-07-2003, 10:32 PM
Oops, forgot about the fog part. Here's the answer, from the Discreet webboard...


Topic: Fog in opacity channel: solutions (1 of 1), Read 1008 times
Conf: FAQ's
From: Martin Foster martingfoster@cox.net
Date: Wednesday, February 14, 2001 11:01 AM

Here's what I've summarized from a recent thread.


lots of renderers exhibit this phenomenon. It's because the fog is a z-buffer effect and doesn't take into account the alpha channel correctly. That's normal. The same phenomenon can be seen in MAX scanline, Mental Ray, PRMan, and BMRT.

Some possible solutions:

1) Activate "exponential" switch in the fog settings. This doesn't do anything but shift the problem into a smaller zone and, if you're really lucky, shift the problem outside the zone where your opacity mapped objects exist. It's a quick thing to try though.

2) (my favourite) Render a beauty pass with fog ON, and render a separate matte pass with fog OFF. Composite together using the matte pass as the new alpha channel.

3) For the most power, put your "fogging" in the material, as a falloff map, distance based. This is my favourite when time allows because it gives you control to fog different objects by different amounts, and excluding objects from fog is simple. Depending on how many different materials are involved, you could be in for a rather small or extremely large task. MAXscript might assist in automating this.

4) Some volume fog plugins by 3rd parties might be worth investigating. Results may vary and the rendering will probably be slower.

Martin G Foster

07-07-2003, 10:36 PM
So, the solution in your case could be, instead of using a separate object, to use the Composite map type to put the snow on the wall.

CGTalk Moderation
01-15-2006, 02:00 PM
This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.