View Full Version : Object Opacity Question

04-19-2007, 09:22 AM
Doing an archviz simulation for a marketplace and client has asked to set the trees at 15% opacity. I'm doing this through object properties. It seems that if there are trees lined up behind one another, there is an 'additive' effect where the trees cross in front of one another. Does anyone have experience with this? It creates an undesireable effect and what we're working toward is 15% opacity no matter if trees are standing alone or lined up behind one another. Thanks for any help!

04-19-2007, 02:38 PM
OK I may be wrong about this but I am going to go ahead and stick my neck out and say that your client is asking for the impossible.

Any transparent object or even surface setting is going to have an additive effect. Think of a stack of tinted glass. Each one you could see though but a stack of 10 or more would be nearly opaque.

The solution would be to get all the trees as one render pass without transparency then make all of them together as transparent.

This is just a very general idea as I know nothing of the scene/animation. Why not post some screen shots and see if somebody has any better ideas. I can't really think of how that would be possible to do without some kind of compositing.

Any ideas for nodes perhaps?

04-19-2007, 02:45 PM
Was typing this as surrealist was answering... I think he's right about the impossibility since it has to do with quantity of light reflecting off objects back through to the viewer.

If limited to LW I would try increasing the transparency of objects behind other transparent objects to get a more pleasing-- less cumulative effect. Also using gradients with an incident angle or slope on the transparency will give a more interesting effect-- though maybe not the effect you are looking for.

However, I would probably render the transparent objects in separate passes and try different blend modes in compositing. You could try one frame and a few objects as a test, before committing a lot of time.

04-19-2007, 03:09 PM
Just incase it's a raytracing problem try increasing your ray recursion limit. I agree transparency will decrease gradually as you overlap more surfaces but if ray recursion isn't high enough you will get a sudden change to opaque/black patches.

04-19-2007, 09:28 PM
Thanks all. Later in the day, after I posted this thread it occurred to me that I could render the trees in a separate pass and composite in After Effects. Not sure why it hadn't come to me sooner. Sounds like we're all thinking the same thing.

In looking at the test animation, I explained to the client that the transparency of the trees was creating an additive effect and in my mind I was thinking that I'd do some research and see if there wasn't something you could activate to get a 20% opacity across the board for example . . .

The ray recursion comment is a great idea too. I'm trying to keep it pretty low because I'm using radiosity. I'll adjust it up and maybe it'll make enough difference that the effect will be acceptable even if some additive effect.

Thanks for the great ideas.

04-19-2007, 10:13 PM
Transparency like this is a real killer, I've had situations in recent projects where the client wanted 2 of the three stages 'ghosted'. Everything, all buildings, trees etc had to be 50% transparent. I easily hit the ray recursion limit easily and it was a pain to get around. In the end I rendered twice (once with everything, once with just stage 1) and mixed the two in post.

04-22-2007, 05:08 PM
I've found LW's perspective camera to handle semi-opaque objects a lot better than the old classic camera...a *lot* better.