Rendering opacity and fog with Mental Ray

[Anonymous]

Hi, I have a scene that uses some shape facing particles utilising a material where a gradient ramp is used to control the opacity. The idea was that each individual shape facing particle would blend from opaque in the centre to fully transparent at the edges so that they blend nicely into the scene.
The scene also uses a Fog environment effect, and it is here where the problem exists. Where ever the particles are rendered against the background opposed to any other 3d object, the opacity does not work correctly, and instead shows up as solid. Where as, any of the particles that are rendered against/in front of any other 3d objects in the scene, the transparency works fine.

I can't seem to find the settings that will sort this out, so has any one else found this problem before and found a solution!

Thanks

[Anonymous]

Something to try, it may work.
Make a big sphere to enclose the whole scene and invert it's normals, as if making a sky dome. Then apply a null material to it. You can do this in mr by starting with a mental ray material, in the surface and shadow slots, put TransMat.
Now the particles will not be against the environment, but some geometry, but the geometry doesn't do anything, so it appears as environment background.
Also, what type of material/opacity map are you using for the particles? Best to use A&D and it's Cutout Opacity for this in mr.

[Anonymous]

Thanks for the reply, I did try what you suggested and didn't really find that it changed things. The problem seems to get worse, the further into the fog the paricles are.
The material applied to the particles is an A&D based material, with a greyscale image placed in the Cutout slot to handle the opacity.
I'm trying to attach a couple of images to show the problem, but it doesnt seem to actually upload them when I click to add attachment...
Maybe I'll just have to try a different method of adding this kind of thing in post. What I'm trying to do is show a subsea scene that is focused on the legs of an offshore platform, where some plantlife has grown on the below water areas of the legs...

[StephenMF]

Hi Guys...
I ran in to this problem on a scene of my own too. Searched the forums and came across this thread, but never really found an answer...

Some of the sugestions worked, but left me with an opaque alpha channel.

So all the answers led me to explore the materials where the transparency got messed up.

I think i found a solution that works with all kinds of fog.

If a promaterials: generic material is used and the transparency is made with the cutout opacity channel, everything seems to work fine.

Did you find any other solution?

enjoy.
Stephen