Multiple Cutout Opacity Cards Creating Alpha Weirdness (MR)

#1

Hi All,

I’m trying to create some little fuzz balls in Maya in a very video game-ey way by using a bunch of single poly planes with fuzzy textures applied to the cutout opacity in the mia_material_x material I’m using (I’m still using Mental Ray), but it’s causing some real weirdness in the alpha channel when the planes overlap.

I’ve tired to distill it down to the simplest possible version with just two planes so you can see what I mean.

Here is the setup in the viewport:

Here is the render:

And here is the alpha channel

You can clearly see the outline of the rear plane over the alpha of the front plane. I’ve checked the fuzz alphas themselves and the cutout parts are definitely pure black. I just don’t understand what’s happening. And it does look fine on black, but as soon as I try to put it over a different background it gets weird because the alpha is so messed up.

Here is an example of a more complicated, more finished version. Looks fine on black:

On light gray, you again see the various planes interacting strangely:

Any help would be really appreciated. Thanks!

Colin

#2

(For what it’s worth, since mental ray is officially discontinued, it might be hard to get help with it…)

#3

Are you using the built-in alpha channel, from the render view?

Hmm, in the Viewport 2 view, you can use “depth peeling” to fix this, but that wouldn’t affect the rendering itself.

Maybe set your shadows to “Segments” in the render settings? I use cutout opacity quite a lot, and have run into stuff like this, but on tree leaves you don’t really notice so much since they’re so tiny compared to the render size, really.

Perhaps try an opacity pass and see if that works? If so, it would be almost no rendering overhead for the pass.

#4

The error appears in both the built-in alpha channel from render view and in my EXR file when I go to composite. I’ll give the shadows settings a try, thanks. And, you’re right, in practice there will be thousands of these little fluffs and they’ll be tiny; after starting to test more finalized renders I think they will be small enough that it won’t be very noticeable. Thanks again for you’re help.

And, to gfk, you’re right I know I’m using an antiquated renderer, I just thought the people that used it for 10+ years like I did probably hadn’t forgotten all their knowledge. I guess it’s time for me to switch to something else, I just didn’t care for Arnold much when I tested it and it wasn’t significantly faster in the test renders I ran. Maybe I should give vray another try, I liked it pretty well when I used it for awhile years ago.

#5

I’ve run into similar issues before in my tree leaf renderings, which are almost always “cutout opacity” shaders (mia_material_x). I haven’t really used the MILA materials much yet, and don’t see any point in learning them now as I’m slowly transitioning to Vray (Arnold is stupid and it’s a stupid name for a rendering engine - my cat is named Stuart and he’s also pretty stupid).

I’m digging through to see what I did to fix this effect, but it doesn’t seem reproducible anymore on my end.