Render Speed – Alpha Vs Geometry

[Raoul]

So i have a bit of a query.

I've been using Megascans textures a lot recently and they have some great grass and leaf scans .. basically textures with an B/W alpha defining the edge of the shape.
The problem i've run into when i scatter a few hundred or so of these textures over a surface.. simple 1x1 polygon planes with texture applied.. it really kills the render times.
from a 2 min frame without the grass to something more like 2 -30 hours with grass. So i can understand its getting very complicated with the shadows and lighting bounces etc..
However.. if i recreate a scene like that and use simple geo to create the grass form, instead of an alpha.. the render times drop back down to very reasonable 3-5 mins etc.

So my question is there something I'm missing about using alpha in the opacity channel of a material? Is there another way to approach this without killing render times..

I've taken some screen grabs of a very simple setup to illustrate my point.

Heres a set up with Geo defining the grass, not an alpha. very reasonable render time of 31seconds.

But then i use an alpha on a 1x1 polygon to define the form.. pretty much exactly the same in every other way. and the render time jumps to 21 minutes..

I'm just curious if this is something that can be addressed.. or if its just best not to use alphas.. (which would be a pain .. as its often the best/only way to do large landscapes with plant detail...
Also curious if Octane had the same performance hits with alphas in previous version of the plugin?

Thanks for your help and thoughts!

[atome451]

Hi,

Yes. It's an old problem. If you uncheck Alpha Shadows in the Kernel, your render will be faster. But it's not a solution of course because Alpha Shadows is often needed for a lot of another things...

That's why Forester is so interresting with Octane: all leaves are polygonal.

[Raoul]

Hey Atome, Thanks for the confirmation.
Yeah its a pity. as there are a LOT of situations where alpha maps for repeating textures are very useful. It does strike me that it could be something that could be optimised for. I don't see why a piece of geo should be that much quicker than a texture.. but hey I'm not a coder.

Forrester looks good but doesn't look to be in the same league as whats possible with Megascans. https://www.youtube.com/watch?v=0EyHSzfDo6c

Would love to know from Aoktar or the broader team if they think theres any chance something like this might get addressed down the line?
Cheers
R

[aoktar]

It's not just a performance drop about alpha. Mixed situation with very dense usage of objects. Note that this is a pathtracer so rays have to travel the world.