Is it normal for opacity to be so slow!?

[RSF]

Hello.
I know opacity is a bit slow in VRay but it seams a little too much.
I made a test with a tree using either opacity mapped leaves (12066 faces) and geometry leaves (821664 faces...that’s 68 times more).
Here are the render times:
Without GI:
Mapped leaves -> 2:04 mins
Geometry leaves -> 0:33 mins

With GI (leaves only receiving):
Mapped leaves -> 6:11 mins
Geometry leaves -> 1:00 mins

To be honest I tried using scanline renderer with raytrace shadows on the mapped tree and it almost stopped...maybe I missed something

Are these timings normal?
I had the idea that opacity maps were used to speed up thing and not the reverse

Well...here's the max file if someone is willing to try to tweak it a little. (sorry for the size)
http://clientes.netvisao.pt/ruquita/tree.zip

Thanks in advance

[DaForce]

Yeah opactiy can sometimes be quite slow. What version of vray are you using?

[vlado]

You can do 2 things:

(*) If you use a newer VRay build, use VRayMtl with opacity mapping
(*) Disable filtering for the opacity map texture. This will not affect the speed of GI calculations, but the final rendering may be a lot faster.

I'll post some examples for this in a few moments.

Best regards,
Vlado

[RSF]

Hi! Thanks for the replies.
I guess I used all the versions but currently I'm using 45.7

Is this issue inherent to the rendering software architecture or can we expect it to be much faster in a hopefully near future?

I mean..is this the way things work? It's slower and that's it!
Should the opacity mapped tree with 68 times less faces be faster in an idealy optimized system or the method of using opacity maps to speed up things are only intended to replace a full 3d tree with a single billboard tree?

As I said...I tried the scanline renderer and it almost stopped when rendering the mapped leaves

Is it possible to do a ratio between the two methods Vlado?
Something like "a face with a 100x100 opacity map" will render as fast as a 100 face mesh"?

Thanks again

[gadzooks]

I was fixing to post this very question. Im currently rendering an exterior animation with archvision trees and they look horrible. I would love to used opacity mapped trees and shrubs but the opacity is so slow that my frame times would go thru the roof. Vlado is the opacity rendering function in 1.5 going to be a lot faster.

[vlado]

Here are some examples:

Standard material with filtering on the opacity map (slowest of all):


Standard material without filtering on the opacity map:


VRay material with filtering on the opacity map:


VRay material without filtering on the opacity map:


VRay material without opacity map at all:


Best regards,
Vlado

[Geddart]

Nice put together vlado. Maybe you can put it in the Troubleshooting Section so people know how they can get faster rendering times with Opacity Mappings.

[RSF]

Hi! Thanks Vlado.

Turning filtering off really helps but the diference between the geometry leaves is strange. Its a lot mory faces in less time.
Using GI on the opacity mapped leaves just blows the timings away.

Where are the timings I got using this opacity map:


Geometry leaves (with and without GI):






and opacity mapped:





and here are some tests using diferent opacity maps:

test 1 (same map as the above):




test 2 (bigger map)




test 3 (same as last one but jpg)





The setings for the GI are very low:
min = -3; max = -2
10 subdivision and 10 interpolation
for secondary bounces is quasi-monte carlo with 8 subdivs and 3 bounces

Is there a way to optimise this?
Why is the diference so great when using opacity?
Is it a non optimized area of the software or is it a mathematical determination?? (hope it's the 1st one)
In a perfect world shoud the opacity tree be faster then the 68 times more geometry tree?
Is this going to change ni future vertions?

Sorry for the insistence but I'm really curius about this.

Thanks again?

[vlado]

Opacity mapping is slower, because it requires more work. Think about it: when tracing a ray, it hits a surface. VRay calls the material for the surface, which also computes its opacity. If the surface is transparent, VRay starts another ray to compute what is behind the surface. If the percentage of perfectly transparent surfaces is large (like in the case of leaves) that means quite a lot of calculations which do not contribute at all to the final result.

Things are very much worse for standard materials, which always compute lighting (GI too), even for perfectly transparent parts of the surface, which does not affect the image in any way, but takes a lot of calculations.

On the other hand, if you have only geometry, things are a lot faster since all those unnecessary steps are omitted. Of course, some time is lost in calculating intersections with the extra geometry, but it's a lot less than the time required to do all the intersection/shading/new ray cycle.

One way to speed things up is to introduce clip mapping, which allows the raytracer to skip perfectly transparent parts while doing the intersections (before any shading occurs). Clip mapping is a property of the geometry itself, not of the material, and would require some extra storage for each triangle. VRay does not support clip mapping, except when doing 2d displacement.

Best regards,
Vlado

[RSF]

Dam....was afraid of that!
It's just that exterior scenes with lots of vegetation are so dificult to speed up
I guess "using opacity maps to speed up things" referes to having a single billboard tree
Well...Thanks again Vlado.

[lander]

Is that a "does not support Clip mapping" period or is it something that we'll see in the future?

Has that question been handled elsewere? In that case sorry