Blend Material Slow Performance

[bkirch]

The Blend Material is quite inefficient when compared to the individual materials that make up the Blend Material. This occurs in both 3dsmax and Maya but I had to choose a forum to post in.

Image named Blend_Mtl_Maya.jpg - rendered in Maya 2014 VRay 2.45.01 (example scene obtained from fullblownimages.com).
Images A and B are standard VRay Materials. Image C is a blend of A and B with a Maya Checker texture as the blend amount. Notice how the render time increases from about 1:00 to 1:43. Where is that extra :43 seconds of render time occurring? This is the most basic example. As shader networks in the Blend Mtl become more complex the render times increase even more. It's as if the Blend Mtl is calculating all parts of every shader, rather than only what is visible through a mask. From what I've read about the new MILA shaders in MR this is not the case.


Image named Blend_Mtl_3dsmax.jpg - rendered in 3dsmax 2013 VRay 2.30.01
Image A was rendered with only a VR Car Paint Material. It is a rather complex shader, with connections in Diffuse, Refl Glossiness, Refl Color, Bump, but rendered in 1:24. Image B was rendered with that same Car Paint material in the Base slot of a VR Blend Mt, plus a couple other simple VR Materials. Render time increased by two minutes! The blend amounts of the other two materials are controlled by a bitmap and a 3dsmax Cellular texture.

I would just like understand why the Blend Material seems to be so inefficient. Are there are any work-arounds, and/or will the Blend Mtl perhaps be more efficient/faster in future versions of VRay?


Thanks for any insight,
-Brian

[bkirch]

After reading through a fair bit of threads on these forums I discovered it is bad form to mention other render engines. Apologies for that, didn't mean to ruffle any feathers, it was my first post.


-Brian

[Morbid Angel]

are you using additive option?

[bkirch]

No, additive option is disabled in both cases. Greyscale procedurals or bitmaps are controlling blend amounts in all cases.

[Morbid Angel]

can you post this scene? I can't reproduce the issue. Regular material 0.06 seconds for me, in blend two materials 0.07 seconds (on simple scene)

[bkirch]

Here is the scene with no texture maps. I tested once more and my render times are :38 and 1:09 for the base material and blend mtl respectively. Thanks for taking a look.

[Morbid Angel]

Alright I had a look at the scene here is what I can conclude. The vray blend material is working correctly. The issue is the map which is being used to blend the two materials. Its not really the issue, but it does take additional time to anti-alias and filter the map borders between the two materials. If you do not use the checker, and instead use solid color at 50%, then the render time is almost the same as the base material alone. If you then try to render vray blend with checker and sampling set to fixed it renders at the same time as the base material. If you simply plug in check into diffuse of the base material (and color it same as both materials in the blend), it then renders at the same time as vray blend.

This is simply the dmc sampler taking more time along the checker borders to produce fine quality nothing more. In your more complex scene, you are dealing with flakes, they naturally like hair will take lots of samples to produce proper AA.

[RockinAkin]

Dmitry hit the nail on the head.

If you took your first comparison image and looked at the SampleRate render element for each render - you'll no doubt see more image samples being taken to resolve the edges of the checker pattern.

In your second comparison image, there are no doubt some grey values in the masks for your blend amount masks, so now V-Ray has to calculate 3 materials in some places where it only had to calculate one - in addition to the extra work the image sampler has to do for any defined edges, again visible in the Sample Rate render element.

[bkirch]

Thanks for the insight guys, this is exactly the type of feedback I was hoping to receive.

Btw, I re-tested the 3dsmax example and I don't know what changed, but the difference between the base car paint mtl and the blend mtl was only 15 seconds this time, not 2 minutes. That's more like what I expected to see based on how little surface area the blend-amount maps are covering.

Thanks for taking the time.

-Brian