change in render outcome going from GPU to CPU

[Armada01]

I'm working on a file with a "sea" material,, basically water with a wave bump map.

When I render in GPU mode (in Rhino6), the outcome is good, however, when I launch the same render in CPU mode, the water becomes nearly black.

Why can there be an issue? I believe only GPU rendering has some more limited functionality (like volumetric)

[Armada01]

here's a visual depiction of the problem:

[Peter.Chaushev]

Hello, and apologies for the delayed response!
I assume you are using a V-Ray Infinite Plane for the water surface and you initially prepared the scene based on GPU render output. The difference in the texture mapping is due to a bug with bump texture mapping on Infinite plane rendering in GPU.
If you use a regular surface instead, the mapping will be identical.

Our developers will investigate the issue based on your report. Thank you for reaching out and be sure to let me know if you have any further questions regarding V-Ray for Rhino.


Kind regards,
Peter

[Armada01]

Hello Peter, thanks for your reply.

In fact, the water shader is already applied to a box shape, so regular Rhino geometry.

[Peter.Chaushev]

Oh, alright. Refractive materials are rendered slightly different between CPU and GPU. I would be able to tell you more if you can share with me your scene (just the water box, without the boat) either through forum PM or an email to support@chaosgroup.com (use V-Ray > Pack Project).

Kind regards,
Peter

[Armada01]

I've packed the rhino file to be sent, but what was essentially a 1.2 MB file Rhino file, became a 280 MB zip file in the process. Seems to the exact opposite of what it's supposed to do...

Any idea what's going on here?

[Peter.Chaushev]

The V-Ray packager collects all asset files, such as image or proxy files, referenced in the project and archives them together with the .3dm project. If you open the 280MB archive you will see all that is being referenced.
You can simply create a copy of your project, then open it, delete all objects and materials not directly related to the water surface, and only then pack the project.

Kind regards,
Peter

[Armada01]

ok thanks, I purged the rhino file already before, but still needed to purge Vray materials separately apparently.

here's the zipped file.

[Peter.Chaushev]

Hello again, and apologies for the delayed response!
Your Water material is slightly transparent (Opacity value < 1) which does not work well with Refraction. It is strongly recommended to keep Opacity = 1 in order to achieve proper material refraction.
In addition to this, set the Refraction Fog Multiplier to a very low value, e.g. between 0.001 and 0.0001 and you will achieve identical results with both GPU and CPU.

Kind regards,
Peter

[Armada01]

Tx, that worked for getting rid of the black water.

One thing I notice, now that I've made the changes to the shader, is that the water is transparent by showing the backrgound (seebed) image. However, the part of the hull that is below the waterline is not visible. (capture.jpg)

Anything to do with making opacity=1? When I make opacity back 0.8, the hull shows through the water, as it would naturally. However, the sea becomes so transparent, that one can see the ocean floor hundreds of meters further. (capture1.jpg)

Any idea of how to control the transparency of water, so that more light rays are bounced back with increasing depth or distance through the water? (attenuation distance)

[palosanto]

Armada01

My experience is that unless you are in pure crystalline turquoise tropical water, and very close to the boat, you don't see the hull under the water.
Just photo-google "boat" and see how many you find where the underwater hull is visible ...
Leave the opacity to 1 and the water color to black. The overall color of the water is mostly a reflection of the environment, sky, clouds, etc. That's why the same water looks totally different on an clear sunny day or on a cloudy stormy day.
Then to play with the color use the fog parameter in refraction.

[konstantin_chaos]

Hi Armada01,

When opacity is used instead of Refraction the 'camera' rays do not bend when they enter the water volume but rather continue in a straight line.
With pure refraction the rays will bend in a natural way sort of shrinking the underwater part of the hull.

This is a natural effect. Check out the following image:


Another thing to note when using opacity is that the ray will not accumulate color while traveling through the water in a natural way, causing you to see the ocean floor.

As a whole 'Opacity' is not a naturally occurring phenomenon.
If you're after realism when working on a material you should stay away from it.
Combining opacity and refraction is also not recommended.

Note that opacity can still be utilized in other situations - masking for example.

Regards,
Konstantin

[Armada01]

Ok, gotcha.

One last question regarding water / waves. I'm currently using a single normal map for the wavelets. If I wanted to use another map with bigger scaled waves onto which the current smaller wavelets are "multiplied" is there a way to do this in Vray?
Ideally I'd use actual 3D modeled "swell" for the wave material to be assigned to for the most realistic wave pattern. So, my waves would ultimately be made out of three layers of "wave information": wavelets / waves / swell.

[palosanto]

Ok, gotcha.

One last question regarding water / waves. I'm currently using a single normal map for the wavelets. If I wanted to use another map with bigger scaled waves onto which the current smaller wavelets are "multiplied" is there a way to do this in Vray?
Ideally I'd use actual 3D modeled "swell" for the wave material to be assigned to for the most realistic wave pattern. So, my waves would ultimately be made out of three layers of "wave information": wavelets / waves / swell.

I often use a combination of wavelets as bump and bigger waves as displacement . Then in either mode you can combine multiple wave sizes by using a "Mix" texture type using a different map in each slot. Each slot in turn can be a "Mix" texture again. That gives you endless possibilities and size control.