RT Cheatsheet - Get up and running fast for Dummies? Questions!

[Morne]

Hi all

I'm seriously considering to start using RT and possibly even switch most workflow over, so like Lele's recent sp3 cheatsheet, is there something like that for RT?

How do you use it?

1) What's the difference between using it in activeshade and setting the actual renderer to RT?
2) If I set it to CPU mode, does it use the GPU at all? What's the point of CPU mode if we have progressive in advanced?
3) I have a GTX, so would GPU in CUDA mode be my best bet?
4) Will GPU be faster than CPU mode?
5) Is there a way to get the CPU to help in GPU mode, or will that just slow things down?
6) I read that my entire scene + textures + proxies need to fit into GPU Ram, Is that a literal disk space to RAM comparison or is there a calculator for this? If in adv my scene is 500mb large and use 2GB to render, does that mean I need at least 2.5GB GPU ram to render same in RT?
7) What optimum settings can I use like I do in adv sp3? Is it same thing or is there some other secret magic going on that only 5 people know about?

Thank you in advance for your patience and help!

[yavor.rubenov]

Hello,

1) Apart from being able to manipulate the scene when in Active Shade mode V-Ray RT uses the viewport representation of geometry that supports it - f.e ParticleFlow, various hair systems etc. Also - when in Active Shade V-Ray RT will not calculate light cache.

2) In CPU mode the GPU is not used. RT CPU can be used in ActiveShade while Adv. Progressive can't.

3) If you have a nVidia GPU (GTX fe) - yes you should use CUDA.

4) Depends on many factors - what are your CPU and GPU, scene type etc.. there are low end GPUs that won't be fast compared to a good Xeon CPU.

5) The CPU does help in GPU rendering - prepares task for the GPU, collects results, displays them in the VFB and etc..

6) There is no calculator, but you can make an rough estimation using this:
1 million polygons around 220MB of GPU memory, motion blurred ones take around 330MB.
Textures take "width x height x numChannels x 4" bytes. NumChannels depends on the RT Settings tab settings and can be 1 (byte), 2 (half) or 4 (float).
There are few hundred mega bytes used by RT GPU on any scene for its buffers as well.
Adding stuff like anisotropy or hairs takes memory on its own.

7) Settings - you can use settings like the progressive Adv.

Hope that helps,

Best regards,
Yavor

[Morne]

1) So then for tests and while working you will use activeshade, but for final render switch the actual renderer to RT? - Final RT should be slightly faster because can use LC?

3, 4) Currently I have a 3930K and GTX 670 4GB. Will get couple of 980 Ti 6GB or couple of Titan X 12 GB later on

[yavor.rubenov]

1) Yes

2) You can even leave the 670 for display purposes and have the 980Ti/Titan only for rendering - couple of them should be faster than 3930K.

[Morne]

2) And between the GTX 690 CUDA and the 3930K CPU, which one should be faster?

[yavor.rubenov]

Would be best to test it

[Morne]

Would be best to test it

Is this the latest one to test with?
http://forums.chaosgroup.com/showthr...228#post445228

[Moriah]

I have the same CPU, and right now 2x 980 Ti for testing RT GPU. From a few scenes i've been testing, while for example the CPU takes around 10 min to render, it takes me ~1m30 with GPU.

[super gnu]

i have the same cpu and the same gpu. (well i now have a titan x too

the 670 4gb is approximately as fast as my two 3930k's at 4.4ghz.. but consider this is comparing rt gpu to rt cpu.




a more fair comparison would be with vray adv. progressive, which is faster for a final render than rt cpu.


fyi the titan x is approximately twice as fast as the 670 (and also has 3x more ram to play with)


so, very approximately, a titan x is equivalent (in RT) to 4x 3930k's (so 24 cores, 48 threads) overclocked to 4.4ghz.

since we can now get machines with 72 threads, but not at such high clock speeds, you can probably see that its -possible- to get a single workstation that is faster than a single titan x, but definitely not cheap.

[Morne]

So instead of getting a new PC with the latest i7, it would cost about the same to get 3 or 4 980Ti and stick that in my current system? (Or 2 x 1080?)