Basic RT / GPU questions about rendering and Active Shade use

[Streetwise]

I've been so focused for a long time on CPU renders via backburner and DBR that I haven't really kept up with the notion of RT and what artists are using it for. 98% of my work is 5K resolution for print, so it seems logical to use CPU rendering. In the meantime, I don't understand the benefits of RT rendering as it pertains to what I do. One of the benefits that I see that is intriguing to me is the use of Active Shade. Right now, I have a single GTX460 card, so it pretty much chokes when I try to use it. I'm thinking about upgrading to a GTX970 to see if that would benefit me for this sort of use. My workflow is to hit 'render', over and over again until I get lighting right, etc. Seems like Active Shade has a lot of benefits to it.

- When does RT rendering come into play as a viable alternative and why isn't it more mainstream?
- How popular is Active Shade in a production environment to see the scene in RT?
- Is one GTX970 sufficient to use Active Shade? I've heard that some are using two cards. Does that make Active Shade even faster to have two cards?

Sorry this is so basic.

Thanks

[savage309]

The benefit is mainly that for most of the scenes, rendering on the GPU is faster.
Render image size does no affect the GPU memory usage (so you can render as large images as you wish). We just don't keep the frame buffer on the GPU.
If you want to use one GPU for monitor and rendering, the UI of the system might become sluggish. You can decrease ray bundle size / rays per pixel from the RT tab until the UI is okay to fix that.
Or if you have another GPU, you can left one of the GPUs for the monitor(s) and to left the other for rendering. The way you do this is simple - just plug the monitor(s) in the GPU you want and de-select it from ocldeviceselect.exe (this tool comes with the V-Ray installation and lets you choose which GPUs should be used. By default, V-Ray will use all the GPUs it can find). You can have 460 and 970 in one system without any problems.
Also, V-Ray RT GPU scales linearly - meaning that if you render with 2 GPUs it will be twice as fast (if they are equal in speed, of course. If they are not it will as faster as each of them is).
And here we have some more info about the GPU rendering - http://docs.chaosgroup.com/display/VRAY3/GPU+Rendering.

[Streetwise]

Interesting in that it's much faster, yet it sounds like to me that more are rendering on CPU's yet. There must still be some downsides to GPU rendering?

[savage309]

Not everything is supported, check that page - http://docs.chaosgroup.com/display/V...rted+Features#
Also, if the scene does not fit in the GPU memory (polygons + textures + etc), the GPU won't be able to render it. And some scenes are better to be rendered on the CPU (like caustics). For some scenes the algorithms of the CPU render are better suited, but it is hard to compare. It will be best to test and see if it works for you (as you already have a GPU).