Section Planes cut down render times

grinch2901

In Iray you can create a section plane under the Create menue. It creates an invisible plane that cuts everything in half. And by that I mean on one side of the plane everything is normal, on the other side it's just gone. Nothing there. If you rotate it around you can change where the "cut" happens. I've found that it can greatly decrease render times if you use a section plan to cut off geometry that's "off camera".  It does mean lighting is sort of messed up, as you won't get shadows from the items cut off. But if your carefull you can work around it.  

I was trying to render some closed sets on my little old laptop and after half an hour it was still way too grainy. Add a section plane, rotate it around and adjust some lighting and I get "good enough" quality in around 5 minutes. Something to consider if you have complex scenes. I suppose you could parent the plane to the camera and just cut everyting behind it all the time.  Thought I'd share since I haven't seen much discussion of this feature.

Subtropic Pixel

grinch2901 said:

In Iray you can create a section plane under the Create menue. It creates an invisible plane that cuts everything in half. And by that I mean on one side of the plane everything is normal, on the other side it's just gone. Nothing there. If you rotate it around you can change where the "cut" happens. I've found that it can greatly increase render times if you use a section plan to cut off geometry that's "off camera".  It does mean lighting is sort of messed up, as you won't get shadows from the items cut off. But if your carefull you can work around it.  

I was trying to render some closed sets on my little old laptop and after half an hour it was still way too grainy. Add a section plane, rotate it around and adjust some lighting and I get "good enough" quality in around 5 minutes. Something to consider if you have complex scenes. I suppose you could parent the plane to the camera and just cut everyting behind it all the time.  Thought I'd share since I haven't seen much discussion of this feature.

Did you mean "decrease" in the highlighted part above?

And...would you list your hardware?  It may help the reader to decide when this technique might be helpful, say for owners of (older?  slower?) systems.

grinch2901

I do mean decrease. I fixed that.  It stands to reason that if you cut off unseen geometry, there are fewer light bounces to calculate, faster to converge.  In a few experiments I've found this to be the case for me when I'm working with rooms that are fully enclosed. Without the cuts, they take forever to converge on my machine. With them, much much faster. Of course of you're using HDRI lighting it will pass right through the now-invisible walls (or buildings or whatever you're cutting out) and so you either need to block that light with a simple plane or work around it some other way. 

I render with a 2 year old laptop that was high end at the time but now, not so much.  Hawrdware details: i7-4700MQ CPU with 8GB or RAM and an NVIDIA GeoForce 750M GPU with 2GB of RAM. 

 

Havos

I suspect the main reason you are seeing faster render times is the fact there is more light around, rather than the loss of the geometry. Closed sets normally take a very long time to render when the only light is coming through any windows, as then pretty much the whole room is lit by ambient light alone, and calculating all those ray traces that bounce around to create the ambient will take a long time. Slice off a wall or two using a section pane, and light will flood into the closed set, meaning it will be quick to render.

Are you aware that there is a tick box on the section plane where you can specify the the plane should still block light, and reflect as normal. The camera can still see through, but otherwise the render will be the same. This is useful when rendering small rooms where you would need a wide angle lens to get the shot you want, but a distant see through camera can have a normal focal lens. This is particularly useful if there are any reflective surfaces that would show the missing walls. Naturally ticking this option would kill off your faster render times.

Subtropic Pixel

Okay, thank you for the extra info.  You definitely have some constraints there.  Users with fairly modern desktops and multiple Nvidia GPUs probably won't need to do this.

grinch2901

Havos said:

I suspect the main reason you are seeing faster render times is the fact there is more light around, rather than the loss of the geometry. 

 Precisely. Less geometry = more light entering AND fewer light bounces to calculate. It runs faster for me to cut open a wall this way. You're right, the light is affected and you get more, I've compensated with tone mapping for the most part. Wasn't aware you could have section planes that made things invisible but still did the shadows/bounces of the unseen geometty. Thanks for the tip.