Extreme rendertime with displacement

[Numse]

I´m working on an interior that has 3 objects with 3d displacement.

This image shows my render settings, displacement setting and the crazy rendertime.
http://www.digitalartist.biz/Tillfal...splacement.jpg

Something is obviously wrong. Seems like the rendertime freaks out when I use higher IRmap settings.

[fabriceb]

same here, I have standard settings, left the image to render all night at 5000 px across and it hasn't even started rendering in the morning. it's fine without GI. but I'm using light cache and the displacement calculation just takes forever...

[jacksc02]

do you need to use 3d displacement?
If its just carpet your displacing 2d displacement should be sufficient

[rmejia]

The critical factors that bring down the render time for me are raising the Dynamic memory limit and raising the Edge length parameter in the VRayDisplacementMod (and/or increasing the subdivisions of the object)

[Numse]

@jacksc02
I tried 2d at first on one of the objects (the carpet) but it became much darker than before, dont know why.
As far as I know 3d displacement shouldn´t take that much more memory than 2d now in the newer versions of vray. If th settings are correct.

@rmejia
Yes, but but with my settings the first irr pass shouldn´t take 289 hours? And I had to use Edge length 2 for the floor.

[rmejia]

@rmejia
Yes, but but with my settings the first irr pass shouldn´t take 289 hours? And I had to use Edge length 2 for the floor.

Do a quick test using a high value to see if that changes anything, maybe 8. It is a pixel dependent value, so the larger the resolution, the larger this value should be.

Is the mesh of the object subdivided, or does it have low subdivisions?

[Numse]

The object is very low res atm. The floor might be just one polygon. I´ll try and tessellate the surface.

[rmejia]

I had a large plane that was not subdivided with VRayDisplacementMod and it was taking forever to calculate before starting to render. I raised the edge length and added a subdivide modifier and then things started to move. I believe for displacement to work the object has to be subdivided into many smaller subtriangles, this is calculated at rendertime and might be one of the factors that slows things down. Reducing the number of triangles (edge length) or pre dividing the object helps VRay out, to speed these calculations up.

From the help:
http://www.spot3d.com/vray/help/150S...Mod_params.htm
Edge length - this determines the quality of the displacement. Each triangle of the original mesh is subdivided into a number of subtriangles. More subtriangles mean more detail in the displacement, slower rendering times and more RAM usage. Less subtriangles mean less detail, faster rendering and less RAM. The meaning of Edge length depends on the View-dependent parameter below.

[Numse]

Ye thanks, I know how it works. I just didn´t think about tessellating the surface, and I actually didn´t know that the subtriangle calculation takes so much time if it´s a low res surface.

Seems to work better now. And I unchecked view dependent to get better control over my displacement, and so that the subtriangles don´t change when I change resolution.

//edit

I tried one region render with edge length 10 and one with 1. The difference in memory consumption was crazy... 4,78gb at length 10 and above 8gb at length 1. This was at 8000px and not View-dependent. Working in mm.