single render vs farm render

[Miguel Tano Adanes]

Hi everybody,

these two images are respectively one single machine render (75 passes) and a farm render (100 passes): what are those shadows in the farm render?

[Ros. Riha]

are you sure pathtracer was turned on in the farm?

[Peter Milner]

It looks to me like an auto exposure problem.

Different machines have rendered different portions independantly and the exposures don't match.

You get this also if you try to render a region to patch into an existing render.

I think, if you render in the AutoCAD view, you can lock the exposure.

[Ros. Riha]

Peter, where could I find that exposure lock?

In the AutoCAD view I cannot render a patch, that is possible from walkabout only...

I am surprised, as I thought the region render takes all its surroundings into account, so there could be difference only if you render different number of passes....without such functionality the region render would be quite useless...

would it be possible to clarify this? thanks

[Roy Hirshkowitz]

To reliably patch a region render into a larger render, you must save both in .nXtImage format and use the Image Editor Patch option.  Both should have the same number of passes.  Any other method will have exposure issues.

When you render a single image using multiple machines on a render farm, the output generated is in .nXtImage format.  There are pieced together using the Image Editor Add option.  There should be no exposure issues using this process.

 

 

 

 

[Peter Milner]

This image was rendered for 50 passes.

I then added a piece of paper and a pen to the desk top and rendered a region without changing any settings.

Both images were saved as nXt format and patched together in the editor.

You can clearly see the exposure difference.

This highlights the need for an exposure lock in the walkabout rendering environmnent.

[Peter Milner]

Same thing happens even if you don't change anything in the model.

[Peter Milner]

Works perfectly with Packet Tracer.

[Roy Hirshkowitz]

It's not related to exposure.  When you patch an nXtImage you're patching luminance data-- data before the tone operator does its thing-- not RGB color data which is the output from tone mapping.  

It's possible there's a bug when you region render using the Path Tracer-- I don't know-- I haven't really tried it.  It's also possible that the solution needs to be closer to convergence for this to work.  I have tried rendering on the farm with multiple machine on the Path Tracer-- although not for a while.  Those pieces, at least in the past, fit together perfectly. (I clearly need to recheck this.)

There is, of course, the additional problem of this being a global rendering algorithm.  Although this is not what you're experiencing here, making changes to the environment, even adding a small piece of paper, will necessarily affect the rest of the environment.

 

[Roy Hirshkowitz]

I think I see the problem here.  I added some code to do adaptive sampling (Build 261).  This causes the Path Tracer to focus a little more on dark areas of the image, making convergence quite a bit faster (the dark areas are usually the most stubborn.)

I can turn this off when you render a single image with the Farm (it will cause path-traced farm images to converge more slowly and may make the farm less beneficial for this application.)  I have no idea if this can be fixed for the Region Render scenario.  I need to think about it a little.

[Miguel Tano Adanes]

@ Ros:

yes, pathtracer was on on both rendering settings (single and farm)

@ Peter:

it was all set up in a nXt walkabout view, everything with same settings. At first renders, the background image was not displayed, neither. So I changed some heavy-textured materials and the background image appeared.

[Roy Hirshkowitz]

On first glance, it does look as if one was generated using the Path Tracer and one used the Packet Tracer w/ indirect lighting on.  I'm just guessing here, however.  Were multiple machines involved in the render farm rendering?  Can you check the farm version you're running on these machines.

 

[Miguel Tano Adanes]

The farm version is the last one on both machines. All settings were exactly the same... as far as I ignored that it is possible to have different lighting settings for single and farm renders without saving different lightting files! (I'll check better all those issues)