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Sampling differences between Path and Branched integrators
Closed, InvalidPublic

Description

System Information
Win7 64-bit

Blender Version
Broken: c8995ec

Short description of error
Branched tracing and normal tracing give different results for the same number of samples taken

Exact steps for others to reproduce the error
Open file, switch between integrators. The difference is visible after only a single sample.

Details

Type
Bug

Event Timeline

Matt Heimlich (m9105826) set Type to Bug.
Matt Heimlich (m9105826) created this task.
Matt Heimlich (m9105826) raised the priority of this task from to Needs Triage by Developer.

The devs can probably speak more to this, but
if they were the same I would think we wouldn't need to
include two separate algorithms for rendering.
I would expect they're going to give two different results.

Julian Eisel (Severin) closed this task as Invalid.Dec 6 2014, 3:43 PM
Julian Eisel (Severin) claimed this task.

Of course two different algorithms means two different results. So everything works as expexcted.

Thanks for the report @Matt Heimlich (m9105826), but (luckily) no bug here :)

I'm going to argue against that. In theory (as I understand it) the branched tracer taking one sample of all ray types should provide the same exact result as the regular path tracer taking a single sample. Changes should only occur (again, at least in theory) when more samples are given to individual ray types. As you can see, a single sample in the BPT does not equal a single sample in the standard tracer, and is in fact apparently much more efficient, which would imply that there are some improvements to be done to the sampling in the progressive sampler. I'm happy to be corrected by someone familiar with the actual cause though.

I found where the disconnect is. The standard path tracer cannot sample all lights per sample, which causes the discontinuity in the results. So I guess this falls more into a feature request than a bug. Sorry for the confusion!