[Bf-cycles] Base 'Render' samples (simple request)

Brecht Van Lommel brechtvanlommel at pandora.be
Mon Nov 14 04:14:34 CET 2011


Could you clarify what you would take as the stopping condition?
Average pixel difference, maximum pixel difference, ...? Average won't
work if only a small part of the image is noisy, maximum makes it very
sensitive to individual pixels.

On a more general note, I'm ok with short term solutions, if I know
how they can evolve into the proper solution in the long term. Adding
a short term solution that you then have to change means breaking
backwards compatibility, or keeping around old code that complicates
adding new features.

The other thing is that I should be able to support it up to a certain
level. If it's there, people will expect to be able to set this value
and send their animations to a renderfarm, and get some predictability
in render times, which I don't think you get with a simple
implementation.

Brecht.

On Sun, Nov 13, 2011 at 3:31 PM, Jordan Miller <jrdnmlr at gmail.com> wrote:
> there is a complicated long term solution, I agree.
>
> but don't get caught up in the individual pixels in the difference image...
>
> what I'm proposing is a simple approach to rapidly get there for the
> short term. if you look at all the difference images you can see they
> follow a very reproducible pattern over time, which I have quantified
> by looking at the histogram. all we have to do is compute this
> histogram along the way after 2, 4, 16, 32.... render samples.
>
> any single pixel "holding things back" still allows the render to
> finish since it is not changing if it is holding things back. this
> would mean the pixel is changing by more than 250 intensity values
> every render. this is not possible, even with fireflies. but they
> could be measured.
>
> if we assume that, given an infinite render time (infinite number of
> samples), the pixels will no longer change, then all we have to do is
> assess the number of pixels that are still changing and by how much to
> pick a reproducible place to stop and have a given quality.
>
> it's a very simple first approximation that I think will work quite generally.
>
> if you have more test scenes I'd be happy to try then out too...
>
> jordan
>
>
>
> On Nov 13, 2011, at 8:49 AM, Brecht Van Lommel
> <brechtvanlommel at pandora.be> wrote:
>
>> Hi,
>>
>> What you're proposing here are basically the first steps to an
>> adaptive sampling algorithm. I think that would be great to add, but
>> implementing this well is not so easy. Which is not to say that I
>> don't think it shouldn't be done, but that it's going to take more
>> than a few days of work to have this working really well.
>>
>> I'm not sure if you're proposing this for per pixel adaptive sampling,
>> or deciding to stop for the entire frame. In the latter case this is a
>> bit problematic, since using the mean value may still mean there is a
>> lot of noise in part of the image, and the min/max may lead to one bad
>> pixel holding back the entire image. So this should really be done on
>> a per pixel level.
>>
>> The difference image you show looks like it shows the noise in the
>> image, but this is deceiving, it only shows part of the pixels that
>> actually have noise. If you're going to make a decision, this pixel
>> needs no more samples, you have to do that with great confidence. If
>> you're wrong 5% or even 1% of the time, that's still many pixels that
>> will be wrong.
>>
>> There's a few ways to improve this, among them using information from
>> neighbouring pixels (blurring the difference image) and using more
>> statistical measures. Tweaking and testing this all takes time :)
>>
>> Brecht.
>>
>> On Sun, Nov 13, 2011 at 2:02 AM, Jordan Miller <jrdnmlr at gmail.com> wrote:
>>> I've added more details here:
>>> http://wiki.blender.org/index.php/Dev:2.6/Source/Render/Cycles/ReducingNoise
>>>
>>> thoughts?
>>>
>>> jordan
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