[Bf-committers] Cycles Progressive Refinement Checkbox Tooltip *somewhat* ambiguous

Simon Repp simon at fdpl.foundation
Thu Nov 13 22:24:42 CET 2014

Here's the scene I initially used to test this:



I rendered on CPU - i7-2920XM quad-core (2.50GHz, 8MB cache), fedora 20
64 bit, blender 2.72b official release

Non-Progressive: 61 seconds
Progressive: 165 seconds

Super coincidentally also 2.7x slower :D


On 11/13/2014 11:49 AM, Sergey Sharybin wrote:
> Brecht, it's multiple factors being involved here, starting from CPU cache
> coherence issues (which we can't easily predict) ending with less efficient
> buffers update (we might be saving all the passes after each of the samples
> in case of progressive refine, which we can fix).
> But yeah, should have asked for a demo file in the original file :) So do
> you guys have a demo .blend file which demonstrates such a major slowdown?
> Brecht, we can (and actually should) do several samples per tile, but i'd
> prefer log(). not sqrt() perhaps. Other idea to improve cache coherence
> would be to use left-to-right, bottom-to-top tile scheduling. That gives
> few %% of speedup, and AFAIR we don't fallback to such a scheduling in
> progressive refine.
> And even after the tweaks, we should probably replace "somewhat" to
> something more deterministic in the tooltip.
> On Thu, Nov 13, 2014 at 10:58 AM, Brecht Van Lommel <
> brechtvanlommel at pandora.be> wrote:
>> If it is really 100% or 300% slower then that sounds like a fixable issue.
>> I'm not sure why it would be this slow, it would be good to find out why,
>> but one thing that would speed it up is to render more samples at a the
>> time and redraw less often as the current sample increases. Due to the way
>> Monte Carlo integration works, one sample barely makes any visible
>> difference in noise after a while anyway.
>> Maybe something like:
>> num_samples_at_once = sqrt(max(current_sample - 4, 1))
>> On Nov 13, 2014 10:13 AM, "Greg Zaal" <gregzzmail at gmail.com> wrote:
>> Just by the way, it's even more noticeable with GPU rendering - I've seen
>> it roughly 300% slower often.
>> "could lead to significant slowdown" sounds good to me.
>> On 13 November 2014 11:06, Sergey Sharybin <sergey.vfx at gmail.com> wrote:
>>> The issue here is that basically slowdown depends on particular hardware
>>> configuration, tile settings and device used to render (GPU/CPU).
>> Meaning,
>>> on modern CPU i've noticed around 20% slowdown peak, which is not that
>> bad
>>> as 100%. So what i'm trying to say here, is that if we'll provide
>>> information "up to 100% slower" it might just scary artists and they
>>> wouldn't use the option at all, even though for their configuration
>>> slowdown wouldn't be so bad.
>>> What about more neutral (in my opinion): "could lead to significant
>>> slowdown"?
>>> On Thu, Nov 13, 2014 at 12:22 AM, Simon Repp <simon at fdpl.foundation>
>>> wrote:
>>>> Dear Renderistas,
>>>> I only recently found out that progressive refinement in Cycles
>>>> rendering (which the corresponding checkbox's tooltip describes to be
>>>> "somewhat slower" than bucket rendering) in fact can impose performance
>>>> penalties of over 100% (aka the same amount of samples takes more than
>>>> twice as long to render).
>>>> Now I don't know if this is just a personal flawed interpretation of
>> the
>>>> english language on my part, but when reading "somewhat slower" I
>> didn't
>>>> realize what I was really in for, and in retrospect I'd rather not
>>>> reconstruct how many days my poor laptop spent in excess to render some
>>>> projects I did in the past.
>>>> I'd hereby like to propose a change of this tooltip to something less
>>>> ambiguous, lest anyone else falls into that same trap that I have - My
>>>> proposal for this would be to include actual figures describing the
>>>> possible speed penalty that progressive refinement can impose, that is,
>>>> something along the lines of "renders [a]% to [b]% slower depending on
>>>> the scene", where figures [a] and [b] are ideally derived from real
>>>> world data we gather (or already have?) about how much speed penalty
>>>> progressive refinement can impose in different scenes. Alternatively,
>>>> only stating "up to [x]% slower" would work as well I guess, as the
>> main
>>>> point is to make people aware that it _can_ possibly affect render
>> times
>>>> _significantly_.
>>>> If the proposal to include figures is not agreeable for some reason, I
>>>> would at least ask for a more indicative wording than "somewhat
>> slower",
>>>> which even after consulting multiple dictionaries I'm not sure if there
>>>> is an official interpretation for. (One dictionary suggests "quite" as
>> a
>>>> synoym, another "slightly" ...) I'd still prefer the figures though, no
>>>> one looks up terms in the dictionary while using Blender and I am
>>>> probably not the worst offender at massacring and misunderstanding the
>>>> english language in the Blender community, and it doesn't get less
>>>> ambiguous than numbers anyway, so I say we use them here? :)
>>>> Best,
>>>> Simon
>>>> _______________________________________________
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>>>> http://lists.blender.org/mailman/listinfo/bf-committers
>>> --
>>> With best regards, Sergey Sharybin
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