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Hi,
Many thanks to everyone for taking time to reply/investigate.
> Thorsten Froehlich
The image I've posted is the actual difference. The difference being extremely
small it may be hard to see to the naked eye without zooming in but making it
easier to see would mean posting a processed image which is not the actual
difference and then irrelevant IMO to anyone wanting to investigate.
I was using 3.6 yesterday because it's the one installed on my main computer and
I'm lazy to install a newer one, my bad, sorry.
> Mike Horvath
Thanks. Disabling jitter actually solved the problem. But, I don't understand
then why the problem doesn't occur for two consecutive rendering in one shot
(just wondering, not a big problem to me as I said before).
> Thomas de Groot
Thanks. Tonight I've tried with another machine on which I have 3.7 installed. I
confirm the problem disappears, even with jitter enable.
> William F Pokorny
Thank you very much for your time. Yes, it's weird. If the difference is never
more than 1/255, that's surely not a big deal. But if the cause is unknown it
leaves the possibility for a greater difference under different circumstances.
My opinion is, partial rendering are probably not the common use case and I
would believe in the smallest difference anyway unless shown the contrary. If I
had to debug it myself I would give it a very low priority. Personally,
disabling jitter to make my unit test run fine in my project is perfectly ok to
me, so it's up to you.
Thanks again to every one.
Pascal
William F Pokorny <ano### [at] anonymousorg> wrote:
> On 6/17/19 11:08 AM, BayashiPascal wrote:
> > Hi there,
> > I've realized something I wasn't aware until now and wanted to ask for your
> > opinion.
> ...
> >
> Hi Pascal,
> I had a little time this morning before being busy with real life. So,
> thought I'd play with what you posted but in the current v3.8 master
> branch as it's the only place anything might be updated.
>
> I rendered your scene at 400x400 instead of 100x100. To my surprise
> using only the default +a type anti-aliasing I see a pixel or sometimes
> a few which are 1/255 by channel in magnitude different(1). In the
> 400x400 render, and just +a, this happens with a single pixel at 231,218
> which isn't even on sub-render boundary. Using still method one but
> with: +am1 +a0.1 +r3 I see again that pixel and an additional 2 in the
> same column above it. All near the edge of the sphere so AA is in play.
> Weird. Guessing maybe some sub pixel offset when rendering sub blocks.
>
> The image attached used the new to v3.8 +am3 mode. In the top row two
> different full renders are compared and they match exactly. In the
> bottom we compare the top left full to an image assembled by chunks. The
> multiplier on the differences is 4x. Looks like +am3 better brings out
> whatever the issue is.
>
> Still, not sure I'll open up an issue. Something like this is going to
> be way down on anyone's to-do list. Opinions?
>
> You could try +am2 in 3.6 as I've not see issues with it in 3.8.
> Probably also faster. The jitter off comments others made stands. I also
> tested with one thread to be sure threads>1 not a problem in v3.8.
>
> Bill P.
>
> (1) - Not thinking about it very much - in 3.6, if not using
> assumed_gamma 1.0, the differences might be a little more dramatic in
> the resultant image though probably still hard to see.
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