POV-Ray : Newsgroups : povray.bugreports : Partial Output does not match Single-Pass Output on some files : Re: Partial Output does not match Single-Pass Output on some files Server Time
27 Mar 2023 07:16:29 EDT (-0400)
  Re: Partial Output does not match Single-Pass Output on some files  
From: clipka
Date: 10 Jul 2018 08:18:00
Message: <5b44a3f8$1@news.povray.org>
Am 10.07.2018 um 09:39 schrieb Lloyd:

> I am interested in using POV-Ray in a distributed computing environment, in
> which I may have many compute nodes available for rendering.  One requirement
> for this project is that an image rendered in a single pass should match exactly
> an image that is rendered in pieces with Partial Output and then properly
> reassembled.

There are certain features that - at least currently - can't be used in
this manner.

Probably the most influential such feature (in terms of its effect on
the result image) is radiosity: During pretrace, POV-Ray decides where
to gather samples, which are then used during the main render; since the
pretrace is limited to the same section of the screen as the main
render, a partial render will lead to different decisions on where to
gather samples, and thus a different sample set, which in turn affects
the render result.

As a matter of fact, radiosity will typically display differences even
between any two given full-frame renders, due to randomness in the
operating system's scheduling and the resulting order in which render
blocks are processed.

(You can force radiosity to give reproducible results by either running
the render single-threaded, or setting `+HR` on the command line to
enforce reproducibility at the cost of a bit of pretrace performance.)

Note however that reproducibility of /individual/ frames is not the only
(and not even the most important) property you want: What you want is
"similarity" (whatever that means) of /subsequent/ frames.

In that, non-reproducibility of individual frames is actually your ally,
not your enemy: If you see differences between two renders of one and
the same frame, that's an indication that you're also likely to see the
same level of differences between subsequent frames - and those can
/not/ be turned off simply with a command-line switch.

Rather, you should make sure that the quality settings of the
problematic features are set high enough so that the residual
differences between renders of the same frame become acceptable.

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