Hi All:

   I found a solution for some artefacts I was suffering, and I thought
perhaps others can benefit from it too. Also, perhaps it can give clipka
some ideas, although I think he already knows the cause of these specific
artefacts.

   When using smooth meshes (mesh2) on scenes with high quality rad
settings, I noticed some black artefacts on curved parts of some meshes.
After two weeks trying everything you can think of, I discovered it was
caused by objects not present on the second pass!

   I use often the trick of "enhancing" the radiosity effect with some
objects which are present on the first pass, to save radiosity, but that I
take out on the second pass with an "if", because I don't want them to be
visible. With medium quality rad settings, the second pass doesn't seems to
suffer any artefacts. But if you crank up the settings, they appear more and
more, even if you are using the exact same settings on both passes.

   Fortunately, the solution is easy and even elegant: *don't take out the
objects on the second pass, just use "no_image" on them!* :)

   As I said, it seems related to smooth triangles, because the artefacts do
not appear on flat-normal versions of the same meshes. I think the real
problem behind it is described by clipka on his radiosity tutorial:

news://news.povray.org:119/web.495b9db71d584ee0483cfa400@news.povray.org

   Regards,

--
Jaime

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Jaime Vives Piqueres <jai### [at] ignoranciaorg> wrote:
>    I use often the trick of "enhancing" the radiosity effect with some
> objects which are present on the first pass, to save radiosity, but that I
> take out on the second pass with an "if", because I don't want them to be
> visible. With medium quality rad settings, the second pass doesn't seems to
> suffer any artefacts. But if you crank up the settings, they appear more and
> more, even if you are using the exact same settings on both passes.
>
>    Fortunately, the solution is easy and even elegant: *don't take out the
> objects on the second pass, just use "no_image" on them!* :)
>
>    As I said, it seems related to smooth triangles, because the artefacts do
> not appear on flat-normal versions of the same meshes. I think the real
> problem behind it is described by clipka on his radiosity tutorial:

This makes sense; the mechanism is probably as follows:

- If you take out objects between passes, you get artifacts everywhere
additional samples are taken, because these new samples "see" a different scene
than those taken previously, and therefore show a different color or brightness.

- Lower-quality radiosity settings typically have samples re-used over a larger
distance, i.e. need a lower sample density, so with the same pretrace (or
pre-render) resolution you will get a higher "sample coverage", causing less
samples to be taken during the final render and consequently less artifacts.

- Surface curvature affects sample re-use, so smooth triangles need a higher
sample density than flat ones, and are more likely to get an insufficient
coverage during pretrace / pre-render.


Your idea to use no_image instead of removing the objects is a near-perfect
solution. I guess it will have performance drawbacks in 3.6 (because
bounding-box intersection tests will probably be done before checking for the
no_image flag), but in 3.7 even this should be a non-issue. (Well, there's
still a performance drawback, but it should be marginal.)

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Just an additional discovery about these "smooth mesh artifacts": don't
increase the radiosity brightness on the second pass, or the artifacts will
appear (this time bright instead dark).

   So, better adjust the brightness on the first pass, or just use your
preferred graphics app to boost it on the final image.

--
Jaime

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clipka wrote:
> Your idea to use no_image instead of removing the objects is a
> near-perfect solution. I guess it will have performance drawbacks in 3.6
> (because bounding-box intersection tests will probably be done before
> checking for the no_image flag), but in 3.7 even this should be a
> non-issue. (Well, there's still a performance drawback, but it should be
> marginal.)

I don't understand why no_image has different radiosity results than taking
the object out...

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Nicolas Alvarez <nic### [at] gmailcom> wrote:
> I don't understand why no_image has different radiosity results than taking
> the object out...

  Because no_image doesn't remove the object, and radiosity samples see it?

-- 
                                                          - Warp

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Nicolas Alvarez <nic### [at] gmailcom> wrote:
> I don't understand why no_image has different radiosity results than taking
> the object out...

Because no_image just hides the object from direct camera view, but not from any
other effects, like reflections, refractions, casting of shadows, or - for that
matter - radiosity sampling rays.

Post a reply to this message

> Nicolas Alvarez <nic### [at] gmailcom> wrote:
>> I don't understand why no_image has different radiosity results than
>> taking the object out...
> 
> Because no_image just hides the object from direct camera view, but not
> from any other effects, like reflections, refractions, casting of
> shadows, or - for that matter - radiosity sampling rays.
> 
> 

Now that you mention it, I note that I didn't think about no_reflection and
no_shadow... :( Of course, these should be used to have the same effect as
removing the object. A quick test reveals the trick works the same when
using the three modifiers at once.

--
Jaime

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