black stroke around sun?

General questions about Indigo, the scene format, rendering etc...
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ViennaLinux
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black stroke around sun?

Post by ViennaLinux » Tue Aug 05, 2008 9:29 pm

Can I somehow remove the black stroke around the sun?
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fused
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Post by fused » Tue Aug 05, 2008 10:25 pm

i beleive that was due to the splat or downsize filter (most likely downsize) being mitchell netravali (mn_cubic). try playing with the settings if your exporter allows or simply use "gaussian".

not sure if you have to increase or lower the values...

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suvakas
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Post by suvakas » Wed Aug 06, 2008 12:12 am

Thanks for the tip fused.
I've always thought, that this is related to some bad color clamping or something.

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Post by OnoSendai » Wed Aug 06, 2008 12:27 am

It's due to the negative lobe in the Mitchell-Netravali filter. try turning on aperture diffraction.

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Post by CTZn » Wed Aug 06, 2008 12:33 am

IIRC mental ray clamps that negative lobe :)
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OnoSendai
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Post by OnoSendai » Wed Aug 06, 2008 12:34 am

clamping the negative lobe would completely change the filter.

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CTZn
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Post by CTZn » Wed Aug 06, 2008 12:35 am

Ok then :)
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ViennaLinux
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Post by ViennaLinux » Wed Aug 06, 2008 1:40 am

why does aperture diffraction solves the problem?
wouldnt it be better to change to another filter?
by the way i tried supersamplefactor 1 and its the same ... shouldnt the be NO FILTER when supersamplefactor is set to 1?
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OnoSendai
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Post by OnoSendai » Wed Aug 06, 2008 1:45 am

aperture diffraction will solve the problem because it will 'spread out' light from the sun, so you won't get such a sharp edge between the sky and the sun.

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Post by ViennaLinux » Wed Aug 06, 2008 2:36 am

ah okay. thanks for the replies.
have a nice day and keep the great work up :-)
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Post by suvakas » Wed Aug 06, 2008 3:29 am

OnoSendai wrote:aperture diffraction will solve the problem because it will 'spread out' light from the sun, so you won't get such a sharp edge between the sky and the sun.
Hmm..I'm almost sure, that I've got the ring with ad enabled too.

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Post by v_mulligan » Wed Aug 06, 2008 6:56 am

Aperture diffraction will cover up or wash out the ring, but it might not eliminate it. Using a splat filter without a negative lobe is your best bet. This is NOT an Indigo bug; it's a limitation of certain types of filters. It's important to use the right tool for the job :) .

Incidentally, mental ray does NOT clamp out negative lobes of these filters. It's possible to get dark areas adjacent to bright highlights in mental ray as well. Again, you have to know the tools and how to use them.

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CTZn
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Post by CTZn » Wed Aug 06, 2008 9:13 am

Ah well I remember mitchell-netralvi filter was causing bad sampling of final gather, causing black blotches, but that's old story (mr 3.3) :)

From the help files (as of 2004):
"The Mitchell and Lanczos filters are both approximations of the theoretically ideal sinc filtering function, cut off after its second lobe. In most cases, the Mitchell filter gives better results."
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Post by v_mulligan » Wed Aug 06, 2008 10:12 am

That means the function used is y(x) = {Mitchell} for 0 <= x <= cutoff, {0} for x > cutoff. That is, the Mitchell function oscillates from positive to negative, then levels off at zero rather than continuing to oscillate. That does NOT mean that y(x) is clamped to a minimum value of 0.

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Post by CTZn » Wed Aug 06, 2008 10:19 am

Yep you are right, I was wrong to use that argument.

Ah, got it:
mr doc wrote:filter clip mitchell|lanczos [width [height]]3.1
These are variants of the regular Mitchell and Lanczos filters that clip the filter result to the range of samples under the filter. Mitchell and Lanczos filters have negative coefficients, which can cause ringing around sharp contrasts. Clipping prevents ringing.
mr 3.1, Q1 2002.

EDIT: I would add: DL3DBlackspotsRemover:
DL3DBlackspotsRemover to remove black squares / spots problem when rendering (especially with final gathering)
You are in this case if when you increase the filter size your black spots' size increases too.
Last edited by CTZn on Sat Aug 16, 2008 5:53 am, edited 1 time in total.
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