[Radiance-general] ar and aa

[Jia Hu]

Thanks, I am more or less understand what you said. I also made some
experiment and discover:

(1) the maximum "influence radius" is not equal to sceneSize/ar but
influenced by aa and ar.
(2) the minimum "influence radius" is equal to sceneSize * aa /ar and it
happens in the complex geometry, e.g, corner.
(3) If a sample ray falls within the "influence radius", it will get ambient
value by interpolation of cached ambient value.

By the way, I can not search such terms as "-ar" or "ar" in the mailing list
archives because the system warns it is not a valid word.

Thanks for patience.
Jia



On Tue, Jul 20, 2010 at 2:53 PM, Greg Ward <gregoryjward at gmail.com> wrote:

> This is just getting so confused (and confusing).  By itself, the -aa
> parameter AFFECTS the distance between ambient values, but it does not
> dictate them.  The spacing actually depends on how close nearby geometry is.
>  If there is nothing nearby, then ambient values may be spaced very far
> apart.  You really need to go back to my original paper and understand what
> is going on with the interpolation:
>
>        http://radsite.lbl.gov/radiance/papers/sg88/paper.html
>
> The -ar setting gets used with -aa and the overall scene dimensions to
> determine a minimum spacing between values.  This avoids having infinite
> calculation density at inside corners and other places where objects are
> right next to and "see" each other.  If you set -ar 0, then it guarantees
> accuracy everywhere, but at a potentially large expense, as you end up
> computing a new hemispherical sampling at each pixel for certain parts of
> the image.  Combined with a -ab setting greater than 1, you can end up with
> a very long calculation, indeed.
>
> Don't worry about what the exact spacing value is.  It doesn't matter.  All
> that matters is the scale over which you maintain the accuracy set by the
> -aa parameter.  Once your objects are closer to each other than the maximum
> scene dimension divided by the -ar setting, you will gradually lose
> accuracy.  There is nothing more to say on the topic.  Read the code in
> src/rt/ambient.c if you want to understand exactly what is happening.
>
> Cheers,
> -Greg
>
>  From: Jia Hu <hujia06 at gmail.com>
>> Date: July 19, 2010 10:47:57 AM PDT
>>
>>
>> Sorry for the typo, I meant to compare "-ar" and "-aa".   I can understand
>> your explanation as to -ar and -aa.
>>
>> The book and some online materials also discussed about the meaning of -ar
>> and -aa. And they seem to explain it from a different perspective. For my
>> understanding of those statements, the interpolation will always happen when
>> the distance between two points is less than minimum spacing distance
>> (sceneSize * aa /ar). When the distance between two points is larger than
>> the minmum spacing distance, interpolation may happen if the point is within
>> the "radius of validity" of another point. In other words, the minimum
>> "radius of validity" is sceneSize *aa /ar?
>>
>> According to Greg's explanation, when the point falls within SceneSize /
>> ar, the accuracy starts to relax. Can I say that when the error gets its
>> maximum, -aa, when the point falls within the minimum spacing distance
>> (SceneSize *aa /ar)?   I know there must be something wrong with my
>> understanding about this issue. But I can not find where the problem is.
>> The book Rendering with Radiance says "  -ar parameter acts as a limiting
>> device and If you are already running up against the -ar limit, increasing
>> the setting will result in a higher density of sampling. If the limit has
>> not been reached, then increasing -ar should have no effect." I also have
>> difficulty in understanding the term "limiting device".
>>
>> Thank you for help.
>>
>> Jia Hu
>>
>
> On Mon, Jul 19, 2010 at 2:31 AM, Greg Ward <gregoryjward at gmail.com> wrote:
>>
>> I assume you meant to compare "-aa" and "-ar" in your first sentence.
>>>  These are not two variables that affect the same behavior -- not much point
>>> in that.  Rather, think of the -ar setting as determining a scene resolution
>>> below which the accuracy of the indirect calculation (as determined by the
>>> -aa parameter) will start to relax.  If you divide your global scene size
>>> (the fourth value reported by "getinfo -d octree") by the -ar setting, you
>>> will get the scene size below which the indirect calculation will begin to
>>> lose accuracy.
>>>
>>> Best,
>>> -Greg
>>>
>>
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