# ar and aa

Hello experts:

Is that right doubling the value of "aa" has the same effect as
decreasing the value of "aa" by half? The minimum distance between sampling
points is the same according to the formula: SceneSize * aa / ar.

Cheer,
Jia

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

···

From: Jia Hu <[email protected]>
Date: July 16, 2010 8:17:48 PM PDT

Hello experts:

Is that right doubling the value of "aa" has the same effect as decreasing the value of "aa" by half? The minimum distance between sampling points is the same according to the formula: SceneSize * aa / ar.

Cheer,
Jia

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
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 <[email protected]> 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

From: Jia Hu <[email protected]>

Date: July 16, 2010 8:17:48 PM PDT

Hello experts:

Is that right doubling the value of "aa" has the same effect as decreasing
the value of "aa" by half? The minimum distance between sampling points is
the same according to the formula: SceneSize * aa / ar.

Cheer,
Jia

_______________________________________________
[email protected]

I share Jia's confusion. Greg, I remember that we had asked you the same
question during the Radiance workshop last year but I still do not get
it? Could you explain the relationship between aa and ar one more time?

Christoph

···

From: [email protected]
[mailto:[email protected]] On Behalf Of Jia
Hu
Sent: Monday, July 19, 2010 1:48 PM
Subject: Re: [Radiance-general] ar and aa

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

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 <[email protected]> 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

From: Jia Hu <[email protected]>
Date: July 16, 2010 8:17:48 PM PDT

Hello experts:

Is that right doubling the value of "aa" has the same effect as
decreasing the value of "aa" by half? The minimum distance between
sampling points is the same according to the formula: SceneSize * aa /
ar.

Cheer,
Jia

_______________________________________________
[email protected]

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:

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 <[email protected]>
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 <[email protected]> > 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

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 <[email protected]> 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:

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 <[email protected]>

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
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 <[email protected]> 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

_______________________________________________