# ambient bounces

Hello all:

If ab (ambient bounces) =3, after a ray reaches three times of diffuse
bounces and does not hit any lighting source, then how should Radiance
define the value of this ray, for example, radiance or irradiance, in order
to calculate the radiance at the viewpoint using backward ray tracing? I
radiance, but still a little confused.

Jia

In simple terms, if any of the three surfaces that the ray has hit have
a direct source shining onto it, the ray will pick up the direct light
reflected from those surfaces.

Otherwise, I can see why you would think a very small light source with
little probability of ray intersection would rarely contribute to the
ambient calculation.

On Behalf Of Jia Hu

Hello all:

If ab (ambient bounces) =3, after a ray reaches three times of diffuse
bounces and does not hit any lighting source, then how should Radiance
define the value of this ray, for example, radiance or irradiance, in
order to calculate the radiance at the viewpoint using backward ray
Rendering with radiance, but still a little confused.

Jia

···

Sent: Tuesday, April 20, 2010 1:11 AM

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Jia.

If the number of bounces is exceeded without hitting a source or
infinity the value specified with "-av" (ambient value) is used. The
default value is "0 0 0" so you won't get any artificial contribution.

You can use rvu to read the average value of a scene on the screen.
This gives you a first approximation of -av.

Regards,
Thomas

···

On Tue, Apr 20, 2010 at 6:11 AM, Jia Hu <[email protected]> wrote:

Hello all:

If ab (ambient bounces) =3, after a ray reaches three times of diffuse
bounces and does not hit any lighting source, then how should Radiance
define the value of this ray, for example, radiance or irradiance, in order
to calculate the radiance at the viewpoint using backward ray tracing? I
radiance, but still a little confused.

Jia
_______________________________________________
[email protected]

Thank you very much, Christopher and Thomas.

If I specify the value of "-av", does that mean the ray without hitting a
source will get some values (or contribution) that is related to "av". In
other words, "av" makes ray without hitting a source have some
contributions?

Regards,
Jia

···

On Tue, Apr 20, 2010 at 5:02 AM, Thomas Bleicher <[email protected]>wrote:

Jia.

If the number of bounces is exceeded without hitting a source or
infinity the value specified with "-av" (ambient value) is used. The
default value is "0 0 0" so you won't get any artificial contribution.

You can use rvu to read the average value of a scene on the screen.
This gives you a first approximation of -av.

Regards,
Thomas

On Tue, Apr 20, 2010 at 6:11 AM, Jia Hu <[email protected]> wrote:
> Hello all:
>
> If ab (ambient bounces) =3, after a ray reaches three times of diffuse
> bounces and does not hit any lighting source, then how should Radiance
> define the value of this ray, for example, radiance or irradiance, in
order
> to calculate the radiance at the viewpoint using backward ray tracing?
I
> radiance, but still a little confused.
>
> Jia
> _______________________________________________
> [email protected]
>
>

_______________________________________________
[email protected]

Sorry, I just want to clarify my question.

For the methods used in Radiance, there are three parts to calculate: direct
component, specular indirect component, and diffuse indirect component.

For the direct and specular components, the value of contribution is got by
hitting the source, is there any limit for the number of bounces for these
two component?

But for diffuse component, how can we define the value after the number of
bounces (the value of "ab" ) is exceeded since the diffuse
component considers those without hitting any source? The rays hitting the
source will be discarded because the other components have considered them.

Thank you,
jia

···

On Tue, Apr 20, 2010 at 7:39 AM, Jia Hu <[email protected]> wrote:

Thank you very much, Christopher and Thomas.

If I specify the value of "-av", does that mean the ray without hitting a
source will get some values (or contribution) that is related to "av". In
other words, "av" makes ray without hitting a source have some
contributions?

Regards,
Jia
On Tue, Apr 20, 2010 at 5:02 AM, Thomas Bleicher < > [email protected]> wrote:

Jia.

If the number of bounces is exceeded without hitting a source or
infinity the value specified with "-av" (ambient value) is used. The
default value is "0 0 0" so you won't get any artificial contribution.

You can use rvu to read the average value of a scene on the screen.
This gives you a first approximation of -av.

Regards,
Thomas

On Tue, Apr 20, 2010 at 6:11 AM, Jia Hu <[email protected]> wrote:
> Hello all:
>
> If ab (ambient bounces) =3, after a ray reaches three times of diffuse
> bounces and does not hit any lighting source, then how should Radiance
> define the value of this ray, for example, radiance or irradiance, in
order
> to calculate the radiance at the viewpoint using backward ray tracing?
I
> radiance, but still a little confused.
>
> Jia
> _______________________________________________
> [email protected]
>
>

_______________________________________________
[email protected]

Yes. That's the idea.

-av is just a way to recover the ambient light that would be lost due
to a limit on ambient bounces. To understand how -av influences the
results you can run a test scene once with a high '-ab' (say 6 or 7)
and '-av 0 0 0' and then reduce the '-ab' to 2 or 3 and increase the
-av value. When the two results are more or less the same you have
picked the right -av.

Regards,
Thomas

···

On Tue, Apr 20, 2010 at 12:39 PM, Jia Hu <[email protected]> wrote:

Thank you very much, Christopher and Thomas.

If I specify the value of "-av", does that mean the ray without hitting a
source will get some values (or contribution) that is related to "av". In
other words, "av" makes ray without hitting a source have some
contributions?

Regards,
Jia
On Tue, Apr 20, 2010 at 5:02 AM, Thomas Bleicher <[email protected]> > wrote:

Jia.

If the number of bounces is exceeded without hitting a source or
infinity the value specified with "-av" (ambient value) is used. The
default value is "0 0 0" so you won't get any artificial contribution.

You can use rvu to read the average value of a scene on the screen.
This gives you a first approximation of -av.

Regards,
Thomas

On Tue, Apr 20, 2010 at 6:11 AM, Jia Hu <[email protected]> wrote:
> Hello all:
>
> If ab (ambient bounces) =3, after a ray reaches three times of diffuse
> bounces and does not hit any lighting source, then how should Radiance
> define the value of this ray, for example, radiance or irradiance, in
> order
> to calculate the radiance at the viewpoint using backward ray tracing?
> I
> radiance, but still a little confused.
>
> Jia
> _______________________________________________
> [email protected]
>
>

_______________________________________________
[email protected]

_______________________________________________
[email protected]

In addition, the "compamb" script will automatically compute the best
ambient value (-av) for a scene based on a rad control (.rif) file and list
of views within it. The script then appends this value in a "render =" line
in the .rif file. (can do color weighted -avs or greyscale, and can also add
an exposure value if you like).

···

On 4/20/10 5:59 AM, "Thomas Bleicher" <[email protected]> wrote:

Yes. That's the idea.

-av is just a way to recover the ambient light that would be lost due
to a limit on ambient bounces. To understand how -av influences the
results you can run a test scene once with a high '-ab' (say 6 or 7)
and '-av 0 0 0' and then reduce the '-ab' to 2 or 3 and increase the
-av value. When the two results are more or less the same you have
picked the right -av.

Regards,
Thomas

I got it, thank you.

"ab" value is the maximum number of bounces for diffuse indirect component.
Another question is that whether there is a limit on the bounces of specular
indirect (not diffuse indirect). I do not find such options in "rtrace".

Jia

···

On Tue, Apr 20, 2010 at 7:59 AM, Thomas Bleicher <[email protected]>wrote:

Yes. That's the idea.

-av is just a way to recover the ambient light that would be lost due
to a limit on ambient bounces. To understand how -av influences the
results you can run a test scene once with a high '-ab' (say 6 or 7)
and '-av 0 0 0' and then reduce the '-ab' to 2 or 3 and increase the
-av value. When the two results are more or less the same you have
picked the right -av.

Regards,
Thomas

On Tue, Apr 20, 2010 at 12:39 PM, Jia Hu <[email protected]> wrote:
> Thank you very much, Christopher and Thomas.
>
> If I specify the value of "-av", does that mean the ray without hitting a
> source will get some values (or contribution) that is related to "av". In
> other words, "av" makes ray without hitting a source have some
> contributions?
>
> Regards,
> Jia
> On Tue, Apr 20, 2010 at 5:02 AM, Thomas Bleicher < > [email protected]> > > wrote:
>>
>> Jia.
>>
>> If the number of bounces is exceeded without hitting a source or
>> infinity the value specified with "-av" (ambient value) is used. The
>> default value is "0 0 0" so you won't get any artificial contribution.
>>
>> You can use rvu to read the average value of a scene on the screen.
>> This gives you a first approximation of -av.
>>
>> Regards,
>> Thomas
>>
>> On Tue, Apr 20, 2010 at 6:11 AM, Jia Hu <[email protected]> wrote:
>> > Hello all:
>> >
>> > If ab (ambient bounces) =3, after a ray reaches three times of diffuse
>> > bounces and does not hit any lighting source, then how should Radiance
>> > define the value of this ray, for example, radiance or irradiance, in
>> > order
>> > to calculate the radiance at the viewpoint using backward ray tracing?
>> > I
>> > read chapters about calculation methods in the book, Rendering with
>> > radiance, but still a little confused.
>> >
>> > Thank you in advance.
>> > Jia
>> > _______________________________________________
>> > [email protected]
>> >
>> >
>>
>> _______________________________________________
>> [email protected]