Zenith Luminance from rvu

Hello Everyone,

I am trying to create falsecolor fish-eye image of just the sky. For that I
am using:

rpict -vta -vp 0 0 0 -vd 0 0 1 -vu 0 1 0 -vh 180 -vv 180 sky.oct |
falsecolor -s XYZ -l cd/m2 > sky.hdr

Dr.Mardaljevic in his dissertation said to set -s argument of falsecolor
close to the Zenith luminance, that can be obtained by rvu's trace command.

To get the Zenith luminance, I am tracing the ray (using rvu) that is
approximately at the centre of the image created with the above view
parameters, and setting this ray's luminance value as the -s option of
falsecolor.

Is this the right way to show/cover the whole luminance spectrum in the sky
? (It doesn't look so in the image..)

Or shall I trace the ray at the apparently at the brightest pixel ? (by
reducing the exposure, and figuring out that pixel)

I think the term Zenith Luminance may mean the luminance at the centre of
the sky, but I must take your opinion before concluding. Thank you!

Best regards,
Vaib

Hi Vaib,

Falsecolor now has an "auto" option to the –s parameter, which will automatically set the top of the scale to the peak value found in the image. Note the peak value will not necessarily be at the zenith (which, yes, is where the vector 0,0,1 points to in a hemispherical fisheye view).

I also find that for other than overcast skies, using a logarithmic scale (-log 2) is very useful for getting the most value out of a falsecolor representation of a sky's luminous distribution.

···

On 5/23/14, 12:14 PM, "Vaib" <[email protected]<mailto:[email protected]>> wrote:

Hello Everyone,

I am trying to create falsecolor fish-eye image of just the sky. For that I am using:

rpict -vta -vp 0 0 0 -vd 0 0 1 -vu 0 1 0 -vh 180 -vv 180 sky.oct | falsecolor -s XYZ -l cd/m2 > sky.hdr

Dr.Mardaljevic in his dissertation said to set -s argument of falsecolor close to the Zenith luminance, that can be obtained by rvu's trace command.

To get the Zenith luminance, I am tracing the ray (using rvu) that is approximately at the centre of the image created with the above view parameters, and setting this ray's luminance value as the -s option of falsecolor.

Is this the right way to show/cover the whole luminance spectrum in the sky ? (It doesn't look so in the image..)

Or shall I trace the ray at the apparently at the brightest pixel ? (by reducing the exposure, and figuring out that pixel)

I think the term Zenith Luminance may mean the luminance at the centre of the sky, but I must take your opinion before concluding. Thank you!

Best regards,
Vaib

Hi Rob,

I missed to mentioned that I tried -s a (auto) also. Also I tried to get
the peak value from pextrem. But I got skeptical as these values were about
100 times more (for sunny sky) than what I saw in some research papers; and
the image was just one color. :slight_smile:

That's why I came back to manually tracing the brightest pixel approach.

Can you please elaborate on the "log 2" scale approach ? Thank you for your
time!

BEst regards,
Vaib

···

On 23 May 2014 23:56, Guglielmetti, Robert <[email protected]>wrote:

Hi Vaib,

Falsecolor now has an "auto" option to the –s parameter, which will
automatically set the top of the scale to the peak value found in the
image. Note the peak value will not necessarily be at the zenith (which,
yes, is where the vector 0,0,1 points to in a hemispherical fisheye view).

I also find that for other than overcast skies, using a logarithmic scale
(-log 2) is very useful for getting the most value out of a falsecolor
representation of a sky's luminous distribution.

On 5/23/14, 12:14 PM, "Vaib" <[email protected]<mailto: > [email protected]>> wrote:

Hello Everyone,

I am trying to create falsecolor fish-eye image of just the sky. For that
I am using:

rpict -vta -vp 0 0 0 -vd 0 0 1 -vu 0 1 0 -vh 180 -vv 180 sky.oct |
falsecolor -s XYZ -l cd/m2 > sky.hdr

Dr.Mardaljevic in his dissertation said to set -s argument of falsecolor
close to the Zenith luminance, that can be obtained by rvu's trace command.

To get the Zenith luminance, I am tracing the ray (using rvu) that is
approximately at the centre of the image created with the above view
parameters, and setting this ray's luminance value as the -s option of
falsecolor.

Is this the right way to show/cover the whole luminance spectrum in the
sky ? (It doesn't look so in the image..)

Or shall I trace the ray at the apparently at the brightest pixel ? (by
reducing the exposure, and figuring out that pixel)

I think the term Zenith Luminance may mean the luminance at the centre of
the sky, but I must take your opinion before concluding. Thank you!

Best regards,
Vaib

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Hi Vaib,

Did you ever get this sorted? All I meant by the logarithmic scale is that by using one, you can get more mileage out of the falsecolor mapping, especially for scenes where there is a huge dynamic range, and especially when the gradients are all crammed into one or two small bands of the total DR. You can try it out by adding a "-log 2" or "-log 3" (whatever decades you need, really, but generally 2 works fine for most scenes) to you falsecolor command.

···

On 5/23/14, 12:57 PM, "Vaib" <[email protected]<mailto:[email protected]>> wrote:

Hi Rob,

I missed to mentioned that I tried -s a (auto) also. Also I tried to get the peak value from pextrem. But I got skeptical as these values were about 100 times more (for sunny sky) than what I saw in some research papers; and the image was just one color. :slight_smile:

That's why I came back to manually tracing the brightest pixel approach.

Can you please elaborate on the "log 2" scale approach ? Thank you for your time!

BEst regards,
Vaib

On 23 May 2014 23:56, Guglielmetti, Robert <[email protected]<mailto:[email protected]>> wrote:
Hi Vaib,

Falsecolor now has an "auto" option to the –s parameter, which will automatically set the top of the scale to the peak value found in the image. Note the peak value will not necessarily be at the zenith (which, yes, is where the vector 0,0,1 points to in a hemispherical fisheye view).

I also find that for other than overcast skies, using a logarithmic scale (-log 2) is very useful for getting the most value out of a falsecolor representation of a sky's luminous distribution.

On 5/23/14, 12:14 PM, "Vaib" <[email protected]<mailto:[email protected]><mailto:[email protected]<mailto:[email protected]>>> wrote:

Hello Everyone,

I am trying to create falsecolor fish-eye image of just the sky. For that I am using:

rpict -vta -vp 0 0 0 -vd 0 0 1 -vu 0 1 0 -vh 180 -vv 180 sky.oct | falsecolor -s XYZ -l cd/m2 > sky.hdr

Dr.Mardaljevic in his dissertation said to set -s argument of falsecolor close to the Zenith luminance, that can be obtained by rvu's trace command.

To get the Zenith luminance, I am tracing the ray (using rvu) that is approximately at the centre of the image created with the above view parameters, and setting this ray's luminance value as the -s option of falsecolor.

Is this the right way to show/cover the whole luminance spectrum in the sky ? (It doesn't look so in the image..)

Or shall I trace the ray at the apparently at the brightest pixel ? (by reducing the exposure, and figuring out that pixel)

I think the term Zenith Luminance may mean the luminance at the centre of the sky, but I must take your opinion before concluding. Thank you!

Best regards,
Vaib

_______________________________________________
Radiance-general mailing list
[email protected]<mailto:[email protected]>
http://www.radiance-online.org/mailman/listinfo/radiance-general