# pcond, luminance or visible radiation?

Hi all, I'd appreciate your help on the following question:

I'm creating a radiance scene using gendaylit to generate the sky, since I
have data of the diffuse horizontal and direct normal irradiance (W/m^2)
I'm using the -W option and then -O 2 to obtain the output in lm/m^2/sr
(luminance). Then I'm using rpict to generate the picture, and pcond -h to
convert it to a human visual response.

But I realized that if I use the default value of -O in gendaylit (output
bright.

So I don't know which one is the right one to use in this case.

I tried to find out if the default input when using pcond would be
that, or maybe there is another command other than pcond, that takes the
input in luminance values, then that would be the one I should use.

Or maybe there is an additional step that I'm missing...

Ch.

Can you clarify the exact process when you say the picture looks less bright? The picture out of pcond is just visually less bright? If you're looking at values prior to using pcond, your process to get values out of your image is probably based on the assumption that the scene is set up using defaults (visible W/m^2/sr). If you convert to lumens in your scene description from gendaylit, then you have to be careful that you're not again converting to lumens (with a 179 multiplier) after your calculation.

···

From: minchaca [mailto:[email protected]]
Sent: Monday, August 05, 2013 8:13 AM
To: [email protected]

Hi all, I'd appreciate your help on the following question:

I'm creating a radiance scene using gendaylit to generate the sky, since I have data of the diffuse horizontal and direct normal irradiance (W/m^2) I'm using the -W option and then -O 2 to obtain the output in lm/m^2/sr (luminance). Then I'm using rpict to generate the picture, and pcond -h to convert it to a human visual response.

But I realized that if I use the default value of -O in gendaylit (output in radiance of the visible radiation W/m^2/sr) the picture looks less bright.

So I don't know which one is the right one to use in this case.

I tried to find out if the default input when using pcond would be luminance rather than visible radiation, but couldn't find an answer for that, or maybe there is another command other than pcond, that takes the input in luminance values, then that would be the one I should use.
Or maybe there is an additional step that I'm missing...

Ch.
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If I select the output of gendaylit -O 2 (lm/m^2/sr) then I generate the
picture using rpict, convert to human visual response using pcond
-h and then to bmp using ra_bmp then I get a picture that is visually more
brighter than if I select the output of gendaylit -O 0 (default) and then
multiply by 179.

I confirm, when I use -O 2 (output lm/m^2/sr) then I don't multiply by 179,
only when I select the default output (W/m^2/sr).

I don't have problem with the results of the values, but with the generated
picture, using the default of the parameter -O the picture looks less
brighter
(even after *179).

I wonder which of the two pictures is the right one, as it doesn't seem to
have anything to do with the results of the values, but with the way how
the image
is created...I guess...

···

2013/8/5 Christopher Rush <[email protected]>

Can you clarify the exact process when you say the picture looks less
bright? The picture out of pcond is just visually less bright? If you’re
looking at values prior to using pcond, your process to get values out of
your image is probably based on the assumption that the scene is set up
using defaults (visible W/m^2/sr). If you convert to lumens in your scene
description from gendaylit, then you have to be careful that you’re not
again converting to lumens (with a 179 multiplier) after your calculation.
****

** **

*From:* minchaca [mailto:[email protected]]
*Sent:* Monday, August 05, 2013 8:13 AM
*To:* [email protected]

** **

Hi all, I'd appreciate your help on the following question:****

I'm creating a radiance scene using gendaylit to generate the sky, since I
have data of the diffuse horizontal and direct normal irradiance (W/m^2)
I'm using the -W option and then -O 2 to obtain the output in lm/m^2/sr
(luminance). Then I'm using rpict to generate the picture, and pcond -h to
convert it to a human visual response.

But I realized that if I use the default value of -O in gendaylit (output
bright.

So I don't know which one is the right one to use in this case.

I tried to find out if the default input when using pcond would be
that, or maybe there is another command other than pcond, that takes the
input in luminance values, then that would be the one I should use. ****

Or maybe there is an additional step that I'm missing... ** **

look..

Ch.****

____________________________________________________________
Electronic mail messages entering and leaving Arup business
systems are scanned for acceptability of content and viruses

_______________________________________________
[email protected]

I'm not an expert on the pcond code, but it's probably applying different tone mapping depending on what it interprets as lux levels in the scene. If you give it a scene from gendaylit -O 2, pcond doesn't know that the pixel value RGB triplets have already been scaled to luminance, and thinks it's a brighter scene, and applies a brighter tone map. If you gave it a scene illuminated to only 10 lux of electric light, it would probably give you a different tone mapping even if the contrast ratios were identical.

Can anyone else corroborate my guess?

···

From: minchaca [mailto:[email protected]]
Sent: Monday, August 05, 2013 11:56 AM

If I select the output of gendaylit -O 2 (lm/m^2/sr) then I generate the picture using rpict, convert to human visual response using pcond
-h and then to bmp using ra_bmp then I get a picture that is visually more brighter than if I select the output of gendaylit -O 0 (default) and then
multiply by 179.

I confirm, when I use -O 2 (output lm/m^2/sr) then I don't multiply by 179, only when I select the default output (W/m^2/sr).

I don't have problem with the results of the values, but with the generated picture, using the default of the parameter -O the picture looks less brighter
(even after *179).

I wonder which of the two pictures is the right one, as it doesn't seem to have anything to do with the results of the values, but with the way how the image
is created...I guess...

2013/8/5 Christopher Rush <[email protected]<mailto:[email protected]>>
Can you clarify the exact process when you say the picture looks less bright? The picture out of pcond is just visually less bright? If you're looking at values prior to using pcond, your process to get values out of your image is probably based on the assumption that the scene is set up using defaults (visible W/m^2/sr). If you convert to lumens in your scene description from gendaylit, then you have to be careful that you're not again converting to lumens (with a 179 multiplier) after your calculation.

From: minchaca [mailto:[email protected]<mailto:[email protected]>]
Sent: Monday, August 05, 2013 8:13 AM
To: [email protected]<mailto:[email protected]>

Hi all, I'd appreciate your help on the following question:

I'm creating a radiance scene using gendaylit to generate the sky, since I have data of the diffuse horizontal and direct normal irradiance (W/m^2) I'm using the -W option and then -O 2 to obtain the output in lm/m^2/sr (luminance). Then I'm using rpict to generate the picture, and pcond -h to convert it to a human visual response.

But I realized that if I use the default value of -O in gendaylit (output in radiance of the visible radiation W/m^2/sr) the picture looks less bright.

So I don't know which one is the right one to use in this case.

I tried to find out if the default input when using pcond would be luminance rather than visible radiation, but couldn't find an answer for that, or maybe there is another command other than pcond, that takes the input in luminance values, then that would be the one I should use.
Or maybe there is an additional step that I'm missing...

Ch.

____________________________________________________________
Electronic mail messages entering and leaving Arup business
systems are scanned for acceptability of content and viruses

_______________________________________________
[email protected]<mailto:[email protected]>

Thank you Christopher...your explanation kind of make sense to me, but yes
would be great to know
what is the real issue behind the differences between the two pictures and
if there is a way of getting
one that really matches the results of the calculations (or which one of
the 2 is that)
so I hope that someone can bring more light to this question...

Chantal.

···

2013/8/7 Christopher Rush <[email protected]>

I’m not an expert on the pcond code, but it’s probably applying
different tone mapping depending on what it interprets as lux levels in the
scene. If you give it a scene from gendaylit -O 2, pcond doesn’t know that
the pixel value RGB triplets have already been scaled to luminance, and
thinks it’s a brighter scene, and applies a brighter tone map. If you gave
it a scene illuminated to only 10 lux of electric light, it would probably
give you a different tone mapping even if the contrast ratios were
identical.****

** **

Can anyone else corroborate my guess?****

** **

*From:* minchaca [mailto:[email protected]]
*Sent:* Monday, August 05, 2013 11:56 AM
**

** **

If I select the output of gendaylit -O 2 (lm/m^2/sr) then I generate the
picture using rpict, convert to human visual response using pcond
-h and then to bmp using ra_bmp then I get a picture that is visually more
brighter than if I select the output of gendaylit -O 0 (default) and then
multiply by 179.

I confirm, when I use -O 2 (output lm/m^2/sr) then I don't multiply by
179, only when I select the default output (W/m^2/sr).

I don't have problem with the results of the values, but with the
generated picture, using the default of the parameter -O the picture looks
less brighter
(even after *179).

I wonder which of the two pictures is the right one, as it doesn't seem to
have anything to do with the results of the values, but with the way how
the image
is created...I guess...

:)****

** **

2013/8/5 Christopher Rush <[email protected]>****

Can you clarify the exact process when you say the picture looks less
bright? The picture out of pcond is just visually less bright? If you’re
looking at values prior to using pcond, your process to get values out of
your image is probably based on the assumption that the scene is set up
using defaults (visible W/m^2/sr). If you convert to lumens in your scene
description from gendaylit, then you have to be careful that you’re not
again converting to lumens (with a 179 multiplier) after your calculation.
****

****

*From:* minchaca [mailto:[email protected]]
*Sent:* Monday, August 05, 2013 8:13 AM
*To:* [email protected]

****

Hi all, I'd appreciate your help on the following question:****

I'm creating a radiance scene using gendaylit to generate the sky, since I
have data of the diffuse horizontal and direct normal irradiance (W/m^2)
I'm using the -W option and then -O 2 to obtain the output in lm/m^2/sr
(luminance). Then I'm using rpict to generate the picture, and pcond -h to
convert it to a human visual response.

But I realized that if I use the default value of -O in gendaylit (output
bright.

So I don't know which one is the right one to use in this case.

I tried to find out if the default input when using pcond would be
that, or maybe there is another command other than pcond, that takes the
input in luminance values, then that would be the one I should use. ****

Or maybe there is an additional step that I'm missing... ****

look..

Ch.****

____________________________________________________________
Electronic mail messages entering and leaving Arup business
systems are scanned for acceptability of content and viruses****

_______________________________________________