capturing with concentrated light sources?

Hi, Greetings to all in the mail list,

I have couple questions in the context of capturing HDR environment
map for image-based lighting, and would like to hear your feedbacks.

When capturing the HDR environment map for a scene with concentrated
light sources (say light bulb or the sun), is it correct that if the
pixel values of the light source are not saturated in the most
under-exposed bracket (but saturated throughout the rest of exposure
brackets), the full dynamic range of the light source is well
preserved?

If I have two, three, or more under-exposed brackets presenting
non-saturated pixels for the same concentrated light source, will the
resulting HDRI be more accurate in terms of representing the
illumination of the light source during IBL than the situation where
the non-saturated pixels of the light source only present in the most
under-exposed bracket?

Thanks,
Jason

Hi Jason,

I assume you are assembling your images using Photosphere or hdrcvt.
Unless the single exposure that doesn't over-expose the light
source(s) has values very close to the maximum (255), you should be OK
as far as accuracy.

The best accuracy is achieved using RAW shots and the raw2hdr Perl
script that calls hdrgen, dcraw, and exiftool. This process avoids
the issue of camera response by going straight from the linear CCD or
CMOS sensor values. This makes more of a difference to color accuracy
than luminance, but it's worth pursuing if accuracy is your goal. If
you're interested in this under Mac OS X, I can post a package for
you.

-Greg

···

On Tue, Jan 25, 2011 at 8:20 AM, jason huang <[email protected]> wrote:

Hi, Greetings to all in the mail list,

I have couple questions in the context of capturing HDR environment
map for image-based lighting, and would like to hear your feedbacks.

When capturing the HDR environment map for a scene with concentrated
light sources (say light bulb or the sun), is it correct that if the
pixel values of the light source are not saturated in the most
under-exposed bracket (but saturated throughout the rest of exposure
brackets), the full dynamic range of the light source is well
preserved?

If I have two, three, or more under-exposed brackets presenting
non-saturated pixels for the same concentrated light source, will the
resulting HDRI be more accurate in terms of representing the
illumination of the light source during IBL than the situation where
the non-saturated pixels of the light source only present in the most
under-exposed bracket?

Thanks,
Jason

Indeed. If by non-saturated you refer to every single channel.
With the sun, for example, it happens quite often that the Luminance histogram indicates you're good, but when you look at the red channel that still clips.

However, there are other problems with environments that have primarily small concentrated light sources.
- Most alignments fail for the most under-exposed images. Well, you shoot your pano on a tripod anyway.
- You need to be very careful editing the HDRs, even a resize with the wrong filter can cause color fringes. Stick to Bilinear to be safe.
- IBL needs sampling settings beyond insane.
Ultimately, these concentrated light sources are perfect examples of direct illumination sources. Brute-forcing this through a GI render pipeline makes little sense. It's much more efficient to represent these concentrated light sources with actual 3d lights, and in turn limiting the DR in the environment map. Also gives you more artistic control over shadows. Some people like to paint out direct light sources completely, but I prefer blurring them out with the rest of the environment to keep some ambient contribution (think hazy spill light).

Hope this helps,
Blochi

···

On Jan 25, 2011, at 8:20 AM, jason huang wrote:

Hi, Greetings to all in the mail list,

I have couple questions in the context of capturing HDR environment
map for image-based lighting, and would like to hear your feedbacks.

When capturing the HDR environment map for a scene with concentrated
light sources (say light bulb or the sun), is it correct that if the
pixel values of the light source are not saturated in the most
under-exposed bracket (but saturated throughout the rest of exposure
brackets), the full dynamic range of the light source is well
preserved?

If I have two, three, or more under-exposed brackets presenting
non-saturated pixels for the same concentrated light source, will the
resulting HDRI be more accurate in terms of representing the
illumination of the light source during IBL than the situation where
the non-saturated pixels of the light source only present in the most
under-exposed bracket?

Thanks,
Jason

_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri

Radiance includes a tool called mksource to address this. Mksource samples
an HDR environment map and creates direct Radiance light sources to
represent bright concentrated regions found in the map, moving much of the
flux to the direct calculation -- more efficient and less artifacts.

···

On 1/25/11 11:47 AM, "Christian Bloch" <[email protected]> wrote:

- IBL needs sampling settings beyond insane.
Ultimately, these concentrated light sources are perfect examples of
direct illumination sources. Brute-forcing this through a GI render
pipeline makes little sense. It's much more efficient to represent these
concentrated light sources with actual 3d lights, and in turn limiting
the DR in the environment map. Also gives you more artistic control over
shadows. Some people like to paint out direct light sources completely,
but I prefer blurring them out with the rest of the environment to keep
some ambient contribution (think hazy spill light).

Hi,

I'd add that when you have point sources, usually the main factor affecting accuracy is lens flare (especially with the sun) and/or "bleeding" of the source in the background. I don't know of any way to avoid this, as it happens as soon as the light enters the camera. With a separate measurement to calibrate image values this can be compensated to some extent. In some cases an illuminance measurement can be useful if it is not possible to measure the source luminance directly.

Cheers,
Santiago

PS: Greg, you mention raw2hdr as a Perl script, but what I found is csh, is there a different version?

···

-----Original Message-----
From: Greg Ward [mailto:[email protected]]
Sent: 25 January 2011 17:08
To: High Dynamic Range Imaging
Subject: Re: [HDRI] capturing with concentrated light sources?

Hi Jason,

I assume you are assembling your images using Photosphere or hdrcvt.
Unless the single exposure that doesn't over-expose the light
source(s) has values very close to the maximum (255), you should be OK
as far as accuracy.

The best accuracy is achieved using RAW shots and the raw2hdr Perl
script that calls hdrgen, dcraw, and exiftool. This process avoids
the issue of camera response by going straight from the linear CCD or
CMOS sensor values. This makes more of a difference to color accuracy
than luminance, but it's worth pursuing if accuracy is your goal. If
you're interested in this under Mac OS X, I can post a package for
you.

-Greg

On Tue, Jan 25, 2011 at 8:20 AM, jason huang <[email protected]> wrote:

Hi, Greetings to all in the mail list,

I have couple questions in the context of capturing HDR environment
map for image-based lighting, and would like to hear your feedbacks.

When capturing the HDR environment map for a scene with concentrated
light sources (say light bulb or the sun), is it correct that if the
pixel values of the light source are not saturated in the most
under-exposed bracket (but saturated throughout the rest of exposure
brackets), the full dynamic range of the light source is well
preserved?

If I have two, three, or more under-exposed brackets presenting
non-saturated pixels for the same concentrated light source, will the
resulting HDRI be more accurate in terms of representing the
illumination of the light source during IBL than the situation where
the non-saturated pixels of the light source only present in the most
under-exposed bracket?

Thanks,
Jason

_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri
____________________________________________________________
Electronic mail messages entering and leaving Arup business
systems are scanned for acceptability of content and viruses

Hi,

This thread is getting interesting to me as well. I was not aware of the mksource tool. Any chance I can find some documentation about how it works?

I tested Paul Debevec's Median Cut approach some years back ... Tested it against another method. The other method demonstrated similar accuracy with half the directional light sources.

My Ph. D. Student has been trying to fuse HDR images from fisheye images of the sky (including the sun). We were using HDRshop. Capturing spanned 13 f-stops and we could not get HDRshop to fuse HDR properly. In the end we gave up.

Do I really need to get a Mac just to fuse proper sky HDRs??

Best
Claus

···

Sent from my iPod

On 25/01/2011, at 19.59, "Guglielmetti, Robert" <[email protected]> wrote:

On 1/25/11 11:47 AM, "Christian Bloch" <[email protected]> wrote:

- IBL needs sampling settings beyond insane.
Ultimately, these concentrated light sources are perfect examples of
direct illumination sources. Brute-forcing this through a GI render
pipeline makes little sense. It's much more efficient to represent these
concentrated light sources with actual 3d lights, and in turn limiting
the DR in the environment map. Also gives you more artistic control over
shadows. Some people like to paint out direct light sources completely,
but I prefer blurring them out with the rest of the environment to keep
some ambient contribution (think hazy spill light).

Radiance includes a tool called mksource to address this. Mksource samples
an HDR environment map and creates direct Radiance light sources to
represent bright concentrated regions found in the map, moving much of the
flux to the direct calculation -- more efficient and less artifacts.

_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri

Hi Claus,

The mksource tool is part Radiance which is available from:

    http://www.radiance-online.org/
    or
    http://radsite.lbl.gov/radiance/

If you are not familiar with it, Radiance is a physically based simulation toolset design for lighting, daylighting and visualization. The image format used in Radiance is the original HDR image format.

Regards,

-Jack de Valpine

···

--
# Jack de Valpine
# president
#
# visarc incorporated
# http://www.visarc.com
#
# channeling technology for superior design and construction

On 1/25/2011 2:29 PM, Claus Brøndgaard Madsen wrote:

Hi,

This thread is getting interesting to me as well. I was not aware of the mksource tool. Any chance I can find some documentation about how it works?

I tested Paul Debevec's Median Cut approach some years back ... Tested it against another method. The other method demonstrated similar accuracy with half the directional light sources.

My Ph. D. Student has been trying to fuse HDR images from fisheye images of the sky (including the sun). We were using HDRshop. Capturing spanned 13 f-stops and we could not get HDRshop to fuse HDR properly. In the end we gave up.

Do I really need to get a Mac just to fuse proper sky HDRs??

Best
Claus

Sent from my iPod

On 25/01/2011, at 19.59, "Guglielmetti, Robert"<[email protected]> wrote:

On 1/25/11 11:47 AM, "Christian Bloch"<[email protected]> wrote:

- IBL needs sampling settings beyond insane.
Ultimately, these concentrated light sources are perfect examples of
direct illumination sources. Brute-forcing this through a GI render
pipeline makes little sense. It's much more efficient to represent these
concentrated light sources with actual 3d lights, and in turn limiting
the DR in the environment map. Also gives you more artistic control over
shadows. Some people like to paint out direct light sources completely,
but I prefer blurring them out with the rest of the environment to keep
some ambient contribution (think hazy spill light).

Radiance includes a tool called mksource to address this. Mksource samples
an HDR environment map and creates direct Radiance light sources to
represent bright concentrated regions found in the map, moving much of the
flux to the direct calculation -- more efficient and less artifacts.

_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri

_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri

A recent paper I did on IBL might be interest to some of you:

Best,
Mehlika

Claus Brøndgaard Madsen wrote:

···

Hi,

This thread is getting interesting to me as well. I was not aware of the mksource tool. Any chance I can find some documentation about how it works?

I tested Paul Debevec's Median Cut approach some years back ... Tested it against another method. The other method demonstrated similar accuracy with half the directional light sources.

My Ph. D. Student has been trying to fuse HDR images from fisheye images of the sky (including the sun). We were using HDRshop. Capturing spanned 13 f-stops and we could not get HDRshop to fuse HDR properly. In the end we gave up.

Do I really need to get a Mac just to fuse proper sky HDRs??

Best
Claus

Sent from my iPod

On 25/01/2011, at 19.59, "Guglielmetti, Robert" <[email protected]> wrote:

On 1/25/11 11:47 AM, "Christian Bloch" <[email protected]> wrote:

- IBL needs sampling settings beyond insane.
Ultimately, these concentrated light sources are perfect examples of
direct illumination sources. Brute-forcing this through a GI render
pipeline makes little sense. It's much more efficient to represent these
concentrated light sources with actual 3d lights, and in turn limiting
the DR in the environment map. Also gives you more artistic control over
shadows. Some people like to paint out direct light sources completely,
but I prefer blurring them out with the rest of the environment to keep
some ambient contribution (think hazy spill light).
      

Radiance includes a tool called mksource to address this. Mksource samples
an HDR environment map and creates direct Radiance light sources to
represent bright concentrated regions found in the map, moving much of the
flux to the direct calculation -- more efficient and less artifacts.

_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri
    
_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri

Hi Jack,

Thanks ... I am somewhat familiar with RADIANCE. I have been using it on and off for 6-8 years ... but never really explored all of its amazing powers. For some reason I just never noticed the mksource tool ...

I was interested in for example being directed to a paper that described the method behind mksource.

Best,
Claus

[cid:[email protected]]

···

From: Jack de Valpine [mailto:[email protected]]
Sent: 25. januar 2011 21:08
To: High Dynamic Range Imaging
Subject: Re: [HDRI] capturing with concentrated light sources?

Hi Claus,

The mksource tool is part Radiance which is available from:
http://www.radiance-online.org/
or
http://radsite.lbl.gov/radiance/
If you are not familiar with it, Radiance is a physically based simulation toolset design for lighting, daylighting and visualization. The image format used in Radiance is the original HDR image format.

Regards,

-Jack de Valpine

--

# Jack de Valpine

# president

#

# visarc incorporated

# http://www.visarc.com

#

# channeling technology for superior design and construction

On 1/25/2011 2:29 PM, Claus Brøndgaard Madsen wrote:

Hi,

This thread is getting interesting to me as well. I was not aware of the mksource tool. Any chance I can find some documentation about how it works?

I tested Paul Debevec's Median Cut approach some years back ... Tested it against another method. The other method demonstrated similar accuracy with half the directional light sources.

My Ph. D. Student has been trying to fuse HDR images from fisheye images of the sky (including the sun). We were using HDRshop. Capturing spanned 13 f-stops and we could not get HDRshop to fuse HDR properly. In the end we gave up.

Do I really need to get a Mac just to fuse proper sky HDRs??

Best

Claus

Sent from my iPod

On 25/01/2011, at 19.59, "Guglielmetti, Robert" <[email protected]><mailto:[email protected]> wrote:

On 1/25/11 11:47 AM, "Christian Bloch" <[email protected]><mailto:[email protected]> wrote:

- IBL needs sampling settings beyond insane.

Ultimately, these concentrated light sources are perfect examples of

direct illumination sources. Brute-forcing this through a GI render

pipeline makes little sense. It's much more efficient to represent these

concentrated light sources with actual 3d lights, and in turn limiting

the DR in the environment map. Also gives you more artistic control over

shadows. Some people like to paint out direct light sources completely,

but I prefer blurring them out with the rest of the environment to keep

some ambient contribution (think hazy spill light).

Radiance includes a tool called mksource to address this. Mksource samples

an HDR environment map and creates direct Radiance light sources to

represent bright concentrated regions found in the map, moving much of the

flux to the direct calculation -- more efficient and less artifacts.

_______________________________________________

HDRI mailing list

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

http://www.radiance-online.org/mailman/listinfo/hdri

_______________________________________________

HDRI mailing list

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

http://www.radiance-online.org/mailman/listinfo/hdri

Hi Mehlika,

Thanks to you as well. I already have your paper ... my student and I studied it in the fall we had problems of fusing our sky capture images (or maybe it could have been another earlier paper of yours with some of the same ideas in it? This one seems a little more detailed than I remember... anyway nice work).

We just didn't feel like taking the jump to photosphere, seing as we have been using HDRShop for so many years.

Best,
Claus

···

-----Original Message-----
From: Mehlika Inanici [mailto:[email protected]]
Sent: 25. januar 2011 21:09
To: High Dynamic Range Imaging
Subject: Re: [HDRI] capturing with concentrated light sources?

A recent paper I did on IBL might be interest to some of you:

Best,
Mehlika

Claus Brøndgaard Madsen wrote:

Hi,

This thread is getting interesting to me as well. I was not aware of the mksource tool. Any chance I can find some documentation about how it works?

I tested Paul Debevec's Median Cut approach some years back ... Tested it against another method. The other method demonstrated similar accuracy with half the directional light sources.

My Ph. D. Student has been trying to fuse HDR images from fisheye images of the sky (including the sun). We were using HDRshop. Capturing spanned 13 f-stops and we could not get HDRshop to fuse HDR properly. In the end we gave up.

Do I really need to get a Mac just to fuse proper sky HDRs??

Best
Claus

Sent from my iPod

On 25/01/2011, at 19.59, "Guglielmetti, Robert" <[email protected]> wrote:

On 1/25/11 11:47 AM, "Christian Bloch" <[email protected]> wrote:

- IBL needs sampling settings beyond insane.
Ultimately, these concentrated light sources are perfect examples of
direct illumination sources. Brute-forcing this through a GI render
pipeline makes little sense. It's much more efficient to represent
these concentrated light sources with actual 3d lights, and in turn
limiting the DR in the environment map. Also gives you more artistic
control over shadows. Some people like to paint out direct light
sources completely, but I prefer blurring them out with the rest of
the environment to keep some ambient contribution (think hazy spill light).
      

Radiance includes a tool called mksource to address this. Mksource
samples an HDR environment map and creates direct Radiance light
sources to represent bright concentrated regions found in the map,
moving much of the flux to the direct calculation -- more efficient and less artifacts.

_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri
    
_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri
  
_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri

Me again ... sorry. Now I am by my computer again and have been looking into the pdf version of the manpages. I don't seem to be able to find a manpage entry for mksource. I have version 4.0 of Radiance installed.

Cheers
Claus

[cid:[email protected]]

···

From: Jack de Valpine [mailto:[email protected]]
Sent: 25. januar 2011 21:08
To: High Dynamic Range Imaging
Subject: Re: [HDRI] capturing with concentrated light sources?

Hi Claus,

The mksource tool is part Radiance which is available from:
http://www.radiance-online.org/
or
http://radsite.lbl.gov/radiance/
If you are not familiar with it, Radiance is a physically based simulation toolset design for lighting, daylighting and visualization. The image format used in Radiance is the original HDR image format.

Regards,

-Jack de Valpine

--

# Jack de Valpine

# president

#

# visarc incorporated

# http://www.visarc.com

#

# channeling technology for superior design and construction

On 1/25/2011 2:29 PM, Claus Brøndgaard Madsen wrote:

Hi,

This thread is getting interesting to me as well. I was not aware of the mksource tool. Any chance I can find some documentation about how it works?

I tested Paul Debevec's Median Cut approach some years back ... Tested it against another method. The other method demonstrated similar accuracy with half the directional light sources.

My Ph. D. Student has been trying to fuse HDR images from fisheye images of the sky (including the sun). We were using HDRshop. Capturing spanned 13 f-stops and we could not get HDRshop to fuse HDR properly. In the end we gave up.

Do I really need to get a Mac just to fuse proper sky HDRs??

Best

Claus

Sent from my iPod

On 25/01/2011, at 19.59, "Guglielmetti, Robert" <[email protected]><mailto:[email protected]> wrote:

On 1/25/11 11:47 AM, "Christian Bloch" <[email protected]><mailto:[email protected]> wrote:

- IBL needs sampling settings beyond insane.

Ultimately, these concentrated light sources are perfect examples of

direct illumination sources. Brute-forcing this through a GI render

pipeline makes little sense. It's much more efficient to represent these

concentrated light sources with actual 3d lights, and in turn limiting

the DR in the environment map. Also gives you more artistic control over

shadows. Some people like to paint out direct light sources completely,

but I prefer blurring them out with the rest of the environment to keep

some ambient contribution (think hazy spill light).

Radiance includes a tool called mksource to address this. Mksource samples

an HDR environment map and creates direct Radiance light sources to

represent bright concentrated regions found in the map, moving much of the

flux to the direct calculation -- more efficient and less artifacts.

_______________________________________________

HDRI mailing list

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

http://www.radiance-online.org/mailman/listinfo/hdri

_______________________________________________

HDRI mailing list

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

http://www.radiance-online.org/mailman/listinfo/hdri

Hi Claus,

If you are grabbing the HEAD release you should have the latest manpages, but the default "makeall" script does not automatically link to the docs not does it attempt to place them in a common location. I usually add the "/ray/doc/man/" directory to my environment. I can send you a pdf of that particular manpage if you are interested, but it is just a manpage — it shows you how to use the tool, not how it works. AFAIK, there haven't been any papers published on it.

Rob Guglielmetti IESNA, LEED AP
Commercial Buildings Research Group
National Renewable Energy Laboratory
1617 Cole Blvd MS:RSF202
Golden, CO 80401
T. 303.275.4319
F. 303.630.2055
E. [email protected]

···

From: Claus Brøndgaard Madsen <[email protected]<mailto:[email protected]>>
Reply-To: High Dynamic Range Imaging <[email protected]<mailto:[email protected]>>
Date: Tue, 25 Jan 2011 13:27:47 -0700
To: High Dynamic Range Imaging <[email protected]<mailto:[email protected]>>
Subject: Re: [HDRI] capturing with concentrated light sources?

Me again ... sorry. Now I am by my computer again and have been looking into the pdf version of the manpages. I don’t seem to be able to find a manpage entry for mksource. I have version 4.0 of Radiance installed.

Cheers
Claus

[cid:[email protected]]

From: Jack de Valpine [mailto:[email protected]]
Sent: 25. januar 2011 21:08
To: High Dynamic Range Imaging
Subject: Re: [HDRI] capturing with concentrated light sources?

Hi Claus,

The mksource tool is part Radiance which is available from:
http://www.radiance-online.org/
or
http://radsite.lbl.gov/radiance/
If you are not familiar with it, Radiance is a physically based simulation toolset design for lighting, daylighting and visualization. The image format used in Radiance is the original HDR image format.

Regards,

-Jack de Valpine

--

# Jack de Valpine

# president

#

# visarc incorporated

# http://www.visarc.com

#

# channeling technology for superior design and construction

On 1/25/2011 2:29 PM, Claus Brøndgaard Madsen wrote:

Hi,

This thread is getting interesting to me as well. I was not aware of the mksource tool. Any chance I can find some documentation about how it works?

I tested Paul Debevec's Median Cut approach some years back ... Tested it against another method. The other method demonstrated similar accuracy with half the directional light sources.

My Ph. D. Student has been trying to fuse HDR images from fisheye images of the sky (including the sun). We were using HDRshop. Capturing spanned 13 f-stops and we could not get HDRshop to fuse HDR properly. In the end we gave up.

Do I really need to get a Mac just to fuse proper sky HDRs??

Best

Claus

Sent from my iPod

On 25/01/2011, at 19.59, "Guglielmetti, Robert" <[email protected]><mailto:[email protected]> wrote:

On 1/25/11 11:47 AM, "Christian Bloch" <[email protected]><mailto:[email protected]> wrote:

- IBL needs sampling settings beyond insane.

Ultimately, these concentrated light sources are perfect examples of

direct illumination sources. Brute-forcing this through a GI render

pipeline makes little sense. It's much more efficient to represent these

concentrated light sources with actual 3d lights, and in turn limiting

the DR in the environment map. Also gives you more artistic control over

shadows. Some people like to paint out direct light sources completely,

but I prefer blurring them out with the rest of the environment to keep

some ambient contribution (think hazy spill light).

Radiance includes a tool called mksource to address this. Mksource samples

an HDR environment map and creates direct Radiance light sources to

represent bright concentrated regions found in the map, moving much of the

flux to the direct calculation -- more efficient and less artifacts.

_______________________________________________

HDRI mailing list

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

http://www.radiance-online.org/mailman/listinfo/hdri

_______________________________________________

HDRI mailing list

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

http://www.radiance-online.org/mailman/listinfo/hdri

If you're PC-based, I recommend Picturenaut.
It's meant to be a modern HDRShop replacement, adding in all the modern features like merging from RAW, ghost-removal, multicore processing. Transition is seamless, because it's compatible with HDRShop plugins, but in addition has an open source SDK.

www.hdrlabs.com/picturenaut

In terms of IBL, me and a team of artists made the "Smart IBL" system, which is exactly what I described earlier: The sun represented with light coordinates, and the lighting split up in diffuse and specular component via differently sized/convoluted HDRs. It's not so much a "scientifically pure" solution, but rather optimized for integration into a production pipeline. So it has no magic algorithm, instead it has to work with whatever commercial renderers can offer. Essentially, it prepares an HDR file into pre-optimized components, that are searchable with tags and keywords, and then it uses templates to feed these to VRay, mental ray, renderman, arnold, Maya, Lightwave, Cinema4d, ect.... trying to take advantage of each renderer's specific capabilities and quirks. Also a free and open source system.

www.hdrlabs/sibl

Representing an HDR only with light sources, well, in real life there are only very few occasions when this is actually more efficient than using the HDR itself for diffuse. There are some, for example, rendering smoke. We just did it this summer in production, using Lightbitch (another one of my creations: www.hdrlabs.com/lightbitch ) to sample an HDR down to 10 spotlights, take it over to 3dMAX and render smoke puffs in FumeFX. Here is a video of it:
http://www.vimeo.com/18607788

.Blochi

···

On Jan 25, 2011, at 12:22 PM, Claus Brøndgaard Madsen wrote:

We just didn't feel like taking the jump to photosphere, seing as we have been using HDRShop for so many years.

Hi Claus!

I am not optimistic about capturing sky and sun using a simple camera. Without worrying about hdr codes - unless you achieve an exposure quick enough to have a non-saturated sun image, the hdr code cannot do the magic and invent pixel values. You may try low-transmission filters, but that costs you the weaker sky luminances. So probably, hardware needed.

Creating hdr images from ldr stacks definitely works without a Mac. Hdrgen and pfstools are just two of the many options available.

Cheers, Lars.

···

--
Dipl.-Ing. Architect Lars O. Grobe

On Jan 25, 2011, at 20:29, Claus Brøndgaard Madsen <[email protected]> wrote:

My Ph. D. Student has been trying to fuse HDR images from fisheye images of the sky (including the sun). We were using HDRshop. Capturing spanned 13 f-stops and we could not get HDRshop to fuse HDR properly. In the end we gave up.

Do I really need to get a Mac just to fuse proper sky HDRs??

You could use filters to get the luminance peaks and extend the series by
re-shooting without the filter. Time and camera handling are definitely a
problem for getting an honest (and alignable) LDR series. Greg has a paper
on this someplace, I think. Can't find it right now...

Rob Guglielmetti IESNA, LEED AP
Commercial Buildings Research Group
National Renewable Energy Laboratory
1617 Cole Blvd MS:RSF202
Golden, CO 80401
T. 303.275.4319
F. 303.630.2055
E. [email protected]

···

On 1/25/11 4:33 PM, "Lars O. Grobe" <[email protected]> wrote:

Hi Claus!

I am not optimistic about capturing sky and sun using a simple camera.
Without worrying about hdr codes - unless you achieve an exposure quick
enough to have a non-saturated sun image, the hdr code cannot do the
magic and invent pixel values. You may try low-transmission filters, but
that costs you the weaker sky luminances. So probably, hardware needed.

There's another good paper on this subject by Jessi Stumpfel et al.

  http://gl.ict.usc.edu/skyprobes/

-Greg

···

From: "Guglielmetti, Robert" <[email protected]>
Date: January 25, 2011 3:50:37 PM PST

On 1/25/11 4:33 PM, "Lars O. Grobe" <[email protected]> wrote:

Hi Claus!

I am not optimistic about capturing sky and sun using a simple camera.
Without worrying about hdr codes - unless you achieve an exposure quick
enough to have a non-saturated sun image, the hdr code cannot do the
magic and invent pixel values. You may try low-transmission filters, but
that costs you the weaker sky luminances. So probably, hardware needed.

You could use filters to get the luminance peaks and extend the series by
re-shooting without the filter. Time and camera handling are definitely a
problem for getting an honest (and alignable) LDR series. Greg has a paper
on this someplace, I think. Can't find it right now...

Rob Guglielmetti IESNA, LEED AP

Hi Rob et al,

one thing I could imagine to avoid setups with mounting internal filters (or, even worse, combining filter wheels and fisheye lenses ...) during the somewhat time-critical process of taking an exposure series... Did anyone try yet to use a short-pass filter at 350nm assuming a smaller direct/indirect ratio at short wavelengths, trying to apply models os spectral angular sky distributions to calculate back luminances? Or is this just a silly idea introducing too many uncertainties to remove a basic mechanical problem...?

Cheers, Lars.

···

--
Dipl.-Ing. Architect Lars O. Grobe

On Jan 26, 2011, at 0:50, "Guglielmetti, Robert" <[email protected]> wrote:

On 1/25/11 4:33 PM, "Lars O. Grobe" <[email protected]> wrote:

Hi Claus!

I am not optimistic about capturing sky and sun using a simple camera.
Without worrying about hdr codes - unless you achieve an exposure quick
enough to have a non-saturated sun image, the hdr code cannot do the
magic and invent pixel values. You may try low-transmission filters, but
that costs you the weaker sky luminances. So probably, hardware needed.

You could use filters to get the luminance peaks and extend the series by
re-shooting without the filter. Time and camera handling are definitely a
problem for getting an honest (and alignable) LDR series. Greg has a paper
on this someplace, I think. Can't find it right now...

Rob Guglielmetti IESNA, LEED AP
Commercial Buildings Research Group
National Renewable Energy Laboratory
1617 Cole Blvd MS:RSF202
Golden, CO 80401
T. 303.275.4319
F. 303.630.2055
E. [email protected]

_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri

There was a brief mention of the source creation algorithm I developed for mksource in the 2005 workshop notes:

  http://www.radiance-online.org/radiance-workshop4/cd/website/PDF/Ward_talk1.pdf

As for man pages, you can always get to these online via:

  http://radsite.lbl.gov/radiance/man_html/whatis.html

I just noticed that the entry for mksource was missing, so I added it. Others are probably missing as well. Sigh. We do need to update our web resources, don't we?

Best,
-Greg

Wauw, this mailing list suddenly lit up, didn't it?

Thanks to Greg for link to manpages (so far I have been looking in the manpages.pdf that comes with the distribution).

I have also studied Jessi Stumpfels paper on sky capture. We were even in contact with the authors to try to get an understanding of how we could not achieve what they achieved.

To sum up on what we tried in terms of attempting to capture nice (and correct) sky light probes:

1) Canon 1Ds Mark II camera
2) Sigma 2.8 fish eye lens, 180+ degrees field-of-view
3) 3.0 neutral density filter, mounted inside lens, used for capture of all exposures, ie., it was not removed for capture of sky luminances
4) darkest exposure (to capture sun pixels without saturation) was achieved with a 1/8000 s exposure, don't remember aperture ...
5) since the sun is roughly 100.000 times brighter than the sky we must have had an exposure sequence spanning 16 or 17 f-stops
6) sun pixels were not saturated in darkest exposure
7) sky pixels were properly exposed in brightest exposure
8) we controlled the camera from a laptop to automate the bracketing sequence acquisition
9) taking one full exposure sequence took more than one minute (getting properly exposed sky pixels with a 3.0 ND filter requires very long exposure time)

At first we attempted simply to HDR fuse all exposures in one go, but the HDR sky probes never came out nice. I suspect there is some numerical resolution problem internally in HDRshop, which resulted in sky colors coming out very ugly. When using HDRshop, regardless of what the actual top luminance of the exposure is, the highest HDR fusion value is always 1.0. With a 16 f-stop sequence, where the sun pixels in the fusion result are roughly 1.0 or 0.9, then sky pixels are going to be on the order of 0.00001 ... maybe that's too small values for HDRshop to handle without numerical problems?

Then we tried fusing the low end and the high end of the exposure sequence separately and then "manually" merging them. I.e., we made a HDR light probe image where the sun and the corona (or whatever the bright area around the sun is called) was definitely saturated, but the sky and cloud pixels were good. Then we thresholded that, and replaced the pixels above the threshold with pixels from a properly scaled version of the HDR fusion result from the top end of the exposure sequence (in order to get the correct sun and corona luminances).

This approach sort of worked. There were some color artifact along the edge of the corona, i.e., where the lower end HDR had been tresholded and pixels were inserted from the higher end HDR.

But it was not really a neat solution.

Best,
Claus

···

-----Original Message-----
From: Gregory J. Ward [mailto:[email protected]]
Sent: 26. januar 2011 03:50
To: High Dynamic Range Imaging
Subject: Re: [HDRI] capturing with concentrated light sources?

There was a brief mention of the source creation algorithm I developed for mksource in the 2005 workshop notes:

  http://www.radiance-online.org/radiance-workshop4/cd/website/PDF/Ward_talk1.pdf

As for man pages, you can always get to these online via:

  http://radsite.lbl.gov/radiance/man_html/whatis.html

I just noticed that the entry for mksource was missing, so I added it. Others are probably missing as well. Sigh. We do need to update our web resources, don't we?

Best,
-Greg
_______________________________________________
HDRI mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/hdri

Hi, Greg,

Thank you for the input. With your confirmation, I can be rest-assured
that I don't have to worry about taking couple more under-exposed
brackets for concentrated light sources, which also make the alignment
and merging process harder.

I am actually on Windows with HDR Shop. Really hope that there is a
windows version of Photoshere such that we can use its flare reduction
feature, which seems not available anywhere else. I have read comments
in this mail list and other places about merging directly from RAW for
accuracy. In my experience, it brought me more troubles than
generating visually obviously better IBL results. At the end, I am not
after pure scientific accuracy. Hence, I just use commercial
applications like Adobe Lightroom to convert RAW brackets to
intermediate TIFFs, with minimum color or luminance related
adjustments applied, for HDR merging.

I have had this "capturing concentrated light sources" question for
quite a while and did came across Stumpfel's paper/thesis and
corresponding section in your book (the section written by Debevec).
My main interest is image-based lighting for visual effects type of
works. As I came across a SIGGRAPH course note by Ben Snow's (ILM) and
read about their on-set lightning acquisition workflow: Canon 1DS +
Sigma 8mm fisheye + 0.6 ND filter for exterior shot. If compared to
Stumpfel's 3.0 filter with varied aperture sizes controlled by laptop
to shorten the exposure time for over-exposed brackets, I wonder if
the former is simply capturing the environment map as a high-ish
dynamic range image (instead of fully recover light sources
illumination) and relying on visual-based indirect approach (via 18%
gray sphere photograph) to calibrate the light source's contribution.
But then what's the point of using that 0.6 ND filter if the indirect
calibration of light source's contribution will be conducted anyway?
Why not just capture an environment map without the filter and
manipulate the pixel values of the light source in floating point
space in post until whatever their calibration approach tells them
that the illumination of the light source is properly represented?

best,
Jason