# How does Pvalue determine CCT in HDR images and how is the EV of a HDR image determined?

Hello everybody,

I’m Kars and I’m new to this Radiance forum.

In the manual of Pvalue (https://www.radiance-online.org/learning/documentation/manual-pages/pdfs/pvalue.pdf) it is stated that inputting a file in XYZE format will give you the luminance values of the image (corresponding to the Y channel). I’ve inserted some .hdr images in Pvalue with the following Primaries (taken from the EXIF data): PRIMARIES= 0.6400 0.3300 0.3000 0.6000 0.1500 0.0600 0.3127 0.3290, where the first two numbers correspond to the R primary (x,y), the third and fourth to the G primary (x,y), the fifth and sixth to the B primary (x,y), and the seventh and eighth to the white point (x,y). The EXIF data also shows an exposure value of the .hdr image.

I’ve got two questions regarding this conversion.

To my knowledge the white point coordinates can be used to calculate the CCT, as described by Inanici in Evaluation of High Dynamic Range Photography as a Luminance Data Acquisition System. The white point coordinates, as found in the EXIF data, correspond to the CIE standard illuminant D65. Does this mean Pvalue assumes a constant CCT for all the .hdr images when converting to luminance? Or is Pvalue making other assumptions / calculations?

My second question refers to the exposure value of the .hdr image, as shown in the EXIF data of the image. How is this value determined? Since a HDR image consists of multiple images (in my case 7) with different exposure values. What is this exposure value of the .hdr image based on and is this used by Pvalue somehow?

I hope I’ve made myself clear.

Kind regards,

Kars Morsink

Bonjour,

Je suis actuellement en vacances. Je serai de retour le 7 août 2017.

En cas d'urgence, vous pouvez toujours appeler le numéro général d'Estia : +41 (0) 21/510.59.59 ou envoyer un mail à l'adresse [email protected].

Pour toutes questions relatives à DIAL+, merci d'utiliser l'adresse mail [email protected].

Cordialement

Julien Boutillier
Estia SA

Hi Kars. Just a friendly tip that there is another mailing list dedicated to HDR image capture/manipulation. See last row of the table on this page: https://radiance-online.org/community/mailing-lists
There's a lot of cross-subscribers between lists, so you may still get a response without re-posting. If your interests lie particularly in HDR imaging, you also might want to subscribe to the HDRI list to keep up with future topics.

-Chris

···

From: Morsink, K. [mailto:[email protected]]
Sent: Wednesday, July 26, 2017 10:35 AM
To: [email protected]
Subject: [Radiance-general] How does Pvalue determine CCT in HDR images and how is the EV of a HDR image determined?

Hello everybody,

I'm Kars and I'm new to this Radiance forum.

In the manual of Pvalue (https://www.radiance-online.org/learning/documentation/manual-pages/pdfs/pvalue.pdf<https://secure-web.cisco.com/1xZYMoIcnbOsaow7kOjcsylubpX3S1Rvfh1LDzJTWhsMxA7baVVdKOuA3HgWhFBFYTT0MB64GYZlcc-T8kHQjagzFlH9fdchH4_DQOKdTme3WdqqdscGM3lHfaEqwOhV45YQHN1yU-ko715qg1V6WF7BUmMCCLFwiSkqP320yjYS3L0yADar6mJPXH2WzEFY373JY2UsJ8rjSRENAHZN25bmZ02ZOK0kwH_12xIQnU7tVFUijBmYv_1phASPb0aNd/https%3A%2F%2Fwww.radiance-online.org%2Flearning%2Fdocumentation%2Fmanual-pages%2Fpdfs%2Fpvalue.pdf>) it is stated that inputting a file in XYZE format will give you the luminance values of the image (corresponding to the Y channel). I've inserted some .hdr images in Pvalue with the following Primaries (taken from the EXIF data): PRIMARIES= 0.6400 0.3300 0.3000 0.6000 0.1500 0.0600 0.3127 0.3290, where the first two numbers correspond to the R primary (x,y), the third and fourth to the G primary (x,y), the fifth and sixth to the B primary (x,y), and the seventh and eighth to the white point (x,y). The EXIF data also shows an exposure value of the .hdr image.

I've got two questions regarding this conversion.

To my knowledge the white point coordinates can be used to calculate the CCT, as described by Inanici in Evaluation of High Dynamic Range Photography as a Luminance Data Acquisition System. The white point coordinates, as found in the EXIF data, correspond to the CIE standard illuminant D65. Does this mean Pvalue assumes a constant CCT for all the .hdr images when converting to luminance? Or is Pvalue making other assumptions / calculations?

My second question refers to the exposure value of the .hdr image, as shown in the EXIF data of the image. How is this value determined? Since a HDR image consists of multiple images (in my case 7) with different exposure values. What is this exposure value of the .hdr image based on and is this used by Pvalue somehow?

I hope I've made myself clear.

Kind regards,

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

Hi Kars,

I am cross-posting my response to the HDRI mailing list per Chris' suggestion.

Specifying the "-o" option of pvalue takes into account the exposure setting, which is determined from the ExIF data if you created the image using Photosphere or hdrgen. The formula below has an empirically derived constant that may not exactly fit your camera, so it is best to add your own calibrating scale factor:

sample_to_nits = 87 * (f-stop)^2 / (ISO * exposure_secs)

The sample_to_nits is converted to a Radiance picture exposure using:

EXPOSURE = 179 / sample_to_nits

where 179 lumens/watt is the agreed-upon efficacy of the equal-energy illuminant E over the visible spectrum.

Unfortunately, pvalue is not very smart about reporting brightness using the "-b" option. It uses a formula based on the standard Radiance color space, which differs from the CCIR-709 color space produced by Photosphere in both the green primary and the white point. It only makes a small difference, but if you are worrying about such things, you had best use the following to report luminance from your image:

ra_xyze image.hdr | pvalue -o -b [other options]

This also takes care of the 179 factor, reporting results in candelas/meter^2.

Cheers,
-Greg

···

From: "Morsink, K." <[email protected]>
Date: July 26, 2017 7:34:45 AM PDT

Hello everybody,

I’m Kars and I’m new to this Radiance forum.

In the manual of Pvalue (https://www.radiance-online.org/learning/documentation/manual-pages/pdfs/pvalue.pdf) it is stated that inputting a file in XYZE format will give you the luminance values of the image (corresponding to the Y channel). I’ve inserted some .hdr images in Pvalue with the following Primaries (taken from the EXIF data): PRIMARIES= 0.6400 0.3300 0.3000 0.6000 0.1500 0.0600 0.3127 0.3290, where the first two numbers correspond to the R primary (x,y), the third and fourth to the G primary (x,y), the fifth and sixth to the B primary (x,y), and the seventh and eighth to the white point (x,y). The EXIF data also shows an exposure value of the .hdr image.

I’ve got two questions regarding this conversion.

To my knowledge the white point coordinates can be used to calculate the CCT, as described by Inanici in Evaluation of High Dynamic Range Photography as a Luminance Data Acquisition System. The white point coordinates, as found in the EXIF data, correspond to the CIE standard illuminant D65. Does this mean Pvalue assumes a constant CCT for all the .hdr images when converting to luminance? Or is Pvalue making other assumptions / calculations?

My second question refers to the exposure value of the .hdr image, as shown in the EXIF data of the image. How is this value determined? Since a HDR image consists of multiple images (in my case 7) with different exposure values. What is this exposure value of the .hdr image based on and is this used by Pvalue somehow?

I hope I’ve made myself clear.

Kind regards,

Kars Morsink

_______________________________________________
[email protected]

Hi Greg,

Thank you for your quick response!

The formula you provided works for me with a single LDR image and thus a single exposure time, but I'm stuck in how I should insert the multiple exposure times (of the 7 LDR images to form the HDR image) in exposure_secs and get the correct EXPOSURE, could you maybe explain this further?

Making use of the equal-energy illuminant E, is it assumed that the white point remains constant (1/3, 1/3)? I'm asking this since I'm taking outdoor photographs, where the CCT (and thereby the white point) changes constantly, assuming a constant white point would therefore affect the accuracy of the conversion to luminance values.

Kind regards,
Kars

···

-----Original Message-----
From: Greg Ward [mailto:[email protected]]
Sent: woensdag 26 juli 2017 19:15
Cc: High Dynamic Range Imaging
Subject: Re: [Radiance-general] How does Pvalue determine CCT in HDR images and how is the EV of a HDR image determined?

Hi Kars,

I am cross-posting my response to the HDRI mailing list per Chris' suggestion.

Specifying the "-o" option of pvalue takes into account the exposure setting, which is determined from the ExIF data if you created the image using Photosphere or hdrgen. The formula below has an empirically derived constant that may not exactly fit your camera, so it is best to add your own calibrating scale factor:

sample_to_nits = 87 * (f-stop)^2 / (ISO * exposure_secs)

The sample_to_nits is converted to a Radiance picture exposure using:

EXPOSURE = 179 / sample_to_nits

where 179 lumens/watt is the agreed-upon efficacy of the equal-energy illuminant E over the visible spectrum.

Unfortunately, pvalue is not very smart about reporting brightness using the "-b" option. It uses a formula based on the standard Radiance color space, which differs from the CCIR-709 color space produced by Photosphere in both the green primary and the white point. It only makes a small difference, but if you are worrying about such things, you had best use the following to report luminance from your image:

ra_xyze image.hdr | pvalue -o -b [other options]

This also takes care of the 179 factor, reporting results in candelas/meter^2.

Cheers,
-Greg

From: "Morsink, K." <[email protected]>
Date: July 26, 2017 7:34:45 AM PDT

Hello everybody,

I'm Kars and I'm new to this Radiance forum.

In the manual of Pvalue (https://www.radiance-online.org/learning/documentation/manual-pages/pdfs/pvalue.pdf) it is stated that inputting a file in XYZE format will give you the luminance values of the image (corresponding to the Y channel). I've inserted some .hdr images in Pvalue with the following Primaries (taken from the EXIF data): PRIMARIES= 0.6400 0.3300 0.3000 0.6000 0.1500 0.0600 0.3127 0.3290, where the first two numbers correspond to the R primary (x,y), the third and fourth to the G primary (x,y), the fifth and sixth to the B primary (x,y), and the seventh and eighth to the white point (x,y). The EXIF data also shows an exposure value of the .hdr image.

I've got two questions regarding this conversion.

To my knowledge the white point coordinates can be used to calculate the CCT, as described by Inanici in Evaluation of High Dynamic Range Photography as a Luminance Data Acquisition System. The white point coordinates, as found in the EXIF data, correspond to the CIE standard illuminant D65. Does this mean Pvalue assumes a constant CCT for all the .hdr images when converting to luminance? Or is Pvalue making other assumptions / calculations?

My second question refers to the exposure value of the .hdr image, as shown in the EXIF data of the image. How is this value determined? Since a HDR image consists of multiple images (in my case 7) with different exposure values. What is this exposure value of the .hdr image based on and is this used by Pvalue somehow?

I hope I've made myself clear.

Kind regards,

Kars Morsink

_______________________________________________
[email protected]

_______________________________________________
[email protected]

Hi Kars,

It's difficult to answer your question if you don't explain why you are asking. If you use the given formulae to compute your EXPOSURE value in some kind of manual conversion of each image to HDR format, the values will correspond, even if the limits are still standard dynamic range. This is sort of what happens if you give Photosphere a single image and tell it to build an HDR result. If you give Photosphere multiple images, it knows how to apply the formula for you. The same is true of hdrgen.

If you are using different software or writing your own, then you need to better explain your expected inputs, or provide a clear example with where you are stuck.

Best,
-Greg

P.S. I am moving the remainder of this thread to the HDRI mailing list.

···

From: "Morsink, K." <[email protected]>
Date: July 27, 2017 8:22:59 AM PDT

Hi Greg,

Thank you for your quick response!

The formula you provided works for me with a single LDR image and thus a single exposure time, but I'm stuck in how I should insert the multiple exposure times (of the 7 LDR images to form the HDR image) in exposure_secs and get the correct EXPOSURE, could you maybe explain this further?

Making use of the equal-energy illuminant E, is it assumed that the white point remains constant (1/3, 1/3)? I'm asking this since I'm taking outdoor photographs, where the CCT (and thereby the white point) changes constantly, assuming a constant white point would therefore affect the accuracy of the conversion to luminance values.

Kind regards,
Kars

-----Original Message-----
From: Greg Ward [mailto:[email protected]]
Sent: woensdag 26 juli 2017 19:15

Hi Kars,

I am cross-posting my response to the HDRI mailing list per Chris' suggestion.

Specifying the "-o" option of pvalue takes into account the exposure setting, which is determined from the ExIF data if you created the image using Photosphere or hdrgen. The formula below has an empirically derived constant that may not exactly fit your camera, so it is best to add your own calibrating scale factor:

sample_to_nits = 87 * (f-stop)^2 / (ISO * exposure_secs)

The sample_to_nits is converted to a Radiance picture exposure using:

EXPOSURE = 179 / sample_to_nits

where 179 lumens/watt is the agreed-upon efficacy of the equal-energy illuminant E over the visible spectrum.

Unfortunately, pvalue is not very smart about reporting brightness using the "-b" option. It uses a formula based on the standard Radiance color space, which differs from the CCIR-709 color space produced by Photosphere in both the green primary and the white point. It only makes a small difference, but if you are worrying about such things, you had best use the following to report luminance from your image:

ra_xyze image.hdr | pvalue -o -b [other options]

This also takes care of the 179 factor, reporting results in candelas/meter^2.

Cheers,
-Greg

From: "Morsink, K." <[email protected]>
Date: July 26, 2017 7:34:45 AM PDT

Hello everybody,

I'm Kars and I'm new to this Radiance forum.

In the manual of Pvalue (https://www.radiance-online.org/learning/documentation/manual-pages/pdfs/pvalue.pdf) it is stated that inputting a file in XYZE format will give you the luminance values of the image (corresponding to the Y channel). I've inserted some .hdr images in Pvalue with the following Primaries (taken from the EXIF data): PRIMARIES= 0.6400 0.3300 0.3000 0.6000 0.1500 0.0600 0.3127 0.3290, where the first two numbers correspond to the R primary (x,y), the third and fourth to the G primary (x,y), the fifth and sixth to the B primary (x,y), and the seventh and eighth to the white point (x,y). The EXIF data also shows an exposure value of the .hdr image.

I've got two questions regarding this conversion.

To my knowledge the white point coordinates can be used to calculate the CCT, as described by Inanici in Evaluation of High Dynamic Range Photography as a Luminance Data Acquisition System. The white point coordinates, as found in the EXIF data, correspond to the CIE standard illuminant D65. Does this mean Pvalue assumes a constant CCT for all the .hdr images when converting to luminance? Or is Pvalue making other assumptions / calculations?

My second question refers to the exposure value of the .hdr image, as shown in the EXIF data of the image. How is this value determined? Since a HDR image consists of multiple images (in my case 7) with different exposure values. What is this exposure value of the .hdr image based on and is this used by Pvalue somehow?

I hope I've made myself clear.

Kind regards,

Kars Morsink

Regarding your second question, I don't believe the assumed illuminant has much influence on luminance readings. You can safely ignore it in most cases. But yes, EE white is CIE (x,y) = (1/3,1/3).

Cheers,
-Greg

···

From: "Morsink, K." <[email protected]>
Date: July 27, 2017 8:22:59 AM PDT

Hi Greg,

Thank you for your quick response!

The formula you provided works for me with a single LDR image and thus a single exposure time, but I'm stuck in how I should insert the multiple exposure times (of the 7 LDR images to form the HDR image) in exposure_secs and get the correct EXPOSURE, could you maybe explain this further?

Making use of the equal-energy illuminant E, is it assumed that the white point remains constant (1/3, 1/3)? I'm asking this since I'm taking outdoor photographs, where the CCT (and thereby the white point) changes constantly, assuming a constant white point would therefore affect the accuracy of the conversion to luminance values.

Kind regards,
Kars

Hi Greg,

I'm using hdrgen to create the hdr images. I'm asking to understand how the EXPOSURE value of the hdr image can be linked back to the different exposure times of the original images. This to better understand the origin of this value and how it is used to compute the luminance values. I'm not using any other software. But from your answer I understand that this is not so straightforward to check?

Thank you for your time and clarification.

Kind regards,
Kars

···

-------- Original Message --------
Subject: Re: [HDRI] [Radiance-general] How does Pvalue determine CCT in HDR images and how is the EV of a HDR image determined?
From: Greg Ward
To: High Dynamic Range Imaging
CC:

Hi Kars,

It's difficult to answer your question if you don't explain why you are asking. If you use the given formulae to compute your EXPOSURE value in some kind of manual conversion of each image to HDR format, the values will correspond, even if the limits are still standard dynamic range. This is sort of what happens if you give Photosphere a single image and tell it to build an HDR result. If you give Photosphere multiple images, it knows how to apply the formula for you. The same is true of hdrgen.

If you are using different software or writing your own, then you need to better explain your expected inputs, or provide a clear example with where you are stuck.

Best,
-Greg

P.S. I am moving the remainder of this thread to the HDRI mailing list.

From: "Morsink, K." <[email protected]>
Date: July 27, 2017 8:22:59 AM PDT

Hi Greg,

Thank you for your quick response!

The formula you provided works for me with a single LDR image and thus a single exposure time, but I'm stuck in how I should insert the multiple exposure times (of the 7 LDR images to form the HDR image) in exposure_secs and get the correct EXPOSURE, could you maybe explain this further?

Making use of the equal-energy illuminant E, is it assumed that the white point remains constant (1/3, 1/3)? I'm asking this since I'm taking outdoor photographs, where the CCT (and thereby the white point) changes constantly, assuming a constant white point would therefore affect the accuracy of the conversion to luminance values.

Kind regards,
Kars

-----Original Message-----
From: Greg Ward [mailto:[email protected]]
Sent: woensdag 26 juli 2017 19:15

Hi Kars,

I am cross-posting my response to the HDRI mailing list per Chris' suggestion.

Specifying the "-o" option of pvalue takes into account the exposure setting, which is determined from the ExIF data if you created the image using Photosphere or hdrgen. The formula below has an empirically derived constant that may not exactly fit your camera, so it is best to add your own calibrating scale factor:

sample_to_nits = 87 * (f-stop)^2 / (ISO * exposure_secs)

The sample_to_nits is converted to a Radiance picture exposure using:

EXPOSURE = 179 / sample_to_nits

where 179 lumens/watt is the agreed-upon efficacy of the equal-energy illuminant E over the visible spectrum.

Unfortunately, pvalue is not very smart about reporting brightness using the "-b" option. It uses a formula based on the standard Radiance color space, which differs from the CCIR-709 color space produced by Photosphere in both the green primary and the white point. It only makes a small difference, but if you are worrying about such things, you had best use the following to report luminance from your image:

ra_xyze image.hdr | pvalue -o -b [other options]

This also takes care of the 179 factor, reporting results in candelas/meter^2.

Cheers,
-Greg

From: "Morsink, K." <[email protected]>
Date: July 26, 2017 7:34:45 AM PDT

Hello everybody,

I'm Kars and I'm new to this Radiance forum.

In the manual of Pvalue (https://www.radiance-online.org/learning/documentation/manual-pages/pdfs/pvalue.pdf) it is stated that inputting a file in XYZE format will give you the luminance values of the image (corresponding to the Y channel). I've inserted some .hdr images in Pvalue with the following Primaries (taken from the EXIF data): PRIMARIES= 0.6400 0.3300 0.3000 0.6000 0.1500 0.0600 0.3127 0.3290, where the first two numbers correspond to the R primary (x,y), the third and fourth to the G primary (x,y), the fifth and sixth to the B primary (x,y), and the seventh and eighth to the white point (x,y). The EXIF data also shows an exposure value of the .hdr image.

I've got two questions regarding this conversion.

To my knowledge the white point coordinates can be used to calculate the CCT, as described by Inanici in Evaluation of High Dynamic Range Photography as a Luminance Data Acquisition System. The white point coordinates, as found in the EXIF data, correspond to the CIE standard illuminant D65. Does this mean Pvalue assumes a constant CCT for all the .hdr images when converting to luminance? Or is Pvalue making other assumptions / calculations?

My second question refers to the exposure value of the .hdr image, as shown in the EXIF data of the image. How is this value determined? Since a HDR image consists of multiple images (in my case 7) with different exposure values. What is this exposure value of the .hdr image based on and is this used by Pvalue somehow?

I hope I've made myself clear.

Kind regards,

Kars Morsink

_______________________________________________
HDRI mailing list
[email protected]

OK, but I'm still not clear what you are hoping to check. Do you want to check values in each original image against the output of hdrgen? The easiest way to do that would be to feed hdrgen a single image along with the response curve (using the -r option). That's not really a check on hdrgen, though, since you would only be comparing it to itself for consistency.

You can use the output of ra_xyze piped into pvalue as we discussed earlier, then pull up individual pixel values in the image in Photoshop or your favorite tool. The luminance should approximately equal this:

luminance = sample_to_nits * (lin(R)*0.213 + lin(G)*0.715 + lin(B)*0.072)

lin(p) = ((p+.5)/256)^2.2

R,G,B = pixel components in 0-255 range

The linearization function above doesn't do a perfect job, since each camera has its own response function, and this is one of the things hdrgen figures out for you. In particular, I would expect larger errors in the shadow regions of each exposure.

Does this help?

-Greg

···

From: "Morsink, K." <[email protected]>
Date: July 27, 2017 9:04:45 AM PDT

Hi Greg,

I'm using hdrgen to create the hdr images. I'm asking to understand how the EXPOSURE value of the hdr image can be linked back to the different exposure times of the original images. This to better understand the origin of this value and how it is used to compute the luminance values. I'm not using any other software. But from your answer I understand that this is not so straightforward to check?

Thank you for your time and clarification.

Kind regards,
Kars

-------- Original Message --------
Subject: Re: [HDRI] [Radiance-general] How does Pvalue determine CCT in HDR images and how is the EV of a HDR image determined?
From: Greg Ward
To: High Dynamic Range Imaging
CC:

Hi Kars,

It's difficult to answer your question if you don't explain why you are asking. If you use the given formulae to compute your EXPOSURE value in some kind of manual conversion of each image to HDR format, the values will correspond, even if the limits are still standard dynamic range. This is sort of what happens if you give Photosphere a single image and tell it to build an HDR result. If you give Photosphere multiple images, it knows how to apply the formula for you. The same is true of hdrgen.

If you are using different software or writing your own, then you need to better explain your expected inputs, or provide a clear example with where you are stuck.

Best,
-Greg

P.S. I am moving the remainder of this thread to the HDRI mailing list.

> From: "Morsink, K." <[email protected]>
> Date: July 27, 2017 8:22:59 AM PDT
>
> Hi Greg,
>
> Thank you for your quick response!
>
> The formula you provided works for me with a single LDR image and thus a single exposure time, but I'm stuck in how I should insert the multiple exposure times (of the 7 LDR images to form the HDR image) in exposure_secs and get the correct EXPOSURE, could you maybe explain this further?
>
> Making use of the equal-energy illuminant E, is it assumed that the white point remains constant (1/3, 1/3)? I'm asking this since I'm taking outdoor photographs, where the CCT (and thereby the white point) changes constantly, assuming a constant white point would therefore affect the accuracy of the conversion to luminance values.
>
> Kind regards,
> Kars
>
> -----Original Message-----
> From: Greg Ward [mailto:[email protected]]
> Sent: woensdag 26 juli 2017 19:15
>
> Hi Kars,
>
> I am cross-posting my response to the HDRI mailing list per Chris' suggestion.
>
> Specifying the "-o" option of pvalue takes into account the exposure setting, which is determined from the ExIF data if you created the image using Photosphere or hdrgen. The formula below has an empirically derived constant that may not exactly fit your camera, so it is best to add your own calibrating scale factor:
>
> sample_to_nits = 87 * (f-stop)^2 / (ISO * exposure_secs)
>
> The sample_to_nits is converted to a Radiance picture exposure using:
>
> EXPOSURE = 179 / sample_to_nits
>
> where 179 lumens/watt is the agreed-upon efficacy of the equal-energy illuminant E over the visible spectrum.
>
> Unfortunately, pvalue is not very smart about reporting brightness using the "-b" option. It uses a formula based on the standard Radiance color space, which differs from the CCIR-709 color space produced by Photosphere in both the green primary and the white point. It only makes a small difference, but if you are worrying about such things, you had best use the following to report luminance from your image:
>
> ra_xyze image.hdr | pvalue -o -b [other options]
>
> This also takes care of the 179 factor, reporting results in candelas/meter^2.
>
> Cheers,
> -Greg
>
>> From: "Morsink, K." <[email protected]>
>> Date: July 26, 2017 7:34:45 AM PDT
>>
>> Hello everybody,
>>
>> I'm Kars and I'm new to this Radiance forum.
>>
>> In the manual of Pvalue (https://www.radiance-online.org/learning/documentation/manual-pages/pdfs/pvalue.pdf) it is stated that inputting a file in XYZE format will give you the luminance values of the image (corresponding to the Y channel). I've inserted some .hdr images in Pvalue with the following Primaries (taken from the EXIF data): PRIMARIES= 0.6400 0.3300 0.3000 0.6000 0.1500 0.0600 0.3127 0.3290, where the first two numbers correspond to the R primary (x,y), the third and fourth to the G primary (x,y), the fifth and sixth to the B primary (x,y), and the seventh and eighth to the white point (x,y). The EXIF data also shows an exposure value of the .hdr image.
>>
>> I've got two questions regarding this conversion.
>>
>> To my knowledge the white point coordinates can be used to calculate the CCT, as described by Inanici in Evaluation of High Dynamic Range Photography as a Luminance Data Acquisition System. The white point coordinates, as found in the EXIF data, correspond to the CIE standard illuminant D65. Does this mean Pvalue assumes a constant CCT for all the .hdr images when converting to luminance? Or is Pvalue making other assumptions / calculations?
>>
>> My second question refers to the exposure value of the .hdr image, as shown in the EXIF data of the image. How is this value determined? Since a HDR image consists of multiple images (in my case 7) with different exposure values. What is this exposure value of the .hdr image based on and is this used by Pvalue somehow?
>>
>> I hope I've made myself clear.
>>
>> Kind regards,
>>
>> Kars Morsink

_______________________________________________
HDRI mailing list
[email protected]
_______________________________________________
HDRI mailing list
[email protected]

The EXPOSURE value in the HDR and the exposure times of the JPGs are
unrelated. You can't derive one from the other.

A

···

On 27 July 2017 at 17:04, Morsink, K. <[email protected]> wrote:

Hi Greg,

I'm using hdrgen to create the hdr images. I'm asking to understand how the
EXPOSURE value of the hdr image can be linked back to the different exposure
times of the original images. This to better understand the origin of this
value and how it is used to compute the luminance values. I'm not using any
other software. But from your answer I understand that this is not so
straightforward to check?

Thank you for your time and clarification.

Kind regards,
Kars

-------- Original Message --------
Subject: Re: [HDRI] [Radiance-general] How does Pvalue determine CCT in HDR
images and how is the EV of a HDR image determined?
From: Greg Ward
To: High Dynamic Range Imaging
CC:

Hi Kars,

It's difficult to answer your question if you don't explain why you are
asking. If you use the given formulae to compute your EXPOSURE value in
some kind of manual conversion of each image to HDR format, the values will
correspond, even if the limits are still standard dynamic range. This is
sort of what happens if you give Photosphere a single image and tell it to
build an HDR result. If you give Photosphere multiple images, it knows how
to apply the formula for you. The same is true of hdrgen.

If you are using different software or writing your own, then you need to
better explain your expected inputs, or provide a clear example with where
you are stuck.

Best,
-Greg

P.S. I am moving the remainder of this thread to the HDRI mailing list.

From: "Morsink, K." <[email protected]>
Date: July 27, 2017 8:22:59 AM PDT

Hi Greg,

Thank you for your quick response!

The formula you provided works for me with a single LDR image and thus a
single exposure time, but I'm stuck in how I should insert the multiple
exposure times (of the 7 LDR images to form the HDR image) in exposure_secs
and get the correct EXPOSURE, could you maybe explain this further?

Making use of the equal-energy illuminant E, is it assumed that the white
point remains constant (1/3, 1/3)? I'm asking this since I'm taking outdoor
photographs, where the CCT (and thereby the white point) changes constantly,
assuming a constant white point would therefore affect the accuracy of the
conversion to luminance values.

Kind regards,
Kars

-----Original Message-----
From: Greg Ward [mailto:[email protected]]
Sent: woensdag 26 juli 2017 19:15

Hi Kars,

I am cross-posting my response to the HDRI mailing list per Chris'
suggestion.

Specifying the "-o" option of pvalue takes into account the exposure
setting, which is determined from the ExIF data if you created the image
using Photosphere or hdrgen. The formula below has an empirically derived
calibrating scale factor:

sample_to_nits = 87 * (f-stop)^2 / (ISO * exposure_secs)

The sample_to_nits is converted to a Radiance picture exposure using:

EXPOSURE = 179 / sample_to_nits

where 179 lumens/watt is the agreed-upon efficacy of the equal-energy
illuminant E over the visible spectrum.

Unfortunately, pvalue is not very smart about reporting brightness using
the "-b" option. It uses a formula based on the standard Radiance color
space, which differs from the CCIR-709 color space produced by Photosphere
in both the green primary and the white point. It only makes a small
difference, but if you are worrying about such things, you had best use the
following to report luminance from your image:

ra_xyze image.hdr | pvalue -o -b [other options]

This also takes care of the 179 factor, reporting results in
candelas/meter^2.

Cheers,
-Greg

From: "Morsink, K." <[email protected]>
Date: July 26, 2017 7:34:45 AM PDT

Hello everybody,

I'm Kars and I'm new to this Radiance forum.

In the manual of Pvalue
it is stated that inputting a file in XYZE format will give you the
luminance values of the image (corresponding to the Y channel). I've
inserted some .hdr images in Pvalue with the following Primaries (taken from
the EXIF data): PRIMARIES= 0.6400 0.3300 0.3000 0.6000 0.1500 0.0600 0.3127
0.3290, where the first two numbers correspond to the R primary (x,y), the
third and fourth to the G primary (x,y), the fifth and sixth to the B
primary (x,y), and the seventh and eighth to the white point (x,y). The EXIF
data also shows an exposure value of the .hdr image.

I've got two questions regarding this conversion.

To my knowledge the white point coordinates can be used to calculate the
CCT, as described by Inanici in Evaluation of High Dynamic Range Photography
as a Luminance Data Acquisition System. The white point coordinates, as
found in the EXIF data, correspond to the CIE standard illuminant D65. Does
this mean Pvalue assumes a constant CCT for all the .hdr images when
converting to luminance? Or is Pvalue making other assumptions /
calculations?

My second question refers to the exposure value of the .hdr image, as
shown in the EXIF data of the image. How is this value determined? Since a
HDR image consists of multiple images (in my case 7) with different exposure
values. What is this exposure value of the .hdr image based on and is this
used by Pvalue somehow?

I hope I've made myself clear.

Kind regards,

Kars Morsink

_______________________________________________
HDRI mailing list
[email protected]

_______________________________________________
HDRI mailing list
[email protected]

Hi Greg and Axel,

Thank you both for your responses. I indeed wanted to find a relation between the EXPOSURE value of the HDR and the exposure times of the JPGs, but as Axel said: they are unrelated. By that I know the answers to my questions.
Thank you for your time and expertise.

Kind regards,
Kars

···

From: Gregory J. Ward [mailto:[email protected]]
Sent: donderdag 27 juli 2017 18:20
To: High Dynamic Range Imaging
Subject: Re: [HDRI] [Radiance-general] How does Pvalue determine CCT in HDR images and how is the EV of a HDR image determined?

OK, but I'm still not clear what you are hoping to check. Do you want to check values in each original image against the output of hdrgen? The easiest way to do that would be to feed hdrgen a single image along with the response curve (using the -r option). That's not really a check on hdrgen, though, since you would only be comparing it to itself for consistency.

You can use the output of ra_xyze piped into pvalue as we discussed earlier, then pull up individual pixel values in the image in Photoshop or your favorite tool. The luminance should approximately equal this:

luminance = sample_to_nits * (lin(R)*0.213 + lin(G)*0.715 + lin(B)*0.072)

lin(p) = ((p+.5)/256)^2.2

R,G,B = pixel components in 0-255 range

The linearization function above doesn't do a perfect job, since each camera has its own response function, and this is one of the things hdrgen figures out for you. In particular, I would expect larger errors in the shadow regions of each exposure.

Does this help?

-Greg

From: "Morsink, K." <[email protected]<mailto:[email protected]>>

Date: July 27, 2017 9:04:45 AM PDT

Hi Greg,

I'm using hdrgen to create the hdr images. I'm asking to understand how the EXPOSURE value of the hdr image can be linked back to the different exposure times of the original images. This to better understand the origin of this value and how it is used to compute the luminance values. I'm not using any other software. But from your answer I understand that this is not so straightforward to check?

Thank you for your time and clarification.

Kind regards,
Kars

-------- Original Message --------
Subject: Re: [HDRI] [Radiance-general] How does Pvalue determine CCT in HDR images and how is the EV of a HDR image determined?
From: Greg Ward
To: High Dynamic Range Imaging
CC:
Hi Kars,

It's difficult to answer your question if you don't explain why you are asking. If you use the given formulae to compute your EXPOSURE value in some kind of manual conversion of each image to HDR format, the values will correspond, even if the limits are still standard dynamic range. This is sort of what happens if you give Photosphere a single image and tell it to build an HDR result. If you give Photosphere multiple images, it knows how to apply the formula for you. The same is true of hdrgen.

If you are using different software or writing your own, then you need to better explain your expected inputs, or provide a clear example with where you are stuck.

Best,
-Greg

P.S. I am moving the remainder of this thread to the HDRI mailing list.

From: "Morsink, K." <[email protected]<mailto:[email protected]>>
Date: July 27, 2017 8:22:59 AM PDT

Hi Greg,

Thank you for your quick response!

The formula you provided works for me with a single LDR image and thus a single exposure time, but I'm stuck in how I should insert the multiple exposure times (of the 7 LDR images to form the HDR image) in exposure_secs and get the correct EXPOSURE, could you maybe explain this further?

Making use of the equal-energy illuminant E, is it assumed that the white point remains constant (1/3, 1/3)? I'm asking this since I'm taking outdoor photographs, where the CCT (and thereby the white point) changes constantly, assuming a constant white point would therefore affect the accuracy of the conversion to luminance values.

Kind regards,
Kars

-----Original Message-----
From: Greg Ward [mailto:[email protected]]
Sent: woensdag 26 juli 2017 19:15

Hi Kars,

I am cross-posting my response to the HDRI mailing list per Chris' suggestion.

Specifying the "-o" option of pvalue takes into account the exposure setting, which is determined from the ExIF data if you created the image using Photosphere or hdrgen. The formula below has an empirically derived constant that may not exactly fit your camera, so it is best to add your own calibrating scale factor:

sample_to_nits = 87 * (f-stop)^2 / (ISO * exposure_secs)

The sample_to_nits is converted to a Radiance picture exposure using:

EXPOSURE = 179 / sample_to_nits

where 179 lumens/watt is the agreed-upon efficacy of the equal-energy illuminant E over the visible spectrum.

Unfortunately, pvalue is not very smart about reporting brightness using the "-b" option. It uses a formula based on the standard Radiance color space, which differs from the CCIR-709 color space produced by Photosphere in both the green primary and the white point. It only makes a small difference, but if you are worrying about such things, you had best use the following to report luminance from your image:

ra_xyze image.hdr | pvalue -o -b [other options]

This also takes care of the 179 factor, reporting results in candelas/meter^2.

Cheers,
-Greg

From: "Morsink, K." <[email protected]<mailto:[email protected]>>
Date: July 26, 2017 7:34:45 AM PDT

Hello everybody,

I'm Kars and I'm new to this Radiance forum.

In the manual of Pvalue (https://www.radiance-online.org/learning/documentation/manual-pages/pdfs/pvalue.pdf) it is stated that inputting a file in XYZE format will give you the luminance values of the image (corresponding to the Y channel). I've inserted some .hdr images in Pvalue with the following Primaries (taken from the EXIF data): PRIMARIES= 0.6400 0.3300 0.3000 0.6000 0.1500 0.0600 0.3127 0.3290, where the first two numbers correspond to the R primary (x,y), the third and fourth to the G primary (x,y), the fifth and sixth to the B primary (x,y), and the seventh and eighth to the white point (x,y). The EXIF data also shows an exposure value of the .hdr image.

I've got two questions regarding this conversion.

To my knowledge the white point coordinates can be used to calculate the CCT, as described by Inanici in Evaluation of High Dynamic Range Photography as a Luminance Data Acquisition System. The white point coordinates, as found in the EXIF data, correspond to the CIE standard illuminant D65. Does this mean Pvalue assumes a constant CCT for all the .hdr images when converting to luminance? Or is Pvalue making other assumptions / calculations?

My second question refers to the exposure value of the .hdr image, as shown in the EXIF data of the image. How is this value determined? Since a HDR image consists of multiple images (in my case 7) with different exposure values. What is this exposure value of the .hdr image based on and is this used by Pvalue somehow?

I hope I've made myself clear.

Kind regards,

Kars Morsink

_______________________________________________
HDRI mailing list
[email protected]<mailto:[email protected]>