[Radiance-general] How does Pvalue determine CCT in HDR images and how is the EV of a HDR image determined?

[Greg Ward]

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." <k.morsink at student.tue.nl>
> 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
> 
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