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

[Christopher Rush]

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:k.morsink at student.tue.nl]
Sent: Wednesday, July 26, 2017 10:35 AM
To: radiance-general at radiance-online.org
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
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