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