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ACR not applying linear segment w/ProPhoto

Dec 2, 2012 5:56 AM

I was using Imatest to check the DR of my Nikon D800e with the Stouffer stepwedge. I was using ACR 7.2 with the Adobe standard profile and PV 2010 with a linear tone curve (sliders set to zero, point curve = linear). When rendering into ProPhotoRGB, I noticed that there was no linear segment at low luminances and the expected linear segment was present with sRGB.

 

The Imatest plot is shown along with the calculated response using the densities of the step wedge and the ProPhoto equations. The linear segment starts at target densities of -2.5. This causes ProPhoto to have a slightly greater total dynamic range than sRGB. This is no big deal, but my investigation was to investigate an assertion that ProPhotoRGB has a greater dynamic range than sRGB. It is true that saturation clipping occurs earlier with the narrower space, but luminance clipping should and DR should be about the same.

 

http://bjanes.smugmug.com/Photography/Imatest/D800e-DR/i-trcSCXv/0/M/DensityResponses-M.png

 

http://bjanes.smugmug.com/Photography/Imatest/D800e-DR/i-5ZnzQSt/0/XL/DR_CompositeB_Flat-XL.png

 
Replies
  • Currently Being Moderated
    Dec 3, 2012 8:00 AM   in reply to Bill_Janes

    ACR/LR does use a linear segment for the initial part (dark shadows) part of the encoding curve for ProPhoto.  The source code is shown in the DNG SDK, file dng_color_space.cpp, if you're interested.  Briefly, the transition point between the linear and non-linear part is (assuming a [0,1] coordinate space):

     

    x = 8.2118790552e-4

    y = 0.019310851

     
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    Dec 3, 2012 9:40 AM   in reply to MadManChan2000

    Emm .... I must admit that I don't entirely understand the code (studied it before, when playing with those profiles, but I'm a delphi programmer), but to me, it looks like the curve actually has two linear segments, both of length zero, one at point (0,0) and another at (8.2118e-4,0.019). However, I see that a kind of spline is used at the bottom (dng_function_GammaEncode_1_8) , unlike for instance sRGB curve which is coded by the book, that's why I think it's like that. And also, there was a difference between my code and ACR output in shadows ...

     
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    Dec 4, 2012 4:44 AM   in reply to MadManChan2000

    Interesting.

     

    I'd never looked at this before, but the ProPhoto inflection point (at least if you take ROMM = ProPhoto) is specified at 0.001953. The above code seems to inflect a lot earlier.....I'm missing something here.....

     

     

     
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    Dec 4, 2012 5:17 AM   in reply to Bill_Janes

    If I understand correctly, function used for this in DNG SDK is dng_function_GammaEncode_1_8. Part of the curve below y = 0.019 is cubical spline, which starts with slope 32 and ends with slope 13.06. Above that, it is power function with exponent 1/1.8. So, there is no linear part at the bottom. Note that x and y axis on my diagram have different scale

     

    Anyway, I don't see the connection of the (non-standard) shape of this curve with dynamic range. It's just a function used to convert linear values to nonlinear, and hopefully, something close to inverse of this function will be used at the end of display chain to transform nonlinear values back to (linear) intensity of colors, displayed on the output device

     

    Edit: added standard photo pro curve (green) - I hope I used the correct formula in my old code libraries ...

     

    Gamma18b.gif

     
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    Dec 4, 2012 5:08 AM   in reply to sandy_mc

    Taking a closer look at the code, I don't think that the ProPhoto implementation in the DNG SDK has a linear segment at the bottom at all - it's actually a spline curve that's designed to smoothly go from a linear slope of 32 at 0 to the same slope as a gamma 1.8 curve at the inflection point. But I haven't actually printed out a curve. However, that's consistent with what Bill is seeing.

     

    Sandy

     
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    Dec 4, 2012 5:11 AM   in reply to sandy_mc

    Looks like Vit and I came to the same conclusion simultaneously.....but Vit's got a curve(!)

     

    Sandy

     
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    Dec 4, 2012 7:10 AM   in reply to Bill_Janes

    I understand that. Yes, some sites are providing some DR measurements for cameras (or software for it), but my personal opinion is that value of those measurements is about the same as length of linear part of the photo pro gamma encoding curve in ACR implementation    for instance, on DxO site, measured DR of some compact cameras is almost the same as DR of DSLR-s, which is a nonsense

     

    Useful measurement should be performed on the raw image directly from the sensor and not on the rendered image. For some sensors, manufacturers are providing this information in a form of signal to noise ratio, but unfortunately, not for DSLR-s ...

     
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    Dec 4, 2012 6:28 PM   in reply to Vit Novak

    Do the charts and tables on this page contribute useful data?

     

    http://www.clarkvision.com/articles/digital.sensor.performance.summary /index.html

     
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    Dec 5, 2012 6:30 AM   in reply to Vit Novak

    > Useful measurement should be performed on the raw image directly from the sensor and not on the rendered image. For some sensors, manufacturers are providing this information in a form of signal to noise ratio, but unfortunately, not for DSLR-s ...

     

    well, the data that those sensor manufacturers provide for their sensors do not actually exactly reflect the data in the raw files written by firwmare of 3rd party camera manufactureres that are using those sensors, more so that those sensors are probably CCDs with much more circuitry added by actual camera manufacturer like ADC off sensor... and DxO is testing the data in raw files (another story that they are shooting targets that you never encounter in a real life and they use "engineering" definitions of DR)... so what is the beef if a modern small sensor has low readout noise and some big sensor does not ? move towards midtones and picture will better for a big guy... there is no replacement for displacement in the end (unless you use totally obsolete technology).

     
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    Dec 5, 2012 9:13 AM   in reply to Bill_Janes

    > The engineering definition of DR is an industry standard and is reported on by Kodak and Dalsa (and their successors) in their data sheets.

     

    true - but you do not deal with sensors directlty and not even with sensors testing equipment that sensor manufacturers have... unlike Kodak/TrueSense and Dalsa you only deal with the data written by camera's firmware and after the signal was through camera (not Kodak/TrueSenses' or Dalsa's) manufacturer's circuitry...

     
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    Dec 5, 2012 9:16 AM   in reply to deejjjaaaa

    but we digress... so what about the curves ? does what Eric say (above) contracit to what you see (in the code) ?

     
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    Dec 5, 2012 9:53 AM   in reply to deejjjaaaa

    Yes, I know that in most cases data from sensor are a bit polished before writing to raw file. Also, I know that DR is usually measured with a reference to certain level of noise.

     

    For me, DR is just a ratio between the brightest signal that can be recorded with the sensor (just before saturation level) and the darkest that can be visible in the noise. How much is enough ? Well, something in the range of a negative film would be nice ...

     
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    Dec 5, 2012 3:53 PM   in reply to Vit Novak

    Sensors aren’t going to have reciprocity failure like film so won’t be able to avoid oversaturation in the non-linear way film does.

     
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