Why is there BPC but no WPC?

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    Topic: Why is there BPC but no WPC? Read 933 Times
  • Sean Anderson
    Sean Anderson
    Participant
    Posts: 9
    Printing Colour Management
    on: September 11, 2022 at 4:29 am

    Why is there a Black Point Compensation  (BPC) option in the printer color management settings but no White Point Compensation (WPC)?  BPC prevents the darkest shadows from blocking up on matte papers, but what prevents the brightest  highlights from clipping?

    Mark D Segal
    Mark D Segal
    Silver Member
    Posts: 655
    Re: Why is there BPC but no WPC?
    Reply #1 on: September 11, 2022 at 9:48 am

    You need to prevent the brightest highlights from clipping at the editing stage of your photographs, by paying attention to both the histogram and the photo under softproof. When you use Relative Colorimetric Rendering Intent, the white point of the source is mapped to the White point of the destination. At the dark end of the tone scale, BPC is optional depending upon whether, especially for matte papers, what style of black shading appearance you want for the photograph: with BPC active and Relative Colorimetric Rendering Intent, there is a remapping of tone values from the black point of the source to the black point of the destination, with relative remapping of lighter shades of Black (gray actually), thus preserving shadow detail. Without BPC active there would be a bunching of all deep gray tones falling at and below the Black point of the paper, rendering them indistinguishable. This can be desirable for some artistic effects, but not for preserving deep gray shadow detail.

    Mark D Segal Author: "Scanning Workflows with SilverFast 8, SilverFast HDR, Adobe Photoshop Lightroom and Adobe Photoshop". Please check the PhotoPXL Store for availability.

    Andrew Rodney
    Andrew Rodney
    Participant
    Posts: 294
    Re: Why is there BPC but no WPC?
    Reply #2 on: September 11, 2022 at 11:40 am

    Let’s begin with what BPC is, why, and where it exists. BPC is part of some CMMs (color conversion engines) like Adobe ACE to fix an issue with some profiles that do not map source black to destination black correctly. IOW, a profile that provides this mapping correctly will not show any effect with or without BPC; there is nothing to compensate for. Toggle BPC on and off, you’ll see no difference.

    So you should be seeing why there is no WPC but to go further: Source to destination white is an attribute of the rendering intents. With an Absolute Colorimetric intent, the mapping is Absolute unlike all others, and is used primarily for proofing (make my Epson using Luster paper look like XXX commercial proof). All other RIs will handle source to destination white as they are designed to do with compensation! The Absolute Colorimetric rendering intent reproduces the exact color that existed in the source—absolutely. If the source was light color on the dingy yellow-white of newsprint, the resulting color on your brilliant coated ink jet paper will be dingy yellow. This intent is designed for making one device simulate the appearance of another device for use in proofing.

    Author “Color Management for Photographers" & "Photoshop CC Color Management" (pluralsight.com)

    Sean Anderson
    Sean Anderson
    Participant
    Posts: 9
    Re: Why is there BPC but no WPC?
    Reply #3 on: September 11, 2022 at 5:41 pm

    What are IOW profiles? I heard of ICC and ICM profiles, but not IOW.

    John Hollenberg
    John Hollenberg
    Participant
    Posts: 56
    Re: Why is there BPC but no WPC?
    Reply #4 on: September 11, 2022 at 6:03 pm

    Click on the IOW link and find out.

    Sean Anderson
    Sean Anderson
    Participant
    Posts: 9
    Re: Why is there BPC but no WPC?
    Reply #5 on: September 11, 2022 at 7:09 pm

     

    So would I be correct if I said,

    Printer/paper profiles are not reliable at mapping source blacks to destination blacks (I’m curious why not).  ICC profiles need a little help. This is where Adobe’s color conversion engine’s BPC program comes to the rescue. Because the Perceptual Rendering Intent already comes with BPC built in, there is no need to click the BPC check box when using Perceptual. However, because the Relative Rendering Intent does not come with BPC already built in, you do need to click the BPC check box when using Relative.

    Contrary to blacks, printer/paper profiles are good at mapping source whites to destination whites, so there is no need for a White Point Compensation (WPC) check box when using either the Relative or Perceptual rendering intent.

     

     

    Andrew Rodney
    Andrew Rodney
    Participant
    Posts: 294
    Re: Why is there BPC but no WPC?
    Reply #6 on: September 11, 2022 at 7:13 pm

    No, it wouldn’t be correct.

    http://digitaldog.net/files/Black_Point_Compensation.pdf

    Author “Color Management for Photographers" & "Photoshop CC Color Management" (pluralsight.com)

    Mark D Segal
    Mark D Segal
    Silver Member
    Posts: 655
    Re: Why is there BPC but no WPC?
    Reply #7 on: September 11, 2022 at 8:21 pm

    From “Real World Color Management” 2nd edition, by Fraser, Murphy & Bunting:

    Definition of BPC (page 534): “A setting in Adobe Photoshop that makes sure that the black point in the source profile is mapped to the black point of the destination profile, and the rest of the tonal range is scaled accordingly.”

    BPC in operation (Page 355): “This proprietary Adobe feature plugs a hole in the ICC profile spec. It ensures that black in the source is always mapped to black in the destination, so that the entire dynamic range of the input is mapped to the entire dynamic range of the output…”

    Mark D Segal Author: "Scanning Workflows with SilverFast 8, SilverFast HDR, Adobe Photoshop Lightroom and Adobe Photoshop". Please check the PhotoPXL Store for availability.

    Sean Anderson
    Sean Anderson
    Participant
    Posts: 9
    Re: Why is there BPC but no WPC?
    Reply #8 on: September 11, 2022 at 11:54 pm

    Is BPC performed differently in relative mode than in perceptual mode?

    Mark D Segal
    Mark D Segal
    Silver Member
    Posts: 655
    Re: Why is there BPC but no WPC?
    Reply #9 on: September 12, 2022 at 7:47 am

    From “Color Management and Quality Output” by Tom Ashe, page 118: ” Black Point Compensation is checked to help you maintain shadow detail when using Relative Colorimetric rendering. If you check or not check the BPC selection with Perceptual rendering, there is no change, since Perceptual Rendering will always map the darkest black of the source space to the darkest black of the destination space, which is what Relative Colorimetric will do when applying Black Point Compensation. ”

    A bit of perspective on this: Rendering intents are designed mainly for controlling gamut compression including, importantly to the distinction between them, how to deal with out of gamut (OOG) colours. Relative rendering clips all OOG colours to their nearest value on the gamut boundary leaving in-gamut colours pretty much unchanged, while Perceptual rendering remaps OOG colours to the gamut boundary but then shifts in-gamut colours to maintain the overall relationships between colours.

    Mark D Segal Author: "Scanning Workflows with SilverFast 8, SilverFast HDR, Adobe Photoshop Lightroom and Adobe Photoshop". Please check the PhotoPXL Store for availability.

    Mark D Segal
    Mark D Segal
    Silver Member
    Posts: 655
    Re: Why is there BPC but no WPC?
    Reply #10 on: September 12, 2022 at 9:06 am

    I can’t copy/paste your links, nor are they hyperlinked, so I can’t conveniently access them without error-free typing of the whole strings. If you would like me to look at them please put them in a more usable format.

    Black and grey are colours, so whatever I say about colours applies to them as well.

    The explanations I provided to you in my above posts should tell you in principle exactly what to expect from any combination of options involving those two R.I. choices and the BPC option.

    Mark D Segal Author: "Scanning Workflows with SilverFast 8, SilverFast HDR, Adobe Photoshop Lightroom and Adobe Photoshop". Please check the PhotoPXL Store for availability.

    Sean Anderson
    Sean Anderson
    Participant
    Posts: 9
    Re: Why is there BPC but no WPC?
    Reply #11 on: September 12, 2022 at 9:17 am

    I tried to link the photos, but it didn’t work. And somehow my post disappeared. I’ll take your answers and move on. Thank you for responding.

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