What exactly is black point compensation?

Discussion in 'Photoshop Tutorials' started by My View, Jul 22, 2006.

  1. My View

    My View Guest

    Yes I 've Googled this and still can't work out what it does when printing
    from Photoshop.

    A simple explanation would be great.

    regards

    PeterH
     
    My View, Jul 22, 2006
    #1
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  2. My View

    Mike Russell Guest

    It means that black in the source image's color space will be mapped to
    black in the destination color space.

    For example, because of paper reflectivity, a printed black is typically
    brighter than black
    on a monitor. Instead of converting printed black to something like
    RGB(10,10,10) value, black will be converted to the black of the destination
    color space, RGB(0,0,0).
     
    Mike Russell, Jul 22, 2006
    #2
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  3. My View

    My View Guest

    Thanks Russell
     
    My View, Jul 22, 2006
    #3
  4. My View

    Dave Guest



    Pity you answered this stupid fool, Mike.
    Maybe we should have a kind of houserule here
    like only reacting to calls placed by people with names.
    Sorry for this explosion, but you carry this kind of weight here,
    that it irritates when a *nobody* does not even have the decency
    of saying thanks to you by your name but calling you by the surname.

    Dave




    Here are the links to those much-talked-about
    Crime Reporting Websites :


    http://www.crimexposouthafrica.co.za

    http://www.southafricaiscrap.blogspot.com

    http://deathofjohannesburg.blogspot.com

    http://www.wea.co.za/
     
    Dave, Jul 22, 2006
    #4
  5. My View

    Mike Russell Guest

    No offense taken for thanks of any kind. At least he spelled it with two
    l's! :)
     
    Mike Russell, Jul 22, 2006
    #5
  6. My View

    My View Guest

    That time of the month Dave? I hope I spelt that correctly.

    Thanks anyway Dave for pointing that out. I'm sure your heart is in the
    right place.

    My apologies Mike for quickly replying with your surname - honest mistake.

    Don't you hate it when people are human and make honest mistakes.

    Mike, your reply was exactly what I wanted to know as I tried Googling it
    and all I got was a list of sites that explained how to print from PS with
    "Black Point Compensation" selected. They did not state what BPC was - now I
    know. I get all my prints done at professional labs and have never used the
    PS print option. Simple printing at home is done using the printer software.
    Hence my lack of knowledge on this particular subject.

    I've received many great replies from you and always take your replies as an
    excellent source to learn the finer points of PS.

    I once again appreciate you realising that the main reason people use
    newsgroups is because firstly they don't know or cannot find the exact
    answer to a simple or complex question and secondly because they are looking
    toward people like yourself to progressively improve their
    knowledge in the use of this great package.

    Mike once again keep up the great work - I always look for your name in
    replies to this newsgroup as a source of good and reliable knowledge on
    these subjects. Your website is on my favourites as well.

    I hope you will continue to reply to "stupid fools" like myself so one day
    we will all be as proficient in PS as Dave is.

    Dave, I will also be looking out for your replies in the future so I can
    make my own judgement as to whether or not you are also worth following like
    Mike.

    regards

    Peter
     
    My View, Jul 22, 2006
    #6
  7. That is correct.

    BPC is a proprietary color management addition to the ICC
    color-management. It can be (but not always is) useful when profiles
    that conform with v.2 ICC spec are being used since the blackpoint is
    not properly managed in them. The v.4 ICC spec does specify device the
    device blackpoint unambiguously so the effect of the BPC could be
    (usually is) different when v.4 profiles are in use.

    Also the v.2 ICC profiles can affect to the conversion of the device
    blackpoint but this goes by approximation (where the v.4 profiles
    provides the accurate blackpoint tag). Some/many profilers (that output
    v.2 profiles) do such approximation, some do not. Therefore it is a very
    good idea to check behavior of a new printing path, in respect to the
    BPC, using a suitable test chart.

    The problem with v.4 ICC profiling is that most of the
    spectrophotometers are not good enough to measure the blackpoint
    accurately, only the most expensive laboratory grade spectros have good
    enough dynamic range.

    Timo Autiokari
    http://www.aim-dtp.net
     
    Timo Autiokari, Jul 23, 2006
    #7
  8. Hi Mike,

    You mean this is done after adjusting the levels of the image, right?

    Hi OP,

    BlacK has always been a real problem as it is not a synthesised ray of
    any color. Rather, we can call it a "blackout" when RGB space is
    involved, ie, the most beautiful black on screen is the black that
    emits no light and only screen's black background shall represent it
    (upon proper monitor calibration).

    Black point compensation issue can only be of real essence when
    printing (or CMYK space) is involved. In Photoshop there is an
    excellent control over black generation which can be adjusted according
    to the required purpose.

    Mohamed Al-Dabbagh
    Senior Graphic Designer
     
    Mohamed Al-Dabbagh, Jul 23, 2006
    #8
  9. My View

    Dave Guest

    It is obvious Peter, that I have been to much in a hurry in replying,
    and not the first time:-(

    2 reasons for it this time, which may be mitigating (w?) -

    I simply hate it when somebody get called by his surname, and Mike's
    answer to you made it very clear that Russel is his surname. It
    recalls the time in the army where officers called a troopy by his
    surname, and getting sent to detention barracks if they do attack
    an officer because of it. When talking about people like mugabe
    (zimbabwe) you do it by his surname because of the contempt he
    deserves.

    You did not even post under a name. There was somebody posting on
    this group with a question regarding something and he needed a file.
    I sent it to him, finding he used a false address, which he should
    have explained, if he was looking for a file. His reaction when I
    pointed it out to him, was sarcastic. Never mind, I then posted this
    file to my homepage, and gave him the URL, and he downloaded it,
    without even coming back with a thank you. Whether he posted
    again, is unknown.

    He posted without a name as well.

    This morning I had two calls, one from a woman (not lady but simply
    woman) and the other from a man, in reaction to an advert in a
    newspaper for property to let. Both telephone numbers showed
    'withheld' on my cell phone, and after telling them the property was
    already taken, they simply dropped the line.
    Why? Because they phoned nameless - without even a number to trace.
    I wish my advice could have been (half as much) worth following like
    Mike's, but it's not. Mike have already forgotten what I still should
    learn.

    Sorry for calling you a stupid fool. If you was one, you would not
    have replied. Don't write without using a name. Even if calling
    yourself *dogman*, use a name.

    Keep well,

    Dave

    Here are the links to those much-talked-about
    Crime Reporting Websites :


    http://www.crimexposouthafrica.co.za

    http://www.southafricaiscrap.blogspot.com

    http://deathofjohannesburg.blogspot.com

    http://www.wea.co.za/
     
    Dave, Jul 23, 2006
    #9
  10. My View

    Dave Guest

    rote:
    It is obvious Peter, that I have been to much in a hurry in replying,
    and not the first time:-(

    2 reasons for it this time, which may be mitigating (w?) -

    I simply hate it when somebody get called by his surname, and Mike's
    answer to you made it very clear that Russell is his surname. It
    recalls the time in the army where officers called a troopy by his
    surname, and getting sent to detention barracks if they do attack
    an officer because of it. When talking about people like mugabe
    (zimbabwe) you do it by his surname because of the contempt he
    deserves.

    You did not even post under a name. There was somebody posting on
    this group with a question regarding something and he needed a file.
    I sent it to him, finding he used a false address, which he should
    have explained, if he was looking for a file. His reaction when I
    pointed it out to him, was sarcastic. Never mind, I then posted this
    file to my homepage, and gave him the URL, and he downloaded it,
    without even coming back with a thank you. Whether he posted
    again, is unknown.

    He posted without a name as well.

    This morning I had two calls, one from a woman (not lady but simply
    woman) and the other from a man, in reaction to an advert in a
    newspaper for property to let. Both telephone numbers showed
    'withheld' on my cell phone, and after telling them the property was
    already taken, they simply dropped the line.
    Why? Because they phoned nameless - without even a number to trace.
    I wish my advice could have been (half as much) worth following like
    Mike's, but it's not. Mike have already forgotten what I still should
    learn.

    Sorry for calling you a stupid fool. If you was one, you would not
    have replied. Don't write without using a name. Even if calling
    yourself *dogman*, use a name.

    Keep well,

    Dave

    Here are the links to those much-talked-about
    Crime Reporting Websites :


    http://www.crimexposouthafrica.co.za

    http://www.southafricaiscrap.blogspot.com

    http://deathofjohannesburg.blogspot.com

    http://www.wea.co.za/
     
    Dave, Jul 23, 2006
    #10
  11. My View

    elementary Guest

    When I soft proof with Epson's 2200 profiles in PS CS, I can't seem to
    notice any difference with BPC on and off. Nor can I see any difference
    in the prints. What does that say about these profiles, or my eyesight?

    Also, when converting from another color space to srgb in PS CS, BPC is
    an option. Similarly, I can't see any difference in Preview with it
    on/off.

    For his profiles, I think Bill Atkinson turns BPC off for Perceptual
    rendering, and on for Relative Colorimetric rendering. So BPC does make
    a difference to him. I could be wrong here.

    Should BPC make any observable difference to the mere mortals?
     
    elementary, Jul 23, 2006
    #11
  12. My View

    KatWoman Guest

    what a whack-job you are Dave!
    never figured you'd get so fired up over a name thing but the explanation
    was really interesting, not the usual NG chat..
     
    KatWoman, Jul 23, 2006
    #12
  13. My View

    Dave Guest

    lol...!!! You did not miss the name I suggested, Katwoman.... surely
    you didn't miss it :-O

    Dave
     
    Dave, Jul 23, 2006
    #13
  14. Sometimes it makes a clearly observable difference sometimes just a
    barely observable difference and sometimes it does not affect at all.

    E.g. when converting between normal/common RGB working-spaces and
    between RGB and Lab the BPC has no affect since in these profiles the
    blackpoint tag is either missing or is set to 0.

    When converting from/to a device ICC profile the BPC can affect (but not
    always affects, depends on the information in the profile).

    You can easily inspect if the BPC has an effect:

    1)make two dupes of an image
    2)convert dupe1 with BPC and dupe 2 without BPC
    3)drag+shift dupe2 as a layer on top of dupe 1
    4)put dupe 2 into difference layer mode
    5)merge the layers, apperance is rather dark after this
    6)do some scaling up using levels
    7)Inspect the histogram. If the BPC affected the histogram shows it,
    else the histogram is just a vertical line at level 0.

    Because the output profiles are results of a huge amount of
    interpolation + approximations it can happen with some profiles that the
    BPC affects in such way that device RGB in the dark end will not be used
    at all, e.g. once I found a profile of an online photoprinting service
    that did not give any device RGB data in the range from level 0 to level
    9. Since then I have been inspecting the behaviour of new output
    profiles rather closely and finally I make a test run using the various
    combinations (mainly intent and BPC).

    I normally use relative intent and never use the perceptual intent.
    Perceptual intent is just a perceptual tweak over the relative intent
    but it is perceptual for the person who created the profile or profiler.
    In my opinion that person has no perceptual say on my printing
    experience (so, when required, I apply my own intent using an Action
    that also convert to the printing space).

    Timo Autiokari
     
    Timo Autiokari, Jul 24, 2006
    #14
  15. My View

    KatWoman Guest

    hmm now that suggestion of nick is that implying you don't care for mine?
    If it's any consolation I fill out all net inquiries with
    first name: Kat
    last name: Woman

    so to insult me according to your rule I would be "woman"
    "hey woman get me some pie!"
    or if you love me I am Kat?
    It is not safe to go on NG with my real name due to perverts and trolls
    BTW DAVE you give no last name
    so we can't call you by the sur
    anyhow NG is not the most polite place as you may have noticed..at least the
    trolls are quieted and not too much spam..
     
    KatWoman, Jul 24, 2006
    #15
  16. My View

    BobLake Guest

    Geez! The guy was excited to find the answer to his question and he
    gets slammed for picking the wrong name from a guy who's name is
    similar to two first names. Wow.
     
    BobLake, Jul 25, 2006
    #16
  17. My View

    Sari Guest

    of course I care for yours, Kat. That indeed is why it came to mind.
    Like you said, I gave no surname, maybe that was clever -
    and you said the trolls are quited, did you not, Kat...
     
    Sari, Jul 25, 2006
    #17
  18. My View

    KatWoman Guest

    I sure hope I am not the reason for the Perkoff Pop OFF of late
    I can't see his posts (he is blocked and I catch his new nicks easily
    now........ ) see he duped a few noobs into sparring.
     
    KatWoman, Jul 27, 2006
    #18
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