white-balancing of TIFF/JPEG

Discussion in 'Retouching and Post Processing' started by Iliah, Aug 30, 2005.

  1. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    It is a known issue that white balancing of tiff and jpeg files brings colour crossover. Each and every day I see photos where a neutral mid-tone object was used to establish white balance, and as a result all colours in shadows and highlights are screwed.

    White balancing is essentially a linear operation of multiplication, and as such it is better to use linear colour space for it. One needs to convert image to 16 bits, and then to an appropriate linear colour space (same matrix, but with gamma = 1); and after that to do the balancing. Same way as it is done in RAW.
     
  2. Hi Iliah

    Could you possibly expand on this. An example of the process in photoshop would be a great help.

    Thanks
     
  3. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Rory,

    Here is how a wall may look under FL light:
    [​IMG]
    Please bring the image to PS, as it is Adobe RGB, and choose eyedropper tool (press "i") to see the anchor point I will use to adjust the tint.

    Here are layered PS files to show the mid-tone correction:
    http://www.rawmagick.us/crossover/shadow_BalancedInRGB.psd
    http://www.rawmagick.us/crossover/shadow_BalancedInLab.psd

    As you see, the file balanced in RGB shows different tints in shadows and highlights, and only the midpoint it is balanced in is really neutral; while file balanced in Lab (which is linear space) maintains neutrality across the gradient.

    Lab is very useful for this purpose also because moves accomplished in "a" and "b" channels will never throw away luminosity information, hence nor shadows nor highlights will be sacrificed for the sake of colour balancing.
     
    Last edited by a moderator: Aug 30, 2005
  4. MontyDog

    MontyDog

    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
     
  5. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Paul,

    To eliminate colour cast one needs only one point if it is done in Lab.

    In non-linear colour space, indeed, separate addressing of highlights, midtones, and shadows is needed. However, it is usually a difficult prcedure, as there are not enough reference points.

    One of the most difficult tasks in colour correction is to determine if the image has a colour cast. Histograms are essentially useless and misleading for this, as histogram shows that details are lost, but does not show what are those details.

    If different tones of the image have different colour casts, like bluish shadows and yellowish highlights (this can be generalized to having 2 or more light sources in the scene, all with different spectrum), then several adjustment points are needed to eliminate the problem - even in Lab, where it is easier, especially using channel blending in overlay mode. But once more, setting perfect colour balance for scenes with complex or distinctive light can kill the atmosphere.
     
  6. MontyDog

    MontyDog

    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
     
  7. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Paul,

    Getting rid of the constant cast in Lab is a cleaner and easier solution - it might be not a subjective, but rather a mathematical observation :)

    One example of intentionally skewed white balance is when I take photos of monotone objects, like red flowers. If I expose for red, I end up with very few details in blue and green. So, I use complimentary (cyan) filter, and/or skew the white balance to warmest. Result is that I have a "tint". If reds are strong, raw processor might blow them up while counteracting the tint with white balance tool. In this case the rest of colour correction should be addressed in PS.
     
  8. Thanks very much Iliah. I really appreciate you're "advanced" education.
     
  9. Iliah, I downloaded your files and tried to duplicate your experiment. After one converts the original image to the lab profile, what does on do next? If I take the eyeballs off all the layers except for the lightness one, my image looks similar to your last one but the black and white in your example are darker and lighter respectively.

    Does this mean my original sampling point is off or is there something else here I should be doing?

    Plese advise.

    Thanks, Virginia
     
  10. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Virginia, you have to use curves (I used an adjustment layer in my example), and compensate "a" and "b" so that they are =zero in the sampling point.
     
  11. Now we're getting somewhere. I suspect only a very few people know what
    compensate "a" and "b" so that they are =zero in the sampling point. means. Would you/or anyone who knows please elucidate, without photoshop jargon. Please understand, I'm not being a smart aleck, but rather than an esoteric discussion among experts simplified wording and not skipping steps would benefit everyone.

    Thanks, this is a very valuable thread; I'd like to learn this technique.

    Rich
     
  12. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Rich,

    If you will open .psd samples, double-click the curves layer, and look at "a" and "b" channels adjustments ... :)

    Some useful shortcuts, if we are starting from scratch:
    To open Info palette, F8
    To convert to 16 bit Lab, Alt-imn Alt-iml
    To choose eyedropper tool, press "i"
    Set eyedropper to 3x3 or 5x5, depending on the amount of noise on the image
    To open curves, Ctrl-M
    To set an anchor point in Info palette, Shift-click on the on the region of the image that is supposed to be neutral
    To go to "a" channel, Ctrl-2
    To make corner point active, Ctrl-Tab
    To make "a" value equal zero (watch the info palette), arrows up/right on the lower left corner point; or left/down on the upper right point
    To go to "b" channel, Ctrl-3
    To cycle through corner points, Ctrl-Tab;
    To make "b" value equal zero, use arrows.

    I will be happy to provide further explanations if it will be necessary.
     
    Last edited by a moderator: Aug 31, 2005
  13. MontyDog

    MontyDog

    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
     
  14. Thanks, Iliah, this last bit helped although I got there via a slightly differently (I'm not as familiar with all the PS shortcut as you are). I didn't initially understand what you meant by corner points. For any one else who is confused (hope I'm saying this right, Iliah), they are the absolute black and white points in the a & b channel.

    Now I have an image that resembles the last one you posted.

    Thank you for your patience, Iliah. You've come to my rescue again!

    Virginia

     

  15. Thanks for your explanations. It works very good i will do some testing on some photo's with color casts thanks for this information Iliah.

    And maybe a off topic question i want to buy studio equipment and i see alot of people use a color chart checker (macgrabeth or kodak). Is it adviced to buy such a card to get colors right and white balance. And would it then be also better if i use such a card to use white balance correction from the first photo that is shot with the color chart in LAB mode?

    Thanks for your help allready.

    Greetings,
    VinceBeus
     
    Last edited by a moderator: Aug 31, 2005
  16. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Virginia,

    You are exactly right, though "a" and "b" are pure colour channels, and I do not know whether "black" and "white" are appropriate terms in English in regards to those. Anyway, here is a picture:

    [​IMG]

    I referred to those points highlighted in green as "corner points"
     
    Last edited by a moderator: Aug 31, 2005
  17. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Dear Vince,

    For digital studio I would say:

    http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=27715&is=REG
    better yet, purchase a sample of N8 from Gretag Macbeth (call them at 1-845-565-7660)

    http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=325038&is=REG

    http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=26662&is=REG

    Very promising, but lacking good software support for digital camera applications/profiling is
    http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=325039&is=REG
    Still, it works very well in studio environment, under constant lighting conditions
    The least useful is
    http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=325035&is=REG
    as it is designed to be a reference for eyeballing, and have not enough reference points for any serious calibration in digital environment.

    "90% method" is heavily criticized, BTW :)
     
    Last edited by a moderator: Aug 31, 2005
  18. Ok thanks for the information i will look it up. And i did not know if i was right about if it was the 90% method. But i use to use that because i started last 2 months focussing more on the White Balance then before. So i try to get the white balance right when i take the photo's. And it is great to learn new things like your explanation of white balance adjustment for Tiff files. I am going to read and look at all the links you gave me.

    Greeatings,
    VinceBeus
     
  19. I saw this one is older version and think i buy this one because it is cheap:

    http://shop.colourconfidence.com/product.php?xProd=1224&xSec=95

    This one is the macgrabeth color chart the image is just standard but the company have confirmed it earlier on they also sell a german version what is 100 procent identical they say:

    http://shop.colourconfidence.com/product.php?xProd=1239&xSec=30

    And the other one you mentioned the white nuetral black kind of car (whitebalance card) i saw at other store.

    I think these cards are ok to work with for the beginning and maybe later on if they are not fully satisfy i look for better solutions. But i think wb metering and setting all equipment good is allready good beginning for working with studiolights.

    Greetings,
    VinceBeus
     
  20. Iliah

    Iliah

    Jan 29, 2005
    nowhere
    Last edited by a moderator: Aug 31, 2005
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