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Thread: help me understand color spaces (part 1?)

  1. #1

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    help me understand color spaces (part 1?)

    My ultimate goal is to thoroughly understand the process of scanning B&W film, processing it, and making inkjet prints. I've been reading a lot about color spaces, and much of what I can find is super-technical, or just compares the gamuts of different spaces, neither of which has been particularly helpful to my understanding. The following are my impressions and questions so far - please tell me where I might be thinking correctly and where I am in error.

    * A digital photograph or a scan is a file containing a bunch of numerical information about the color (or tones) in the image. The scanner (or scanning software?) or camera attaches a color space to that numerical information?

    * The color space allows software and a monitor to interpret the numerical information in such a way that the image appears "correctly" on the monitor?

    * I have the impression that some software could take an image with one color space attached and output the image with a different color space?

    * Processing software might change the color space to some other color space (the "working space") for the purposes of editing? I use Lightroom, and have the impression that might be the case with it, that it works in ProPhoto RGB?

    I think that is the end of part 1...

  2. #2

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    Re: help me understand color spaces (part 1?)

    The answer does not fit in a forum post. But... a starting point, hopefully useful.
    - Real colors live in perceptual color space (PCS)
    - Numerical colors live in their respective space (sRGB, AdobeRGB, etc...) characterized by (a) their gamut, i.e. how much of the PCS they cover; (b) how the numbers are to be mapped interpreted as "real" colors" in PCS. This is the function of the profile, kind of a numerical recipe without which the numbers are meaningless.

    The scanner (or scanning software?) or camera attaches a color space to that numerical information?
    correct; see just above.

    I use Lightroom, and have the impression that might be the case with it, that it works in ProPhoto RGB
    That is internal to Lightroom and you can ignore it.

    Personal opinion for which I may be flamed. Beyond sRGB, advanced color spaces differ mostly in coverage of saturated greens. Such extended coverage is not purely beneficial (satisfaction of using ProPhotoRGB?). Covering more space in PCS, the elementary increments in colors are farther apart, increasing the risk of visible banding. Fully saturated colors are easy to create in illustration software, but hardly occur in nature. My monitor, that probably does not even cover sRGB, nevertheless has a wide enough gamut to display greens that hurt the eyes when they are meant to be foliage. And some end uses (e.g. photo bok creation) ignore color spaces other than sRG.

  3. #3

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    Re: help me understand color spaces (part 1?)

    Quote Originally Posted by h2oman View Post
    My ultimate goal is to thoroughly understand the process of scanning B&W film, processing it, and making inkjet prints.
    In my humble opinion, correct me please:

    If we're scanning B&W film, choice of bit-depth is more important: 16-bit is preferable to 8-bit, to avoid banding as adjustments are made.

    When scanning B&W (or converting to B&W from a color file) consider making TIF files in 16-bit Adobe Gray Gamma 2.2 color space. If you can't specify the color space, at least make a 16-bit B&W TIF, which has lossless 16-bit depth.

    When saving images for display on the internet, you can always convert to a compressed (and lossy) format like 8-bit JPG.

    If you already have 8-bit B&W scans, you can convert them to 16-bit before making further adjustments, as described in this short article:How (and why) to Convert Images from 8 to 16 bit in Photoshop. I don't know how to do this in Lightroom. I recommend Photoshop and 30 Days of Photoshop, a free series from PHLEARN is a terrific place to start.

    Speaking of 16-bit, you might want to investigate the Quadtone RIP and Print-Tool (and Piezography which uses them) because (as far as I know) unlike the OEM drivers, they can send 16-bit data to the printer, while typical OEM printer software is 8-bit only.

  4. #4

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    Re: help me understand color spaces (part 1?)

    Quote Originally Posted by h2oman View Post
    my ultimate goal is to thoroughly understand the process of scanning b&w film, processing it, and making inkjet prints.

    ditto!

  5. #5
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    Re: help me understand color spaces (part 1?)

    Quote Originally Posted by Ken Lee View Post
    In my humble opinion, correct me please:

    If we're scanning B&W film, choice of bit-depth is more important: 16-bit is preferable to 8-bit, to avoid banding as adjustments are made.

    When scanning B&W (or converting to B&W from a color file) consider making TIF files in 16-bit Adobe Gray Gamma 2.2 color space. If you can't specify the color space, at least make a 16-bit B&W TIF, which has lossless 16-bit depth.

    When saving images for display on the internet, you can always convert to a compressed (and lossy) format like 8-bit JPG.

    If you already have 8-bit B&W scans, you can convert them to 16-bit before making further adjustments, as described in this short article:How (and why) to Convert Images from 8 to 16 bit in Photoshop. I don't know how to do this in Lightroom. I recommend Photoshop and 30 Days of Photoshop, a free series from PHLEARN is a terrific place to start.

    Speaking of 16-bit, you might want to investigate the Quadtone RIP and Print-Tool (and Piezography which uses them) because (as far as I know) unlike the OEM drivers, they can send 16-bit data to the printer, while typical OEM printer software is 8-bit only.
    +1. Why worry about color gamuts and spaces with BW film? Just scan 16 bit grayscale setting. That's what I do.

  6. #6

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    Re: help me understand color spaces (part 1?)

    Thank you, Ken. I think it would be helpful, for me at least, if I am more specific, instead of discussing this in terms of generalities. The bottom line is that I will have a set of Piezography inks arriving next week, and I'm trying to figure out my workflow, based on the minimal tools and understanding that I currently have. As I've indicated elsewhere, it is my preference to work with what I have unless I find myself dissatisfied, at which point I'll get more stuff. Let me start with my current workflow:

    1. Scanning I've inherited an Imacon Flextight scanner, with a computer that has the Flextight software. The first choice I have when scanning (other than negative and resolution) is whether I want the scan to be RGB, RGB 16 bit, Grayscale, or Grayscale 16 bit. I've been using Grayscale 16 bit for B&W negatives. When the scan is completed, I'm told that there is an "Embedded Profile Mismatch." The embedded profile is Flextight Input Gray, and the working profile is Gray Gamma 2.2. I have the choices of (a) use the embedded profile, (b) convert document colors to the working space, or (c) discard the embedded profile (don't color manage). So far I have been using the embedded profile, although I'm beginning to think I should have been converting to Gray Gamma 2.2

    2. Processing I save the file to a USB and take it to another computer, on which I have Lightroom and Picture Window Pro. I use Lightroom for everything but spotting/cleaning and sometimes some local adjustment, for which I use Picture Window Pro. I believe that both of those softwares leave my 16 bit file as 16 bit. I import the cleaned file to Lightroom to print.

    3. Printing At this point I select Print in Lightroom and tell Lightroom that I'm going to let the printer manage the color, and I use Epson "Advanced" Black and White. My understanding is that Lightroom resizes the image for me and does some print sharpening. (There is also rendering, which I know nothing about right now - I choose "perceptual" because someone once told me to do so!) My understanding is that to use QTR and Piezography, I will need to size the images appropriately myself, which is one of the few things I can figure out on my own.

    So let's suppose I have my tiff file from the scan, saved in whatever space you all tell me I should use. I'll import into Lightroom and process. When I export to use Picture Window Pro, or to print with QTR, I can choose 16 bit tiff. The color space choices I have are sRGb, Adobe RGB (1998), and ProPhoto RGB. From what I've been reading here and elsewhere, I guess I'll choose sRGB?

    I have some more general questions, but they aren't pressing. Thanks to all for your help.

  7. #7

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    Re: help me understand color spaces (part 1?)

    Quote Originally Posted by h2oman View Post
    So let's suppose I have my tiff file from the scan, saved in whatever space you all tell me I should use. I'll import into Lightroom and process. When I export to use Picture Window Pro, or to print with QTR, I can choose 16 bit tiff. The color space choices I have are sRGb, Adobe RGB (1998), and ProPhoto RGB. From what I've been reading here and elsewhere, I guess I'll choose sRGB?
    The Piezography documentation states that the Adobe RGB profile supports an embedded 16-bit black and white image. From what I can tell, the standard sRGB color space is 8-bit only, so should be avoided for this purpose. (It's fine for JPG files on the web, but not the best for printing.)

    If you use Piezography you can drive the process with Print-Tool. The settings are documented in the Piezography manual. I also have a short page here, which merely suggests settings for warm-toned images on Canson Platine paper.

    Piezography has several forums for Q&A. See https://forums.piezography.com/.

  8. #8

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    Re: help me understand color spaces (part 1?)

    Thanks, Ken. I had the impression, from the NEW Piezography User's Manual, that one would use Print-Tool with a Mac, but not a Windows machine.

    I think once I get set up I'll need to just start experimenting with a few things and see what happens!

  9. #9

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    Re: help me understand color spaces (part 1?)

    * A digital photograph or a scan is a file containing a bunch of numerical information about the color (or tones) in the image. The scanner (or scanning software?) or camera attaches a color space to that numerical information?
    Each device has a color space. It is usually called a device color space. An ICC profile of that device describes device's color space (boundaries) and, in addition, may provide set of rules for conversion from and\or into that color space. ICC Profiles do not have bit depth.
    Images produced by input devices such as scanners and cameras contain information about each pixel's color in device's (scanner or camera) native color space. Without association with the particular device profile, color information contained in the digital image file makes no sense.

    * The color space allows software and a monitor to interpret the numerical information in such a way that the image appears "correctly" on the monitor?
    It allows to define each pixel's true color. It also allows to predictably transfer the image colors from one color space to another . Displaying a picture from a file on one's monitor usually undergoes the following transformations (most of them happen under the hood by the imaging software and\or OS that is used to visualize the picture on the monitor's screen such as Lightroom PS, etc.):
    1. From the source color space via embedded into the image file ICC profile to Profile Connection Space (PCS)
    2. From PCS to Monitors color space via Monitor's ICC Profile
    PCS could be either LAB or XYZ color space

    Needless to say that in order to transform color information from color space of one device to another without distorting the colors , each color from the source color space should fit into the destination color space. When color don't fit into the destination space they have to be altered to fit. When color alteration is applied the colors get distorted or "lost".

    * I have the impression that some software could take an image with one color space attached and output the image with a different color space?
    Same as above

    * Processing software might change the color space to some other color space (the "working space") for the purposes of editing? I use Lightroom, and have the impression that might be the case with it, that it works in ProPhoto RGB?
    That's correct. There are some "universal" or also called "working space" profiles that do not describe any particular devices but simply define the primaries and boundaries instead. They are commonly used for image editing. sRGB, aRGB, ProPhotoRGB are all such profiles. In fact one can create their own color spaces in PS.
    Usually (depending on PS settings), when an image from a scanner is opened, PS check the embedded image profile and if does not match to the set in PS working color space it offer to either one on the following:
    1. Keep using the embedded profile. Ok for a visual inspection of a scan and may be some dust cleanup . In general not suitable for significant editing as the device color space might not be gray balanced or linear.
    2. Convert (transform the color values ) into working space (profile). Usually the best option for editing.
    3. Assign a profile. Usually a no-no option unless:
    * the file is tagged with an incorrect profile to begin with
    * a better profile is available for the same device
    * the file did not have an embedded profile at all(that is rare)
    Once the image is edited to our liking and ready to be printed it needs to undergo another round of conversion. Now from the working color space into printers color space. This type conversion happens with the usage of 2 ICC profiles the working space and printer's and is done either explicitly or implicitly by the editing software PS. Lightroom, etc, or printer driver. The same steps as before
    1. From working space via its ICC profile to Profile Connection Space (PCS)
    2. From PCS to Printers color space via printers ICC Profile
    PCS could be either LAB or XYZ color space

    For B&W printing it is not that important which RGB color space will be used sRGB, aRGB or ProPhotoRGB. They all should allow to produce comparable edits. The quality of printer's profile is absolutely important.
    Bit depth of image files 16bit per channel vs 8bit per channel is absolutely important as it will define the range of edits that the image data may sustain during editing.
    I prefer to scan B&W into 16 bit per channel color RGB files. This is how scanners output the data (3 channels RGB) so why throw 2/3 of the info mid-way? At the end of my edits I can add a Channel Mixer layer set to Greyscale to make the tones look absolutely neutral if needed.
    For B&W when printing I would stick to the Relative Colorimetric rendering intent as there are no out of gamut colors to deal with.
    Perceptual may help with Color printing in some cases ( a lot of out of gamut colors) but that is mostly a wishful thinking. I am yet to see real-life examples where that theory finds a solid proof.

  10. #10

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    Re: help me understand color spaces (part 1?)

    Here's a "non-technical" explanation-- I am not an expert, so if I get anything hideously wrong, feel free to correct me, but as I understand things:

    A color space, in the simplest terms, is the mapping of numbers to colors. Typical 8-bit RGB is three numbers from 0 to 255 representing the red, green and blue channels for a total combination of 16.7 million colors. The most common "sRGB" space defines those 16.7 million colors, primarily for use with a screen ("s").

    Cameras these days have 10 or 14 bit RAW data per channel per pixel, so they can record more finely gradated colors than sRGB can handle. Software can work with 16 bits per channel. Inkjet printers, however, might struggle to match sRGB.

    So we have color spaces, and ICC profiles, which define which colors a particular device can handle, and how.

    Ultimately, the goal is to be able to take a photo of a scene, process it in <application>, and print / display it on <device>, in a consistent, predictable manner that preserves as much color detail as possible.

    However-- for B&W, if you're scanning in 16 bit grayscale, that means the scanner is processing each pixel and turning it into one of 65,536 values, which seems far simpler than dealing with actual "color spaces"-- But again, a screen and a printer might display the same set of 65,536 shades of gray differently, so gamma allows you to compensate.

    I believe most of the major color spaces also include data for 16-bit grayscale as well (and black, for some printers, is far more difficult to print than color).

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