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Author Topic: Generating a Kodachrome profile from an IT8 target  (Read 24772 times)
guyburns
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« on: May 09, 2011, 10:00:46 AM »
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I'm trying to generate a Kodachrome profile from a Kodak Kodachrome IT8 target on a Nikon Coolscan V ED. I've actually generated a successful profile, and now that I know how to do it, I want to go back to the start and make sure I have the optimum profile. So I'm scanning the target again – and here's where I've come unstuck.

I assume that the most accurate profile will be based on a scan that is itself the most accurate scan of the target. What I mean is: if you scan a target and it is way off, then the profile-generating software should, in theory, be able to generate an accurate profile – but it may have to make large corrections. My theory (correct me if I'm wrong) is that if you scan a target and the scan is very accurate, you will get a better profile because the corrections required will be smaller.

So, I'm going back to square one, and am trying to get the most accurate scan. That's where the problems lie.

The Coolscan allows a variety of profiles to be applied to the scan, or you can turn off the colour management. Since Nikon gives no information about this in their literature, I have done some tests. I placed the IT8 target in the scanner and scanned it with various settings. Then I opened the image in Photoshop, and checked the assigned profile (Edit > Assign Profile). The Assign Profile window has three options:

A. Don't Colour Manage This Document
B. Working RGB
C. Assign Profile

To ensure we are all on the same wavelength, my understanding is that if the image has an embedded profile, option C is highlighted with the name of the profile. If the image does not have an embedded profile, option B is highlighted, and the image is automatically assigned Photoshop's "Working RGB" profile (in my case sRGB 2.1). Option A is provided in case you want to turn off colour management. I assume that if you select option A, you are seeing the image on screen based solely on it's RGB numbers with no corrections applied.

I scanned my target several times with various options selected:

1. CM on/Apple RGB profile. This scan had an assigned profile which I'll call Apple RGB.

2. CM on/Adobe RGB profile. This scan had an assigned profile which I'll call Adobe RGB.

3. CM on/Nikon sRGB profile. This scan had an assigned profile which I'll call Nikon sRGB.

4. CM on/Scanner RGB profile. When this image was loaded into Photoshop, it arrived with a "Working Profile" of sRGB 2.1 (my system default). I assume this means the scanned image had no embedded profile and PS applied one (the default sRGB).

5. CM (colour management) off. As per 4 – the image arrived in PS with a "Working Profile" of sRGB 2.1 (my system default).

At the end of the scanning, I therefore had 5 images. Which one should I use as the basis for generating a Kodachrome profile? It gets rather complicated. These are the characteristics of the images.

• Scans 1, 2 & 3, with their profiles applied, look identical on screen.
• Scans 4 & 5 look different from each other, and different from 1, 2 & 3.
• Scans 3, 4 & 5 appear identical (compared to themselves) no matter which of the A, B & C options are applied in Photoshop under "Assign Profile". This must mean that the native PS display on my computer, and sRGB, and Nikon sRGB all result in the same colours.
• When I choose "Don't Colour Manage This Document" for all scans, none of the images (as scanned, without profiles) looked the same. All five differed from each other.

Five scans resulted in five different sets of RGB numbers. In the case of Scans 1, 2 & 3, it seems the scanner distorts the RGB values its sees so that when the profile is applied, the result looks the same. The RGB numbers are distorted even when no profile is applied at the scanner stage – Scans 4 & 5 arrived in PS without embedded profiles (PS applied sRGB), but for some reason, the scanner sent different RGB values.

Any suggestions as to which of the scans would be the most accurate? Scans 1, 2 & 3 are probably not in the running because the RGB values have been distorted; so it's down to 4 & 5. But 4 does not have as good a rendition of "Kodak yellow" as 1, 2 & 3, but it does have better definition on the woman's shoulder (her shoulder is quite clearly defined on the slide).

The scans can be downloaded as a zip file (1.6 MB) from within this folder: http://www.mediafire.com/?thogddgfozxi2 (Five Kodachrome IT8 Scans.zip)
« Last Edit: May 22, 2011, 06:21:21 AM by guyburns » Logged
Ethan_Hansen
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« Reply #1 on: May 09, 2011, 10:48:30 AM »
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Although I have a Nikon Coolscan, I admit it's been some years since I even took it out of the box. I'll need to go from memory here, but the process was not that involved.

Enabling the scanner's color management is not what you want. This converts the image using Nikon's default profile to the desired color space. Option #5, CM off, is what you want. This gives the raw (albeit gamma-corrected) RGB values. Photoshop needs to assume some color space for an image to display it properly. With nothing else to go on, PS assigns your working color space. The software you use to build your scanner profile should pay no attention to embedded image profiles. The raw numbers are the best place to start.

Reading your post, it also appears that you are using the Nikon software. If you have not done so already, download an evaluation copy of VVueScan. This program manages to get the absolute best performance out of film scanners. It also allows extracting the truly raw RGB values read from the scanner and using them to build a profile. Doing so often produces superior results than beginning with gamma-corrected values.
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dmerger
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« Reply #2 on: May 09, 2011, 03:07:50 PM »
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Here is an article from Hutch Color that may be helpful: http://www.hutchcolor.com/PDF/Scanning_Guide.pdf
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Dean Erger
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« Reply #3 on: May 09, 2011, 04:37:22 PM »
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... extracting the truly raw RGB values read from the scanner and using them to build a profile. Doing so often produces superior results than beginning with gamma-corrected values.

Here is an article, though not exactly on point, that may help in producing a "raw" linear scan with the Nikon scanning software.  As Ethan states, try using a "raw" linear scan for profiling, then after you create your profile, scan your film using the same settings (i.e. to produce "raw" linear scans), then assign your profile to your scans in PS. The next step is up to you, but I usually convert my scans to ProPhoto then do the bulk of my adjustments in Adobe Camera Raw.

http://www.colorneg.com/nikonscan.html?lang=en
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guyburns
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« Reply #4 on: May 09, 2011, 10:27:23 PM »
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Thanks for the responses. The "colorneg" link was particularly informative, though not correct in its description of how to set gamma to 1.0. You have to turn off CM first, otherwise gamma correction is grayed.

Now that I have set gamma to 1.0, CM off, I have one more question. The colorneg site suggested activating the "auto exposure for positive film" option, whereas I had already turned that off, thinking that the best results would be obtained if the exposure was constant when scanning the target and when subsequently scanning other slides.

Is it desirable to turn auto exposure on?
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guyburns
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« Reply #5 on: May 10, 2011, 02:17:54 AM »
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Okay, I've scanned the target at gamma = 1, CM off; generated two profiles (an XYX look-up table, and a gamma-matrix type) and applied them to an image that was scanned with identical scanner settings, and…

1. The colour difference between this profile and several other profiles I have generated using a variety of scanner settings is imperceptible.
2. However, using the gamma = 1 profile results in banding in gradations, banding that is almost non-existent when I applied all the other profiles to the relevant image (i.e profiles applied to an image that was scanned with the same settings that originally generated that particular profile).

I'm using Coca, based on Argyll, to generate profiles.

I'll go back to the idea I posited in my original post: that it is best to start from a target scan that looks on screen to be a close match to the target slide when the target is viewed on a slide viewer. The gamma = 1 scans came out exceptionally dark, but there was nothing wrong with them: when I applied a gamma of 2.2 in PS they looked fine. But it seems to me that if you start with a linear scan there may be problems: (1) large corrections are required by the profile because the image will ultimately be displayed on a monitor that effectively has a gamma of 2.2 [?]; (2) linear scans require more bits to accurately map the luminance range of an image.

I came across an explanation about digitising images in Reproduction of Colour (RWG Hunt). I don't know how applicable this is, but he says (p546, slightly edited for clarity):

Digital imaging systems often operate with 8 bits in each of three channels, making a total of 24 bits. This nymber of bits generates 224 or 16,777,216 different colour signals. The number of colours that can be distinguished by human colour vision is a matter of some debate, but it has been quoted as 10 million… For a 24-bit system to reproduce all these colours, it would have to sample colour space almost uniformly in visual terms. But many systems digitise linearly, and these are notoriously non-uniform perceptually.

A perceptually uniform grey scale which is widely used is that provided by the L* function of the CIELAB colour space… The range of L* values used in imaging depends on the display and the viewing conditions, but the maximum range can be taken as extending from 10 (for a black) to 100 (for a white). If a just-noticeable difference is taken to be one unit of L*, then the number of tonal levels required to avoid spurious contouring in gradually changing areas would be 100-10 = 90, which would need 7 bits… But, when linear signals are used… the number of tonal levels rises to 733, requiring 10 bits.


Hunt certainly knows his stuff, but he is not the best teacher, and he doesn't clearly explain how he arrived at those figures. I've spent a few hours reading some of his other chapters, and this is how he derived those figures. As he describes on p109 (edited for clarity):

…a uniform linear scale of luminance does not represent a uniform visual scale: for example, the apparent difference between two luminances of 10% and 15% is much greater than that between luminances of 70% and 75%. To allow for this, a quantity called CIE 1976 lightness, L*, is calculated as:

 L* = 116 (Y/Yn)0.33 - 16


Where Y is the luminance being examined, and Yn is the luminance of the reference white. Below a certain value of Y/Yn this formula is replaced by another, so that L never goes negative (as I understand it).

A luminance scale of L*, ranging from black (~10) to white (100) requires only 7 bits (128 values possible). Therefore, 8-bits will be more than ample to avoid banding, if the levels are non-linearly spaced (i.e. if gamma is higher than 1.0). Problems arise when you use a linear scale, because depending how you set up the scale, a jump from one level to the next may exceed what humans can detect – and we will see banding. Humans have greatest sensitivity to changes in luminance when the luminance is low. Therefore, to limit banding to being just perceptible, you choose the change in luminance between L*(10) and L*(11) as the basis for the increments between linear levels for the entire range from black to white. What this means, however, is that as the brightness increases, you have a lot of unnecessary levels (requiring more bits). The advantage is: now matter what the luminance (black, white, grey), the change from one level to the next will never be more than just perceptible, and then only in the darker regions of the image. And that is exactly where I saw the banding in my profile generated from a linear scan – in the darker regions.

Now, where did Hunt get his number 733 from? Rearrange the formula, we have:

Y/Yn = ((L* + 16)/116)3

For L* = 10, relative luminance is 0.01126
For L* = 11, relative luminance is 0.01261

a difference of 0.00135. So, to get from a luminance of L*(10) to L*(100), which required 7 bits in non-linear L* space, we would have to go from 0.01126 to 1.0 in linear space, using steps of 0.00135 (otherwise in the darkest areas we would see banding). Number of levels, N, required in linear space:

N = (1.0 - 0.01126)/0.00135 = 732.4 levels.

Therefore 10 bits (1024 possibilities) are required in linear space to avoid banding. My scans were 14 bit, but how many bits does the profiling software use? If less than 10, and if based on a linear IT8 target scan, it would seem to me that banding may sometimes become visible in the darker areas of an image.

I have generated a few dozen test profiles from my six targets (scanned non-linearly), and only on one occasion so far have I noticed banding – and then at an insignificant level – when the profiles were applied to an image. But the first time I applied a profile based on a linear scan to one of my test images, banding was obvious and obtrusive. Just a coincidence, or the result of linear scanning?








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dmerger
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« Reply #6 on: May 10, 2011, 07:40:49 AM »
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Your scanner CCD, like all digital cameras, produces linear data.  That linear data is the starting point, whether you make your gamma adjustment in your scan software, in PS or via a profile.  I’ve never experienced the banding you describe.  Are you sure you’re scanning in 16 bit and starting with a properly exposed scan?

Regarding exposure, so long as you get a proper exposure, it doesn’t matter whether you use auto exposure or manual exposure (using the master analog gain).  Manual exposure is often necessary to get the best exposure.  A good exposure should clip neither the highlights nor the shadows.  As a general rule, follow the “expose to the right” method commonly used for digital cameras.  (Your scanner is just a specialized digital camera.)
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Dean Erger
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« Reply #7 on: May 10, 2011, 01:51:11 PM »
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[iTherefore 10 bits (1024 possibilities) are required in linear space to avoid banding. My scans were 14 bit, but how many bits does the profiling software use? If less than 10, and if based on a linear IT8 target scan, it would seem to me that banding may sometimes become visible in the darker areas of an image.[/i]

I don't know about Argyll/Coco as far as scanned bit depth goes. This is not only whether the software accepts high-bit input, but whether it makes use of the information. This is a problem with many scanner profilers.

The built-in profiling software for VueScan and SilverFast do not have this limitation, however I have not found their profiles to be of as high quality as, for example, ProfileMaker produced. Profiles are, by their nature, more suited for performing color adjustments that the gross tonal shifts required to shove linear gamma input into gamma 2.2 output. Again, I have never used Argyll for scanner profiling, so I can't say how well it performs here, even if high-bit data are supported.

When you see banding in profiles made from the raw, linear gamma data, it is better to let the scanner software perform the tonal adjustments to move the input from linear to either 1.8 or 2.2 gamma. With the previous generation of Nikon film scanners combined with ProfileMaker 4.something, I had the best results scanning into Wide Gamut RGB and building a profile from there. Newer flavors of Nikon hardware + PMP5.10 allowed using either linear raw data (via VueScan) or NikonScan with CM off, but gamma 2.2 output.
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dmerger
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« Reply #8 on: May 10, 2011, 02:55:33 PM »
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It’s possible that Argyll/Coco is creating a faulty profile using linear data, but I suspect that Guy’s banding problem could be the result of a problem somewhere in his workflow; maybe double profiling, incorrect assigning or conversion of the profile or some other cause.  In any event, I doubt that there is any significant difference between (a) first applying a gamma adjustment and then profiling versus (b) profiling directly from the linear data.  I’ve done it both ways.  The results are a little different, but not dramatic.  So, if you’re having a problem using linear data, Guy, go ahead and do your gamma adjustment first, but I’d try keeping CM off.

By the way, does Nikon Scan include an option to use the scanner’s native color space (i.e., no CM) and include a corresponding profile that you can assign in PS?  Just curious.


Profiles are, by their nature, more suited for performing color adjustments that the gross tonal shifts required to shove linear gamma input into gamma 2.2 output.

Interestingly, the Minolta scanning software for my Minolta 5400 includes an option to profile directly from the scanner's linear data. e.g., I can assign the included Minolta linear profile to the 16 bit linear output.  This profile has no problem doing the gamma and color adjustments in one step. I've also used IT-8 targets to make my own profiles directly from the scanner's linear output without any problems. So, at least in my experience, there doesn't seem to be any problem using a profile to perform both the gamma and color adjustments.
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Dean Erger
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« Reply #9 on: May 10, 2011, 03:07:45 PM »
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Dean - I should have clarified the point. It requires additional handling above and beyond the normal number crunching to perform large shifts to tonal values without introducing artifacts. It is possible, and the newer software for producing scanner profiles is adept at this.
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guyburns
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« Reply #10 on: May 10, 2011, 09:00:17 PM »
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By the way, does Nikon Scan include an option to use the scanner’s native color space (i.e., no CM) and include a corresponding profile that you can assign in PS?  Just curious.

You have the option to choose no CM, and you have the option to choose Scanner RGB, both come into PS unprofiled, but they have different data and look different.
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dmerger
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« Reply #11 on: May 10, 2011, 09:09:33 PM »
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Do you have a profile for Scanner RGB?  I assume Nikon included one with Nikon Scan, and it sounds like the native color space for your scanner. If so, try scanning with Scanner RGB, then when you open the scan in PS assign the Scanner RGB profile.

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guyburns
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« Reply #12 on: May 10, 2011, 10:17:31 PM »
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There does not appear to be a profile for Scanner RGB, but I'll hunt around and see what I can find.

When using Coca, there are 4 options for generating a profile:

• Lab clut
• XYZ clut
• Shaper + matrix
• Gamma + matrix

My limited understanding of the above says that:

• Lab clut is an Lab based look-up table i.e. the profile is based on a non-linear set of values contained in a look-up table.
• XYX clut is a linear version of Lab clut.
• Shaper + matrix sounds like there is an equation involved somewhere, with the parameters contained in a matrix.
• Gamma + matrix sounds very similar to the shaper + matrix.

Anyone know about these various options? The only info I have been able to find is from the Argyll site:

Similar to a display profile, an input profile can be either a shaper/matrix or LUT based profile. Well behaved input devices will probably give the best results with a shaper/matrix profile, and this may also be the best choice if your test chart has a small or unevenly distributed set of test patchs (ie. the IT8.7.2). If a shaper/matrix profile is a poor fit, consider using a LUT type profile.

When creating a LUT type profile, there is the choice of XYZ or L*a*b* PCS (Device independent, Profile Connection Space). Often for input devices, it is better to choose the XYZ PCS, as this may be a better fit given that input devices are usually close to being linearly additive in behaviour.
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dmerger
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« Reply #13 on: May 11, 2011, 09:15:21 AM »
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There does not appear to be a profile for Scanner RGB, but I'll hunt around and see what I can find.
[/i]

Look in the Nikon program files.  You should find a list of quite a few profiles.  Is there one or more that doesn't appear to match the options given when using the software, such as maybe "NKLS5000_P.icm".  If so, that could be the profile for Scanner RGB.

Anyway, back to creating your own profile.  I've read that when you turn off CM, or select Scanner RGB in CM, that the scan opens in PS with sRGB imbedded, which is a defect in the software.  If so, you need to be sure not to use sRGB and instead you should delete the profile tag. If what I've read is correct, maybe Coca was seeing that incorrect sRGB tag and it caused your faulty profile.

Also, it appears that no CM and Scanner RGB are the same, just that you can control gamma (and other features) with one and not the other. See: http://www.nikonusa.com/pdf/manuals/Scan4/NikonScan-4_CMS_en.pdf

No CM or Scanner RGB is what you probably want to use for creating your profile since they are the native color space of your scanner and have the widest gamut. Of course, you then need to scan your film with the same settings, then when you open the scans in PS be sure not to use the incorrect imbedded sRGB tag, but instead assign the profile you created.

My hunch is that the particularities of Nikon Scan led you to use some incorrect setting when you created your profile with linear data, which caused the banding problem. 
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guyburns
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« Reply #14 on: May 12, 2011, 06:29:02 AM »
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If what I've read is correct, maybe Coca was seeing that incorrect sRGB tag and it caused your faulty profile.  
I'm reasonably certain the Coca ignores embedded profiles because, as a test, I generated two profile from a scanned target, one with and one without an embedded profile and the Coca profiles came back the same (as far as I could tell by applying them to an image).

TESTING PROFILES
I've been thoroughly putting Coca through its paces and have found some anomalies. This is what I have done:

A. Scanned a Kodachrome IT8 target at 2000 dpi, 14-bit, with seven different settings…

1. CM off, Gamma 1.0
2. CM off, Gamma 1.5
3. CM off, Gamma 1.8
4. CM off, Gamma 2.2
5. CM on, Adobe RGB (gamma 2.2, embedded profile called Adobe RGB)
6. CM on, Apple RGB (gamma 1.8, embedded profile called Apple RGB)
7. CM on, sRGB (gamma 2.2, embedded profile called sRGB)

… and gave the resulting images names such as Kodachrome IT8 Gamma 1.0, Kodachrome IT8 Adobe RGB, and so on.

B. I saved the targets without profiles.

C. I generated 4 profiles for each target, using these options in Coca: Lab, XYZ, Gamma+Matrix, Shaper + Matrix, and gave the profiles names such as: Kodachrome IT8 Apple RGB XYZ. The name reflects the way the target was scanned, and the way the profile was generated. There are 28 profiles, generated at high quality, and two generated at ultra-high quality to see if there was any difference (there wasn't).

D. Then I chose five of my test slides (slides that are a challenge to scan) and scanned each of those with the seven settings given in A. The images had names such as: Ref 02 Gamma 1.0, Ref 05 Apple RGB, the name reflecting the slide number and the scan method. This gives me 35 images with which to test the profiles.

E. Then starting with Ref 02, I opened the seven scanned versions of Ref 2 in PS – I had seven tabs open, each tab showing the same image but scanned a different way.

F. Then for each tab, I applied the four profiles applicable for that image, compared them, and took notes. So, for example, the first tab might have been Ref 02 Adobe RGB (scanned with Adobe RGB), and then I applied the profiles: Lab, XYZ, Shaper and Gamma to see the effect. Then I moved to the next tab, and repeated, until all seven versions of Ref 2 had been compared.

G. Then I closed all the Ref 2 images, and opened the seven Ref 5 images, and applied four profiles to each.

A pretty thorough test to determine which profile worked the best. And one did work the best – and it wasn't Gamma 1.0.

RESULTS OF TESTS
Something very interesting turned up: with CM off, the profiles resulting from Gamma 1.5 and Gamma 2.2 scans of the IT8 target generated nonsense profiles in Coca. When applied to the appropriate image, nonsense colours appeared. Any suggestions why? Gamma 1.0 worked, Gamma 1.8 worked, Adobe RGB worked … but not Gamma 1.5 and 2.2. Same scanner, same process, same profile generator, same application to an image, but nonsense results.

FILES TO REPLICATE MY TESTS
Just in case anyone wants to try and work out what went wrong I have uploaded all that is needed to replicate my testing. Coca is free, so if you've never used it, here is your chance. In this folder, http://www.mediafire.com/?thogddgfozxi2, can be found:

• Coca installation file (Coca_setup.exe, 10.7 MB)
• 7 IT8 scans + data file required by Coca (Kodachrome IT8 Targets.zip, 14.8 MB)
• 7 Kodachrome scans (Reference Scans.zip, 11.5 MB)
• 30 profiles, 28 at high quality, 2 at ultra high (Kodachrome Coca Profiles.zip, 6.7 MB)

To reduce file size, the IT8 scans have been resampled to 1000 dpi, 8bit (from the original 2000 dpi, 14-bit). This resampling has no effect on generating profiles, as far as I could tell when I re-profiled some of them at the reduced resolution.

BANDING
If you want to see the banding problem, check out the deep blue of the water in the foreground of the Kodachrome scans. Banding is subtle, but definitely present in all images except Gamma 1.8. It was present in most of the other reference images as well, to various extents. The slides I have chosen as reference slides are a serious challenge to scan. They show extreme contrast, or gentle gradations, or saturated colours against other saturated colours. Most scanners do a reasonable job scanning an "easy" slide – and you won't see banding – but I've dived in the deep end.

COCA HELP
If you need help with Coca, the home site is here: http://www.nla.gov.au/preserve/dohm/coca.html. You do need to put a sensible name into the "Internal Profile Description" field (Section 7), because that is what appears as the profile name in PS. I ran Coca from Windows XP. I don't know if it works on later versions.






« Last Edit: May 12, 2011, 06:37:42 AM by guyburns » Logged
dmerger
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« Reply #15 on: May 12, 2011, 01:48:48 PM »
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Guy, your test results are bizarre. I don’t know the reason, but it’s a red flag that something is amiss.  I wouldn’t trust any of your profiles to be accurate.  The problem could be software, but it’s much more likely that you have a glitch in your workflow. It could be an incorrect setting in Nikon Scan, PS or how you’re processing your scans to create your profiles.

You’ve obviously put a lot of time into your tests.  I wish I had an answer for you, but all I can offer is a suggestion that you try to keep your tests very limited until you solve the mystery.  For example, work solely with your IT-8 target slide, CM off, gamma 2.2, and one profile method.  Create your profile, and then instead of testing with a regular slide, use your IT-8 slide. Open your IT-8 scan in PS and assign the profile you created. The result doesn’t have to be perfect, but it should be very good.  If not, you probably have a problem in your workflow.  In such case, make only one change in Nikon Scan, PS or your workflow, and retest.  Repeat as necessary. (The reason I suggest you use CM off and gamma 2.2 is because that combination now gives nonsense results and thus may make it easier to track down the problem.)

There is one PS setting you may want to change.  It sounds like you don’t have PS set to automatically alert you to profile mismatches or missing profiles.  If so, you may want to change your PS color settings by checking all the boxes under “Color Management Policies”. 
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« Reply #16 on: May 12, 2011, 04:04:40 PM »
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I took your suggestion about limiting the profile generation to one type and applying it to the IT8 target. But I went one step further: I did the lot on the PC this time (because I'm a Mac man, I do everything I can on the Mac, but Coca only runs on a PC). The interesting thing is: Gamma 2.2 worked first time, and worked with all the reference images as well.

Something strange must have happened to Coca just for Gamma 1.5 and Gamma 2.2, because there is nothing wrong with the target scans. Might have been just a blip in the system.

Anyway, just a short summary of what I have found.

COLOUR DIFFERENCES – Negligible
There is no real problem with any of the various profiles I have generated (apart from the gamma 1.5 & 2.2 glitch). The colour differences between all the dozens of tests are not that much. And since I intend editing further anyway, the differences would mostly disappear in the editing.

BANDING
The banding problem appears to be real and occurs here and there. Increasing the gamma when scanning the target, and using XYZ profile seems to reduce the problem.

SHADOW CONTRAST
There is a slight, but definite improvement in the deepest shadow contrast, sometimes matched by better colour rendition in the shadows, as the gamma of the target scan is increased – the higher the gamma, the greater the improvement. I doubt this can be compensated for by the profiling program as effectively as increasing the gamma during the target scan. One possible reason for this is the gamma of Kodachrome, which is approximately 1.5 according to Hunt – set to that figure to improve shadow contrast in dark-surround viewing (the conditions in which slides are normally viewed). This might be fine for viewing, but causes problem when scanning, because it means the darker areas are darker than they were in real life as compared to the lighter areas. This is the prime reason that you can't copy slides with the same type of film as the original (unless the original film had a gamma of 1) – the shadows become even darker. And it explains why when scanning slides the shadows have to be lifted out of their blackness. Lifting them before profiling seems to give better shadow detail – the profile software has less correction to do.

The above statements are open to correction. I have to whittle down all the test results to the most promising profiling method – at this stage XYZ consistently gives the best results – and then regenerate profiles at Gamma = 1.0, 1.5, 1.8 and 2.2 and apply them to all my reference slides.

The aim of all this testing is to get the best possible profile, accompanied by the easiest editing method in PS, to get superb results from Kodachrome. I expect the resulting PDF, tentatively named The Art and Science of Kodachrome Scanning (and which will be made freely available), will be a lengthy document full of test images, graphs, and quotes from authoratative sources. And I'm hoping volunteers from this forum will offer to proof read it and offer comments.

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dmerger
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« Reply #17 on: May 12, 2011, 05:54:20 PM »
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Now, when you open an IT-8 scan in PS and apply the profile you created, do you get a scan without major color casts?  I ask because the IT-8 scans you posted, with CM on and aRGB or sRGB profiles embedded, open in PS, using the respective profile, with very large color casts.  In other words, does your profile provide a significant improvement over the Nikon CM?

"COLOUR DIFFERENCES – Negligible" -- This is also my experience with my scanner and profiles, even among the "canned" profiles that came with my scanner software (gamma adjusted as well as linear), third party profiles created for my film type and model of scanner, and the profiles I created myself for specific film types using both linear and gamma adjusted targets. I've only used Fuji Velvia and Provia, however, never Kodachrome. But, they all usually have some color cast that needs correction in PS or ACR, as well as tone, contrast and other adjustments.

As far as banding and shadow contrast, I've never see any banding with my scanner (using Velvia and Provia) regardless whether I started with linear or gamma adjusted files, nor have I noticed any significant difference in shadow contrast. Perhaps your experience is different because you're scanning Kodachrome. 

Anyway, glad you got the mystery solved.  Good luck with your project.
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Dean Erger
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« Reply #18 on: May 13, 2011, 03:07:22 AM »
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Now, when you open an IT-8 scan in PS and apply the profile you created, do you get a scan without major color casts?

There is an obvious change in colour when I apply the IT8 profile to itself for all scans. I tested like this:

1. I opened all seven IT8 scans (Adobe RGB, Apple RGB, Gamma 1.0 and so on) with their original as-scanned profiles. Adobe, Apple, and sRGB looked identical; the other four were different (Gamma 1.0, 1.5, 1.8 & 2.2) but had the same general look, though of different brightnesses.

2. I then applied the appropriate IT8 profile to each. Except for the weird colours of Gamma 1.5 and 2.2, they all looked identical.

3. Then I applied the new "correct" profiles for Gamma 1.5 and 2.2, and all seven images looked identical – except Gamma 1.0 which had slight banding in a few of the darker patches.

Given that all seven had no perceptible difference, I think I'm safe in assuming that my profiling of this scanner is about as good as it is likely to get. But I'm going to tweak a little more just for the learning experience.


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dmerger
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« Reply #19 on: May 13, 2011, 08:56:32 AM »
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Given that all seven had no perceptible difference, I think I'm safe in assuming that my profiling of this scanner is about as good as it is likely to get.

I think that's a safe assumption.  

« Last Edit: May 13, 2011, 09:00:21 AM by dmerger » Logged

Dean Erger
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