Generating a Kodachrome profile from an IT8 target

[guyburns]

Mentioned are two options for the colprof command that might be worth trying: -u and -r.

Thanks for the interesting info. I have purposely avoided the Argyll mailing list, assuming it would be way beyond my understanding. But filtered through you, there might be something of interest. Are you able to tell what defaults Coca uses for -u and -r when it calls on Argyll? Can they be changed by me by altering a file, similar to the way I can alter Box Shrink? Do you know whether Coca uses absolute intent? I was hoping the profiled highlights would not be showing any significant errors, but from what you've quoted, highlights on a real slide that are brighter than GS0 may be clipped. I don't want that. Maybe I should be faking GS0 as well.

I read somewhere in a Hutchcolor document that he recommended manually inserting a pure black patch to improve the profile. I'll see if I can find it again.

Smoothing by -r might help get rid of the reversal at GS21-23?

The only way I'm comfortable with of getting rid of the GS22/23 reversal is by faking GS23 at its target value, or a value can can be demonstrated to be reasonable. Unless my scanner is reading the value of GS23 incorrectly, it seems to me that GS23 has been compromised beyond being useful.

[guyburns]

Just going through all your recent posts to respond.

1. http://www.luminous-landscape.com/forum/index.php?topic=54040.msg445565#msg445565
Re your editing of Ref 18 at gamma 1.0 and gamma 1.8: A difficult slide to digitise accurately. The original is nowhere near as dark as it scans, but because it is underexposed, Kodachrome sent the lower exposures (most of the slide) into darkness – Hunt's 1.5 gamma thing – and the detail is hard to retrieve. The original slide shows obvious detail in the shadow on the tree; the pack is not solid black, it has a definite gradation to dark gray on the top half, and the red of the pack is not mottled with black as in the gamma 1.0 version, but more like the gamma 1.8 version. On a bright lightbox under a loupe this is a very pleasant-looking, moody slide. When projected, it loses a bit; and when scanned and presented on a monitor, it needs a lot of work before it appears as it should.


2.

Apparently Kodak measured the target with some highly-specialized equipment, as the Q60 data are calculated from spectral measurements. The spectral data is available from Kodak in a QSP file.

Thanks for that suggestion. I already had a copy of the QSP file, but I didn't take much notice of it. I've just had a detailed look. So, the numbers are spectral data! I could sit down and plot some spectrums, like the ones that appear in Hunt's book. I might just do that, for interest.


3.

Another option is the Silverfast Kodachrome targets.

When I purchased my non-Kodachrome targets from a particular supplier (I probably shouldn't include the name], I asked about Kodachrome. This is the reply:

There are currently two manufacturers offering 35mm Kodachrome targets. Last time I checked none of the targets managed to be within the allowed tolerances of the IT8 standard. However, the fault of the Kodak target was small and shouldn't affect scanning quality. The Kodak target faults were much lower than those of the other manufacturer. Also, the Kodak chart has a decent color gamut unlike the target from the second manufacturer, which should avoid problems, especially with some of the better profilers I know. The rather old age of the reference file I got with my Kodak Kodachrome test charts didn't cause any problem and my measurement of that target was within the expected tolerances of the Kodak reference file. Another impressive example of the great aging perfomance of Kodachrome film. Modern dye films are much worse when it comes to aging.


4.

I updated the files. Looks better.
diagnostic image 7.7
measurements 7.7

The numbers in the measurement files: I assume the XYZ values come from the IT8 data file, and the RGB values (scaled 0-100) come from the 16-bit scan. Is that correct?

[crames]

Are you able to tell what defaults Coca uses for -u and -r when it calls on Argyll? Can they be changed by me by altering a file, similar to the way I can alter Box Shrink?

I don't think that Coca uses either option, which means no -u extrapolation and the value for -r  defaults to 0.5 (percent). It would be great if CoCa had an editable text file that would allow you to add Argyll options that are not available on the CoCa interface, but I looked and it doesn't.

There is another program like CoCa called Argyll CMS GUI. In contrast to CoCa it enables most, if not all Argyll options. There is a Mac version available. This might be the answer to your needs to have access to more of the Argyll command options. I tried the Windows version, but could not get it to work. Maybe you will have better luck with the Mac version.

Do you know whether Coca uses absolute intent? I was hoping the profiled highlights would not be showing any significant errors, but from what you've quoted, highlights on a real slide that are brighter than GS0 may be clipped. I don't want that. Maybe I should be faking GS0 as well.

The Coca profiles contain the choice of Absolute or Relative intent, to be chosen when you Convert to Profile in the workflow. (Note that if -u were available in Coca, it would result in a profile that allowed only Absolute intent)

The K3 Q60 has a relatively dark DMIN GS0 patch at L* 88. It might be possible to get higher L* on some Kodachrome scans, but I don't know. It depends on how consistent Kodachrome DMIN is from type to type and roll to roll. Have you seen any of your scans blow out the whites? I think that if you keep your scanner exposure setting the same when scanning slides as when you scan the target, you might be ok for the most part. On the other hand, your ref 03 scan seems close to the edge of clipping, so you might want to build a little margin for error into the profile.

I read somewhere in a Hutchcolor document that he recommended manually inserting a pure black patch to improve the profile. I'll see if I can find it again.

The only way I'm comfortable with of getting rid of the GS22/23 reversal is by faking GS23 at its target value, or a value can can be demonstrated to be reasonable. Unless my scanner is reading the value of GS23 incorrectly, it seems to me that GS23 has been compromised beyond being useful.

The Hutch Color RGB Scanning Guide - there's a wealth of info there about dealing with scanner flare, modifying patches, etc. I need to read it again.

[crames]

Re your editing of Ref 18 at gamma 1.0 and gamma 1.8: A difficult slide to digitise accurately. The original is nowhere near as dark as it scans, but because it is underexposed, Kodachrome sent the lower exposures (most of the slide) into darkness – Hunt's 1.5 gamma thing – and the detail is hard to retrieve. The original slide shows obvious detail in the shadow on the tree; the pack is not solid black, it has a definite gradation to dark gray on the top half, and the red of the pack is not mottled with black as in the gamma 1.0 version, but more like the gamma 1.8 version. On a bright lightbox under a loupe this is a very pleasant-looking, moody slide. When projected, it loses a bit; and when scanned and presented on a monitor, it needs a lot of work before it appears as it should.

I see what you are saying. I think it boils down to differences in black point and contrast. I feel that editing can make them equivalent. Which is the better starting point for editing? Which is easier to edit?

To me, looking at the individual color channels, the Blue channels look the same, the gamma 1 Green channel looks slightly more detailed, and the gamma 1 Red channel definitely looks less noisy and more detailed. Like I mentioned before, I think the >1 gammas can be noisier. Is the extra red noise visible? Maybe not. I attached a screen cap. of the red channel comparison.

It's a very nice image, and satisfying to see how much quality you have brought out of such a dark scan.

4. The numbers in the measurement files: I assume the XYZ values come from the IT8 data file, and the RGB values (scaled 0-100) come from the 16-bit scan. Is that correct?

Correct.

[crames]

The Hutch Color RGB Scanning Guide - there's a wealth of info there about dealing with scanner flare, modifying patches, etc. I need to read it again.

I tried some of the flare-reducing techniques that Hutcheson describes and am encouraged by the results, but have to play around with it more. It seems that the deep shadows can be made very neutral, which would make them easier to edit, compared to the standard-preparation profiles that almost all go red in the dark tones.

[crames]

The Hutch Color Extended Range (XR) technique has been very useful.

In a nutshell, it amounts to subtracting a constant from all pixel RGB values of the profiling target scan. When your target scan has flare, (that is, more flare than normal scans) the profiler compensates by reducing the response to make the target scan match the Q60 reference. When the profile with this built-in flare compensation is used on an image with less flare, the darker tones can be clipped, making ugly artifacts. Hutcheson's technique is to subtract a constant value from the RGB values of the target scan before feeding it to the profiler. The profiler responds to the reduced RGB values by making a profile that does not reduce or clip the dark tones of a low-flare image.

In practice, if you subtract a lot from the RGBs, the profile can actually introduce a boost in the dark tones. If instead of subtracting, you add a value to all RGBs, the profile will cut or clip more. So the control over the making of the profile is, if you want to boost the dark tones, subtract a value, if you want to lower the dark tones, add a value.

In Hutcheson's XR method, he describes subtracting a single constant integer from all RGB values by using the black point slider of Levels. The black point slider in Levels does not offer much precision - it limits you to subtracting 8-bit values. If your target scan image is 16-bit (as it should be), Levels does not allow you to subtract or add small 16-bit integers. The Exposure tool has an Offset slider that offers the increased precision needed for 16-bit scans.

I found that even better is to adjust each R,G,B channel individually with its own value. Adjusting the R,G,Bs individually allows you to control the tint in the dark tones to achieve neutrality. Unfortunately Exposure does not offer a way to offset the 16-bit RGB channels individually - as far as I know there is no facility in Photoshop that can.

To address this I made a simple PS compatible plug-in RGB Offset 16 that allows adding and subtracting from 16-bit RGBs. Sorry, Windows only. Copy to the Plug-Ins folder. It will appear in the Filter menu under "Color Tools."

My testing so far has been with 16-bit linear files, and with the commercial profile software that I've owned for several years, inCamera by PictoColor. It is much faster to generate a profile than Argyll, and the profiles are accurate. inCamera responds very predictably to target scans with the added and subtracted offsets. After several iterations I was able to come up with profiles, both for my SS4000 and for the GB Coolscan, that do not clip in the shadows, do not have excessively-elevated black points, and are neutral at the black point. I think they are easy to edit with, compared to the usual Kodachrome profiles which tend to give the dark tones a reddish tint, and have apparently crossed-curves where the red channel crosses over the green and blue channels at some point, resulting in a color balance that differs along the tone scale.

I just started testing the same XR procedure with Argyll/CoCa, but I'm not yet finding the degree of control I have with inCamera. It might be a matter of finding the right profile type: shaper + matrix vs Lab clut vs XYZ, etc. It's time consuming going through the various profile types and testing a range of offsets for each. Not to mention, eventually testing each at various gammas.

(But I'm thinking XR might not work well with gammas other than 1.)

Edit: Still seems to be crossed-curve in the red channel. Need to work on it some more...

[guyburns]

There is another program like CoCa called Argyll CMS GUI. In contrast to CoCa it enables most, if not all Argyll options.

No good for me. It requires OSX 10.5 running on an Intel Mac. I'm still on OSX 10.4.

I feel that editing can make them equivalent. Which is the better starting point for editing? Which is easier to edit?

After several days of comparing editing techniques, I'm coming to the conclusion that the overall problem of getting the best result from a scanned slide lies not with the selection of the best profiling technique, but in the editing itself. Most of my reference scans require serious editing, even given perfect profiling, and I'm finding that sometimes I cannot obtain similarly good results if I select a different scan gamma to start with, or if I change the colour space. i.e. if after editing I choose a particular image as the optimum (to be used as a reference), most of the time I cannot reach that optimum with a different scan technique or in a different colour space.

Like I mentioned before, I think the >1 gammas can be noisier. Is the extra red noise visible?

I've seen lots of noise if I try and bring the shadow detail out, and I'll take your results as definite – that Gamma 1.0 has the least noise, as would be expected. Although, I have found that extra noise in the darkest shadows, as long as it is not obviously coloured, improves the image because the noise appears to be detail within the black. i.e. solid blacks are not desirable in most images. If they are broken up with noise, it can give the impression of detail, even though it is false detail.

It seems that the deep shadows can be made very neutral, which would make them easier to edit, compared to the standard-preparation profiles that almost all go red in the dark tones.

I have a technique that easily removes the colour cast in deep shadows, but it would be easier to edit if it wasn't there.

I found that even better is to adjust each R,G,B channel individually with its own value. Adjusting the R,G,Bs individually allows you to control the tint in the dark tones to achieve neutrality.

Do you think that will work for a variety of slide images? The shadows on my slides show a range of colour casts because of the lighting conditions: red (sunsets or bright red shirts), green (forest), blue (sky). They have to edited away for a more natural look, and such editing may swamp the small improvements available by adjusting the profile. It's good to have the most accurate profile, so I'll give your new plugin a go to see the effect.

When the profile with this built-in flare compensation is used on an image with less flare, the darker tones can be clipped, making ugly artifacts.

I'm seeing some of this clipping, I think. It is difficult to edit away, and if the original slide had a lot of shadow detail, it makes for an unsatisfactory image. I am still finding that editing gamma 1.8 scans sometimes gives the best end-result. Not all the time. Gamma 1.0 comes out on top here and there, and so does sRGB. Which is rather annoying. I was hoping for a single method to give optimum results for all slides.


Clever Kodak?
What are the chances that Kodak purposely designed those flaws into GS18-23 so that when profiled, such a target gave the most pleasing shadow result? Monitors, for example, only have 256 levels. If the end result is to be displayed on a digital screen (in my case, eventually, BluRay via a monitor or projector), there must come a point when you have to say: for the best image when viewed, those extra 16-bit levels have to be compressed in a certain way for optimal results on an 8-bit display. I need convincing that all my attempts at better profiling have not resulted in a degraded 8-bit image when viewed – because blacks on the digital image are now closer to the slide-blacks. Deeper blacks are difficult to reproduce digitally (obtaining rich blacks is the Holy Grail of all digital projectors), and having an excess of them, as does Kodachrome, can only cause problems.

Given a perfect profile applied to a perfect scan of a Kodachrome slide, you will still end up with a very poor digital image because Kodachrome has been optimised for projection, not digitising. Kodachrome scans will always require significant editing because of the way they are, and I'm hoping that such editing doesn't render profiling unnecessary. It's been a good learning experience, but I hope it hasn't all been wasted.

What's happening in my testing (a couple of hours a day for six months), is that I'm moving away from the "science" of scanning Kodachrome, and into the "art" aspect. And that may pose more serious difficulties than any of the science.

P.S. Thanks for the emailed profiles. Will test and get back to you.

[guyburns]

I have a present for you, Cliff: Gamma 1.0 scans of two separate unexposed Kodachrome 64 slides: http://www.mediafire.com/?d38ugsiwkc8a7cm

[crames]

After several days of comparing editing techniques, I'm coming to the conclusion that the overall problem of getting the best result from a scanned slide lies not with the selection of the best profiling technique, but in the editing itself. Most of my reference scans require serious editing, even given perfect profiling, and I'm finding that sometimes I cannot obtain similarly good results if I select a different scan gamma to start with, or if I change the colour space. i.e. if after editing I choose a particular image as the optimum (to be used as a reference), most of the time I cannot reach that optimum with a different scan technique or in a different colour space.

I agree that the profile only does a little of the work, the editing builds upon the starting point that the profile gives you and is what makes the image.

I think that the different effects of the CoCa/Argyll profiles on the crucial shadow region can be summarized as:

1. The higher gamma profiles give a lower black point. With the lower black point, the higher gamma profiles can sometimes over-emphasize or clip the blacks in slides with heavy shadows. The gamma 1 profiles have higher black points that give a milky and low-contrast appearance to the shadows, but can be corrected with a black point adjustment.

2. The higher gamma profiles of type Lab clut and XYZ have the most neutral shadows. Of the gamma 1 profiles, only the Lab clut profile is neutral. I think the most accurate is the higher gamma ( ~1.8 ) XYZ profile which has both maximum neutrality and the lowest black point. It's not necessary to jump through hoops, modifying the target scan, etc. to get a profile with neutral shadows. The CoCa Lab clut profiles and the higher-gamma XYZ profiles will give you neutrality.

The shadows on my slides show a range of colour casts because of the lighting conditions: red (sunsets or bright red shirts), green (forest), blue (sky). They have to edited away for a more natural look, and such editing may swamp the small improvements available by adjusting the profile. It's good to have the most accurate profile, so I'll give your new plugin a go to see the effect.

The profile should give a neutral starting point when the slide is exposed in photographic daylight, (D55). In other lighting there will color casts that can be tricky to handle, because of the way the color channels are shifted. (This is true for transparencies in general, not just Kodachrome.) See section 14.17 in Hunt. Fig. 14.11(d) shows what can happen when you attempt to correct the color of the lighting with a white balance adjustment.

I'm seeing some of this clipping, I think. It is difficult to edit away, and if the original slide had a lot of shadow detail, it makes for an unsatisfactory image. I am still finding that editing gamma 1.8 scans sometimes gives the best end-result. Not all the time. Gamma 1.0 comes out on top here and there, and so does sRGB. Which is rather annoying. I was hoping for a single method to give optimum results for all slides.

If there is clipping, then a gamma 1 Lab clut profile, with it's higher black point, might be better to start with. Then you can drop the black point (Levels, Curves, Exposure/Offset, etc), stopping before clipping occurs?

Clever Kodak?
What are the chances that Kodak purposely designed those flaws into GS18-23 so that when profiled, such a target gave the most pleasing shadow result? Monitors, for example, only have 256 levels. If the end result is to be displayed on a digital screen (in my case, eventually, BluRay via a monitor or projector), there must come a point when you have to say: for the best image when viewed, those extra 16-bit levels have to be compressed in a certain way for optimal results on an 8-bit display. I need convincing that all my attempts at better profiling have not resulted in a degraded 8-bit image when viewed – because blacks on the digital image are now closer to the slide-blacks. Deeper blacks are difficult to reproduce digitally (obtaining rich blacks is the Holy Grail of all digital projectors), and having an excess of them, as does Kodachrome, can only cause problems.

Maybe Kodak was expecting the target to be drum-scanned so the flaws wouldn't occur? I think the flaws are mainly due to limitations of the desktop scanners we are using.

When it comes time to project your images, are you going to profile your projector? There will probably have to be some gamut mapping and black point compensation. More profiling for you to do - good luck with that!

Given a perfect profile applied to a perfect scan of a Kodachrome slide, you will still end up with a very poor digital image because Kodachrome has been optimised for projection, not digitising. Kodachrome scans will always require significant editing because of the way they are, and I'm hoping that such editing doesn't render profiling unnecessary. It's been a good learning experience, but I hope it hasn't all been wasted.

Good point, and probably why Kodachrome didn't survive the digital era. I'm wondering if some of the Hutcheson techniques can be used to enhance the profile to do more of the work for Kodachrome, such as neutralizing the whole grayscale and reducing overall contrast.

What's happening in my testing (a couple of hours a day for six months), is that I'm moving away from the "science" of scanning Kodachrome, and into the "art" aspect. And that may pose more serious difficulties than any of the science.

If you're game, a discussion on editing Kodachrome scans would be great.

[crames]

I have a present for you, Cliff: Gamma 1.0 scans of two separate unexposed Kodachrome 64 slides: http://www.mediafire.com/?d38ugsiwkc8a7cm

The average RGBs of those scans are:
#01 141, 63, 53
#02 141, 62, 52

Lower than DMIN or anything on the Q60 target scans by almost half. Your reference scan 18 has shadow areas this dark or even slightly darker (left sides of the backpack and tree trunk.)

Goes to show how much lower the flare is in actual slides.

Edit: typos

[guyburns]

If you're game, a discussion on editing Kodachrome scans would be great.

Good idea. I'll start it soon. But this thread has a little distance to run yet.

Thanks for the summary of the various gammas and their effects. I won't comment on them here, but I intend taking note of what you said and do a little further testing on the various gammas and their effect on black point.

[crames]

Something is bothering me about most of these profiles.

Going back to the Kodachrome characteristic curves (attached below), a neutral subject should have RGB numbers with Blue highest, Green lower or equal to Blue, and Red lowest. In other words a profiled scan should reproduct the Kodachrome blue cast. Looking at the shadows of Ref 18, for example, the raw scanner RGBs have this reversed, with Red being the strongest. When a profile is then assigned and converted to a working space, it should result in a correction that brings the RGBs more in line with the characteristic curves, changing the raw red cast in the shadows to blue (or possibly neutral).

However, what is happening is that most of the test profiles are not restoring a blue cast to the shadows (although they reproduce the blue cast farther up in the tone range). The exceptions are the CoCa/Argyll gamma 1.8 XYZ profile, the Coca/Argyll gamma 1.0 Lab clut profile, and possibly the inCamera profiles.

The following table shows the 16-bit RGB numbers of a small patch in the deep shadows of Ref. 18, after assigning the various profiles and converting to Prophoto RGB.

My thinking is that the profiles that restore the blue cast to the shadows might be easier to edit, since it should then be possible to align and neutralize the entire gray-scale using simple gamma adjustments. In contrast, an image with a red shadows and blue mid-tones will need more complicated adjustments to fix it.

Gamma 1.8 R G B Gamma 1.0 R G B

raw RGB 1437 993 891

Argyll XYZ 77 72 182 1274 1008 867

Argyll Lab 167 60 632 1156 1278 1417

SCARSE.4 1570 1348 1102 1633 1452 1141

LPROF 1399 1010 673 1477 1032 661

inCamera3.1 311 217 316 673 714 652

[crames]

Guy - two more profiles for your collection, made with SCARSE for your Coolscan V:

Kodachrome Gamma 1 SCARSE
Kodachrome Gamma 1.8 SCARSE

They seem to be pretty smooth in the darker tones, so might offer an advantage when editing, despite a bit of a red cast in the deep shadows.

[guyburns]

Cliff,

Thanks for the extra profiles. Something more for me to play around with. The red cast in the raw scans of deep shadows is an interesting phenomenon. It could be a part of the scene itself, or it could be a problem with the scanner. How is it that a Kodachrome slide taken in near perfect blackness (my two "black" slides of a previous post) show a significant red cast when scanned? As you stated:

The average RGBs of those scans are:
#01 141, 63, 53
#02 141, 62, 52

The only explanation must be that the Coolscan is throwing a red cast, which profiling should remove. These dense blacks are a very touchy area.


Removing the non-linearity of the D-H Curve
My next series of tests is to try and remove the non-linear response of the Kodachrome D-H curve, by applying a correction curve in Photoshop's Curves. This would have to be applied after profiling (and the scan would have to be a linear scan for this to work), and before editing. Here I am not trying to correct colour, I am trying to remove the non-linear density characteristic of Kodachrome. This is my thinking:

1. The D (density) axis corresponds to what the scanner sees on the slide. For the red channel of Kodachrome 25, the density range, for example, ranges from about 0.2 to 3.8. These values could be scaled and converted to linear relative-luminance, and would become the horizontal axis (the "input" axis) for Photoshop's Curves, ranging from 0-255.

2. For any value of D on the slide, the actual scene "brightness" for that particular D could be derived from the D-H curve. I am assuming that "brightness" and luminance are directly related to exposure. The problem I have is that there is no upper limit to real-life exposure, whereas there is a limit to Photoshop's 8-bit "brightness" levels: 255. How do you relate the two? My solution is to set the minimum exposure from the D-H curve as being equivalent to Photoshop's brightness of 0, and the maximum exposure from the D-H curve as being equivalent to Photoshop's 255.

3. It is a bit confusing to explain, but when I linearised D and H for the Kodachrome 64 red-channel under the assumptions given above, I came up with the figures listed below [removed 20 June as there were errors in the figures]…

Neglecting the complications of dim and dark surrounds and so on, and assuming that the scanned image will be a faithful reproduction of the scene if the relative luminances between scene and image are linear, then the above figures should allow a curve to be set up in Curves to correct for Kodachrome's non-linear density characteristic.

Can you see any problems with this approach?

Another problem: I could go through all the bother of correcting for the D-H characteristic, only to find at projection time (dark surround) that I should have kept the original Kodachrome D-H characteristic as optimum for dark surrounds – assuming that as far as dark surrounds and optimum gamma are concerned, there is no difference between projecting digitally and projecting by a slide projector.

[Bart_van_der_Wolf]

The only explanation must be that the Coolscan is throwing a red cast, which profiling should remove. These dense blacks are a very touchy area.

Hi Guy,

When you look at the Characteristic Curve for slide films in general, and Kodachrome is no exception, then you'll see that the D-max for R/G/B is different, quite a bit different. Check the specifications for the type of Kodchrome you are looking at. Therefore one should stay away from the D-max for calibration purposes, except for perhaps a clipping level or a smoother TRC adjustment.

Cheers,
Bart

[crames]

The red cast in the raw scans of deep shadows is an interesting phenomenon. It could be a part of the scene itself, or it could be a problem with the scanner. How is it that a Kodachrome slide taken in near perfect blackness (my two "black" slides of a previous post) show a significant red cast when scanned?
-snipped-
The only explanation must be that the Coolscan is throwing a red cast, which profiling should remove. These dense blacks are a very touchy area.

I get similar results with my SprintScan. the red channel for unexposed Kodachrome is much higher than the other channels.

The Argyll Lab and the inCamera profiles for my scanner both reduce the excess red and increases the blue, but to different extents:

        Raw         Argyll Lab    inCamera
K64   53 10 20   0  9  141    6  8  8  
K200  20  2  8    0  4  145    2  2  5

Removing the non-linearity of the D-H Curve
My next series of tests is to try and remove the non-linear response of the Kodachrome D-H curve, by applying a correction curve in Photoshop's Curves. This would have to be applied after profiling (and the scan would have to be a linear scan for this to work), and before editing. Here I am not trying to correct colour, I am trying to remove the non-linear density characteristic of Kodachrome. This is my thinking:

1. The D (density) axis corresponds to what the scanner sees on the slide. For the red channel of Kodachrome 25, the density range, for example, ranges from about 0.2 to 3.8. These values could be scaled and converted to linear relative-luminance, and would become the horizontal axis (the "input" axis) for Photoshop's Curves, ranging from 0-255.

Did you convert from Density to Transmittance by the formula Transmittance = 10^(-Density), then scale Tranmittance by the maximum RGB value? Likewise convert LogE to linear Exposure? The curves will have a completely different shape in a linear-linear plot compared to the log-log characteristic curve.

2. For any value of D on the slide, the actual scene "brightness" for that particular D could be derived from the D-H curve. I am assuming that "brightness" and luminance are directly related to exposure. The problem I have is that there is no upper limit to real-life exposure, whereas there is a limit to Photoshop's 8-bit "brightness" levels: 255. How do you relate the two? My solution is to set the minimum exposure from the D-H curve as being equivalent to Photoshop's brightness of 0, and the maximum exposure from the D-H curve as being equivalent to Photoshop's 255.

I think you can just scale Transmittance x 255.

Neglecting the complications of dim and dark surrounds and so on, and assuming that the scanned image will be a faithful reproduction of the scene if the relative luminances between scene and image are linear, then the above figures should allow a curve to be set up in Curves to correct for Kodachrome's non-linear density characteristic.

Can you see any problems with this approach?

Another problem: I could go through all the bother of correcting for the D-H characteristic, only to find at projection time (dark surround) that I should have kept the original Kodachrome D-H characteristic as optimum for dark surrounds – assuming that as far as dark surrounds and optimum gamma are concerned, there is no difference between projecting digitally and projecting by a slide projector.

Now I see why you want to do this - in order to have the option to make your own compensation for surround, etc. specific to your projector and viewing conditions. I would think that a global gamma adjustment would do the trick, while preserving Kodachrome's toe and shoulder, which might be worth preserving.

If you're going to remove Kodachrome's non-linearity, you might as well do it for each channel, in order to get rid of the blue cast at the same time.

Here's a text file of the K25 curves digitized from a graph in a Kodak publication. The columns are LogE, Red Density, Green Density, and Blue Density. LogE is in 0.05 density increments. It should save you a lot of trouble reading numbers off the graph.

[crames]

Did you convert from Density to Transmittance by the formula Transmittance = 10^(-Density), then scale Tranmittance by the maximum RGB value? Likewise convert LogE to linear Exposure? The curves will have a completely different shape in a linear-linear plot compared to the log-log characteristic curve.

Guy, I think you have done this. Sorry!

Does Curves have enough resolution to linearize the dark end of the tone scale?

[guyburns]

Does Curves have enough resolution to linearize the dark end of the tone scale?

Not if you want to do it accurately – but it may not need to be done accurately. No point worrying too much about that aspect until I prove that the idea works.

If the idea works and I need to generate accurate Curves in Photoshop, would you be able to write a script that accepts Curves "input" and "output" values and passes those values to Curves so that it can draw smooth curves through the values? Could a polynomial equation be sent to Curves?

[crames]

If the idea works and I need to generate accurate Curves in Photoshop, would you be able to write a script that accepts Curves "input" and "output" values and passes those values to Curves so that it can draw smooth curves through the values? Could a polynomial equation be sent to Curves?

I read that Curves uses cubic splines to draw between points. Curves gives you this automatically, unless you are using the Pencil.

Have you tried using the Pencil in Curves? You can set 256 points individually, and have the option to apply smoothing. With the Pencil, the curve will be saved as an .amp (arbitrary map) file.

Alternatively, you can create a Photoshop amp format file and read it in to Curves. Amp files are very simple, a list of the 256 output numbers, but to make one you will probably have to use a hex editor, unless someone knows of an easier way to make a binary file. It will be possible to make a plugin to do it, if necessary.

[guyburns]

I've given up on the idea of correcting the D-H curves. This is the second time I've tried it (my first attempt was a month ago) and both times no improvement came. Both times I tried two methods: an absolute approach where I tried to correct the D-H curves point-for-point; and a relative approach where I left the blue channel unchanged and applied alterations only to the green and red. I think the absolute approach failed because I didn't know how to handle the end points – for example, the D-H curves show a minimum density of ~0.21 (0.62 relative luminance), yet my scanned slides go much lighter than that. And if you don't know how the end points of the original luminance of the scene translate to density on the slide, I think you're stuck. I was more certain of getting useful results from the relative method, but the end points were again a problem.

Below are the Input/Output figures I tried to put into Curves. The first column is the input, the second is Green output, and the third is Red output. Because PS doesn't allow an input of less than 4 into Curves, I combined the first four settings into one for green and red: 4,7, then rounded the others. Note how the differences disappear at Input = 157 (equal to density of 0.21, the lightest shown on the D-H curves). So the corrections only apply to half the brightness range of the slide. Didn't seem right when I calculated the figures, and it certainly didn't remove the blue cast.

In      G      R
0.1   0.3   0.5
0.2   0.3   0.4
0.4   0.5   0.6
1.5   1.8   2.3
6.0   7.1   9.0
24.4   27.6   34.2
102   102   125
157   157   157