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Author Topic: Calibration to L* TRC and photography  (Read 3609 times)

MirekElsner

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Calibration to L* TRC and photography
« on: December 11, 2013, 10:39:31 am »

I noticed that some of the more sophisticated display calibration packages offer and recommend calibration to L* TRC instead of Gamma 2.2. This seems to show more detail in shadow areas. The most typical editing step is to tamper with shadow detail. This is convenient that I can see more of the shadows during editing, but I am not sure if this is useful for the actual presentation.

  • Most users won't have their displays calibrated to L*. They will see darker shadows than I intended, correct?
  • For soft proofing I don't see much difference, if any between L* and Gamma 2.2 and prints have good match to soft proof displayed on monitor calibrated to L*. Is this expected?
« Last Edit: December 11, 2013, 10:41:12 am by MirekElsner »
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Scott Martin

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Re: Calibration to L* TRC and photography
« Reply #1 on: December 11, 2013, 11:18:26 am »

In theory, a color managed app like Photoshop would compensate for the TRC and display an image identically on a bunch of displays calibrated with a variety of TRCs. In actual real world usage this isn't quite the case. Calibrate a display with a gamma of 1.0 and then 3.0 and compare the results. Photoshop doesn't *quite* compensate all the way and there are some differences.

IMO, it all comes down to the final printing process and customizing the display calibration to match it. With my work I've found it useful to calibrate displays to L* with matching paper white luminosity and a custom black point only for silver halide processes. Silver halide has the ability to show subtle shadow detail like no other and L* helps me see those subtitles. For most inkjet and press printing processes it's harder to see those 96-100% shadow density values and gamma 2.2 along with a matching paper white and black points is the more successful approach.
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digitaldog

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Re: Calibration to L* TRC and photography
« Reply #2 on: December 11, 2013, 12:21:37 pm »

This has been a debate for years. One of the best posts on the subject comes from the ICC user's list from Lars Borg March 2008:

Quote
L* is great if you're making copies. However, in most other
scenarios, L* out is vastly different from L* in.  And when L* out is
different from L* in, an L* encoding is very inappropriate as
illustrated below.

Let me provide an example for video. Let's say you have a Macbeth
chart. On set, the six gray patches would measure around  L* 96, 81,
66, 51, 36, 21.

Assuming the camera is Rec.709 compliant, using a 16-235 digital
encoding, and the camera is set for the exposure of the Macbeth
chart, the video RGB values would be 224,183,145,109,76,46.

On a reference HD TV monitor they should reproduce at L* 95.5, 78.7,
62.2, 45.8, 29.6, 13.6.
If say 2% flare is present on the monitor (for example at home), the
projected values would be different again, here: 96.3, 79.9, 63.8,
48.4, 34.1, 22.5.

As you can see, L* out is clearly not the same as L* in.
Except for copiers, a system gamma greater than 1 is a required
feature for image reproduction systems aiming to please human eyes.
For example, film still photography has a much higher system gamma
than video.

Now, if you want an L* encoding for the video, which set of values
would you use:
96, 81, 66, 51, 36, 21 or
95.5, 78.7, 62.2, 45.8, 29.6, 13.6?
Either is wrong, when used in the wrong context.
If I need to restore the scene colorimetry for visual effects work, I
need 96, 81, 66, 51, 36, 21.
If I need to re-encode the HD TV monitor image for another device,
say a DVD, I need 95.5, 78.7, 62.2, 45.8, 29.6, 13.6.

In this context, using an L* encoding would be utterly confusing due
to the lack of common values for the same patches.  (Like using US
Dollars in Canada.)
Video solves this by not encoding in L*. (Admittedly, video encoding
is still somewhat confusing. Ask Charles Poynton.)

When cameras, video encoders, DVDs, computer displays, TV monitors,
DLPs, printers, etc., are not used for making exact copies, but
rather for the more common purpose of pleasing rendering, the L*
encoding is inappropriate as it will be a main source of confusion.

Are you planning to encode CMYK in L*, too?

Lars
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digitaldog

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Re: Calibration to L* TRC and photography
« Reply #3 on: December 11, 2013, 12:25:10 pm »

In theory, a color managed app like Photoshop would compensate for the TRC and display an image identically on a bunch of displays calibrated with a variety of TRCs. In actual real world usage this isn't quite the case. Calibrate a display with a gamma of 1.0 and then 3.0 and compare the results.

Native is probably the right answer for a heck of a lot of people. The less you have to muck around, the better.
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Tim Lookingbill

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Re: Calibration to L* TRC and photography
« Reply #4 on: December 11, 2013, 01:26:07 pm »

Standards are good in this regard especially with the optics involved with transmissive displays.

Something else that wasn't mentioned though Scott mitigates for in matching to silver halide is that black points density appearance vary wildly on various print media that makes calibrating to L* seem even more confusing with regard to standards.

Below are images of prints showing various effects of black density variances that require a side by side comparison to be made aware, not possible on quite a few workflows for reproduction.

The first is a closeup shot of my LG 27" 000RGB black next to the surrounding bezel of Lab gray ramp (data already in L* visual scaling) with the number "1" representing 10RGB gray.

The second shows the black point variance of the original PDI skin target with Lab grayramp showing a match to color and density roll out off black scaling on Epson Glossy paper even though the black density on the print doesn't match viewed under light level that provides an overall match.

The third shows the black density difference between an overall print match between my Epson and a Fuji DL430 inkjet I had printed at Walgreens. The Fuji's gamut is far and away larger than my Epson NX330 as well as are the black densities.

Again standards are needed especially when viewing and editing on a transmissive display that more than likely retains a constant black density level depending on ambient lighting which will affect our perception of black over it's actual measured value.
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Christoph C. Feldhaim

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Re: Calibration to L* TRC and photography
« Reply #5 on: December 11, 2013, 01:29:46 pm »

Since I calibrated my Eizo CG 243 W to L* instead of Gamma 2.2 my "shadows too dark" problems have vanished.
Temp is 5500 K and Lum 100 cd/m².
But I still had problems with too dark printed shadow up to the point where I changed to L*.

Tim Lookingbill

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Re: Calibration to L* TRC and photography
« Reply #6 on: December 11, 2013, 01:37:18 pm »

Since I calibrated my Eizo CG 243 W to L* instead of Gamma 2.2 my "shadows too dark" problems have vanished.
Temp is 5500 K and Lum 100 cd/m².
But I still had problems with too dark printed shadow up to the point where I changed to L*.


We don't know what light level you're viewing your prints under.

Why don't I have the same problem as yours? I've never had to calibrated to L* since I started in digital imaging in 1998 printing to an Epson Stylus Photo EX.
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Scott Martin

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Re: Calibration to L* TRC and photography
« Reply #7 on: December 11, 2013, 01:38:58 pm »

Since I calibrated my Eizo CG 243 W to L* instead of Gamma 2.2 my "shadows too dark" problems have vanished.
Temp is 5500 K and Lum 100 cd/m².
But I still had problems with too dark printed shadow up to the point where I changed to L*.

OK, but have you calibrated your display for matching DMax density? How does a solid black compare onscreen vs your printing process? And what is your printing process?
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Christoph C. Feldhaim

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Re: Calibration to L* TRC and photography
« Reply #8 on: December 11, 2013, 01:48:13 pm »

I am printing on an Epson 7890.
The blacks pretty much match - I am viewing my prints under indirect tungsten halogenide light from a flood light against a white ceiling, sometimes under a normal tungsten bulb.
I have no standardized D50 Lightsource.
I realized, that in softproofing in LR and PS the darks started to match better the prints when I switched to L*.
I don't understand this myself - when I tuned down the display from 120 cd/m² to 100 /m² it got much better, of course, but the change to L* made it perfect.

Tim Lookingbill

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Re: Calibration to L* TRC and photography
« Reply #9 on: December 11, 2013, 01:54:49 pm »

Quote
I am viewing my prints under indirect tungsten halogenide light from a flood light against a white ceiling, sometimes under a normal tungsten bulb.

How many lights? What's the wattage of each? And how far away are the lights from the print?

So when I look at your images online on my calibrated display and I see much darker shadows because you'ld rather let your display show you your shadows instead of edit them that way Soft Proofing, if I buy a print from you, it won't match to what I see on my display.

I don't see that as workable. There's got to be some other way to mitigate this.

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Scott Martin

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Re: Calibration to L* TRC and photography
« Reply #10 on: December 11, 2013, 01:56:39 pm »

Native is probably the right answer for a heck of a lot of people. The less you have to muck around, the better.

I like your concept here but on practical level, I've found PS doesn't handle a small percentage of displays in native TRC as well as when calibrated to 2.2.

If you have 3 displays (different brands) that are, lets say, calibrated to Gamma 1.0, 2.2 and 3.0, PS displays the same image differently on these displays. If they are all calibrated to native they are different (since the native gammas are different). If the are all calibrated to the same TRC they all look the same (at least as far as density goes). So that predicitable consistency is important to me. Native gamma calibration works really well in many situations but I think it's dangerous to recommend it as a policy to the masses.

On a practical level, I think it's productive to encourage the masses to start by calibrating to 2.2 customizing the white and black points to match the printing process. Just getting that far is a challenge for many! People overlook the importance of customizing the white and black points and good lighting. Worrying about TRC and other minutia when they haven't mastered the basics is an all too common mistake. Only once they've mastered these things should the gear heads consider experimenting with native and L*. With the white and black points properly customized I've personally only found L* practical for silver halide users.
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Scott Martin

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Re: Calibration to L* TRC and photography
« Reply #11 on: December 11, 2013, 02:00:04 pm »

I am printing on an Epson 7890.

But on what paper? I'm sure you can imagine how incredibly different the black Dmax is on a cotton rag matte paper vs a Baryta paper and how differently you might calibrate your display to match one or the other…
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digitaldog

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Re: Calibration to L* TRC and photography
« Reply #12 on: December 11, 2013, 02:03:36 pm »

I like your concept here but on practical level, I've found PS doesn't handle a small percentage of displays in native TRC as well as when calibrated to 2.2.
That's fine if 2.2 is better (and probably not a whole lot different than Native). The L* controversy doesn't help with respect to all the other issues of getting a good screen to print match. It is probably the least important target to be concerned with.
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Scott Martin

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Re: Calibration to L* TRC and photography
« Reply #13 on: December 11, 2013, 02:12:04 pm »

That's fine if 2.2 is better (and probably not a whole lot different than Native).

Yes, exactly. It's not likely to be very different most of the time.

The L* controversy doesn't help with respect to all the other issues of getting a good screen to print match. 

Agreed! Let's make sure the other stuff is nailed down before we start talking about L*.
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Czornyj

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Re: Calibration to L* TRC and photography
« Reply #14 on: December 11, 2013, 02:17:09 pm »

That's fine if 2.2 is better (and probably not a whole lot different than Native). The L* controversy doesn't help with respect to all the other issues of getting a good screen to print match. It is probably the least important target to be concerned with.

+1, target TRC is completely irrelevant IMO. Viewing conditions are the key factors, with the most impact on screen to print match.
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Christoph C. Feldhaim

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Re: Calibration to L* TRC and photography
« Reply #15 on: December 11, 2013, 02:25:10 pm »

Honestly, I don't understand why switching from 2.2 gamma to L* helped me so much with my shadows, apart from lowering from 120 cd/m² to 100 cd/m².
I had this happen on various papers, matte and glossy - all used with their appropriate profiles.
I think I'll re-do some more systematic testing, since it appears quite illogical to me too.

MirekElsner

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Re: Calibration to L* TRC and photography
« Reply #16 on: December 11, 2013, 02:46:07 pm »

Thanks for the great discussion so far.

Display profile with L* TRC shows the shadows differently than 2.2 profile - at least in color managed browsers (I did not check in PS). This is nicely demonstrated on the gamma test ramp from drycreekphoto. If I want to ensure that web images in color managed browsers look like the author intended, do I have to forget L* TRC?
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MirekElsner

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Re: Calibration to L* TRC and photography
« Reply #17 on: December 11, 2013, 02:48:01 pm »

Honestly, I don't understand why switching from 2.2 gamma to L* helped me so much with my shadows, apart from lowering from 120 cd/m² to 100 cd/m².
I had this happen on various papers, matte and glossy - all used with their appropriate profiles.
I think I'll re-do some more systematic testing, since it appears quite illogical to me too.

If you darkened the display, perhaps you are pulling the shadows more by siders in your post processing app?
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Christoph C. Feldhaim

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Re: Calibration to L* TRC and photography
« Reply #18 on: December 11, 2013, 03:30:56 pm »

If you darkened the display, perhaps you are pulling the shadows more by siders in your post processing app?

The shadows look differently with the different calibrations, it has nothing to do with sliders.
I'll do some testing, because I always thought luminance alone should do the trick, but it doesn't.

Scott Martin

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Re: Calibration to L* TRC and photography
« Reply #19 on: December 11, 2013, 08:13:47 pm »

If I want to ensure that web images in color managed browsers look like the author intended, do I have to forget L* TRC?

Yes. Gamma 2.2 is the standard there.

I'll do some testing, because I always thought luminance alone should do the trick, but it doesn't.

The white point luminance or black point luminance? :-] Both need to be set to match the printing conditions.
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