Adobe Photoshop Training with Dan Margulis

Dan Margulis Applied Color Theory

Is Gammaless Better?

Date: Sun, 23 Jan 2005 00:26:39 +0100

From: "Francisco Bernal"

Subject: ¿It is better to work in programming in an linear or gamma corrected image?

And I wonder ¿It is better to manipulate an image in an lineal space or in a

gamma corrected ones? I am talking about programming, not about use a

retouching software...

¿Any advice?

/*--------------------------------------*/

Francisco Bernal Rosso

Webpage at:

http://www.geocities.com/pacorosso

http://www.fotoforum.net/socios/b/b_f/fotos.htm

http://www.michelle7.com/contributors/r/paco_rosso.htm

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Date: Sat, 22 Jan 2005 17:52:30 -0800

From: Paul D. DeRocco

Subject: RE: ¿It is better to work in programming in an linear or gamma corrected image?

It depends upon the specific function. Anything that involves the additive combination of weighted values, e.g. blurring, should be done in the gamma 1 domain. Other functions can be done directly in the image's color space. There are some functions that _must_ be done in a domain that roughly matches the brightness scale of the eye, although off-hand I don't remember specifically which.

There's a guy, Timo Autiokari, who swears one should use linear for everything, and he's a real crank about it, to the point of making himself infamous in various newsgroups and mailing lists. In reality, he's right about some things, and wrong about others. You may learn some things from his web site (http://www.aim-dtp.net), but don't take everything you read there as gospel.

Ciao, Paul D. DeRocco

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Date: Sat, 22 Jan 2005 18:28:21 -0800

From: Mike Russell

Subject: Re: ¿It is better to work in programming in an linear or gamma corrected image?

From a strictly software standpoint, linear space is assumed by most - if not all -graphics algorithms, e.g. quarter red plus quarter red equals half red. Color fringing occurs, to some extent, any time you apply a linear operation in gamma space. Does this mean we should all start working in 1.0 gamma space? No.

Here's an interesting 1998 article, by Alvy Ray Smith that touches on this issue. Click on the small red square to get the pdf. http://alvyray.com/Memos/MemosMicrosoft.htm#NonlinearAlphaQuestion .

Mike Russell

www.curvemeister.com

www.geigy.2y.net

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Date: Sat, 22 Jan 2005 22:18:43 -0800

From: Paul D. DeRocco

Subject: RE: ¿It is better to work in programming in an linear or gamma corrected image?

From: Mike Russell

Does this mean we should all start working in 1.0 gamma space? No.

Fortunately, PS does this automatically for some operations, if you turn on the "Blend RGB Colors Using Gamma" checkbox in the color settings, and type in 1 for the value.

Ciao, Paul D. DeRocco

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Date: Sun, 23 Jan 2005 15:12:17 +0100

From: "Francisco Bernal"

Subject: Re: ¿It is better to work in programming in an linear or gamma corrected image?

Thanks Paul.

My thinking (probably wrong) is: if gamma correction is to bring some perceptual uniformity to th ewhole range of lighting in the image ¿Why aply the manipulation in linear -non uniformity perceptual- space? If I apply a weihgted value in the shades, and the same in the lights, in an linear space the efects are non uniform perceptualy, but (maybe) in a gamma corrected one. As I say I am only wondering, I try to un derstand this issues.

/*--------------------------------------*/

Francisco Bernal Rosso

Webpage at:

http://www.geocities.com/pacorosso

http://www.fotoforum.net/socios/b/b_f/fotos.htm

http://www.michelle7.com/contributors/r/paco_rosso.htm

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Date: Sun, 23 Jan 2005 16:24:19 EST

From: Dan Margulis

Subject: Re: ¿It is better to work in programmi ng in an linear or gamma corrected image?

For most manipulations that don't involve gradients it makes little difference whether the space is perceptually uniform or not, or whether the data is completely linear or very close to linear (like, gamma 1.1). However, certain things work better at a relatively high gamma and others at a relatively low one. Sharpening is somewhat better at a high gamma and blurring and blending better at a low gamma. Curves and similar maneuver are more controllable in a high gamma environment with respect to detail, but in a low gamma environment with respect to color. If a curve is applied and then the user wishes to reduce its effect by 50 percent, this move works better in low gamma.

To see the drawback of high gamma, imagine you must find the RGB color that is midway between bright green and bright red. Clearly this needs to be some kind of subdued yellow, as that is the hue they both share. That subdued yellow is what you get if you blend in LAB, where gamma is not a factor. If you blend in RGB, however, 255r0g0b and 0r255g0b average to 128r128g0b. Because of the gamma factor the child will be much darker than either parent, as well as being too green. This has considerable implications for painting tools and for blurring.

Dan Margulis

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Date: Sun, 23 Jan 2005 18:38:35 -0800

From: Paul D. DeRocco

Subject: RE: ¿It is better to work in programmi ng in an linear or gamma corrected image?

You can experiment with gamma 1 manipulations fairly easily, using a custom

RGB space in PS.

Ciao, Paul D. DeRocco

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Date: Mon, 24 Jan 2005 09:38:38 EST

From: Dan Margulis

Subject: Re: ¿It is better to work in programmi ng in an linear or gamma corrected image?

Paul DeRocco writes,

You can experiment with gamma 1 manipulations fairly easily, using a custom

RGB space in PS.

Indeed. I am the originator of that method, first publishing it in 2000.

Dan Margulis

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Date: Mon, 24 Jan 2005 09:26:40 -0800

From: Paul D. DeRocco

Subject: RE: ?It is better to work in programmi ng in an linear or gamma corrected image?

Somewhere along the line, I managed to save the linear color space as an ICC profile. Now, I can't figure out how I did it. I don't recall using a separate program to extract it from an image. Do you know of a way of getting PS to spit out a .icc file, based on a custom color space?

Ciao, Paul D. DeRocco

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Date: Mon, 24 Jan 2005 12:31:39 -0500

From: Lee Clawson

Subject: Re: ¿It is better to work in programmi ng in an linear or gamma corrected image?

on 1/23/05 4:24 PM, Dan Margulis at Dan Margulis wrote:

Sharpening is somewhat better at a high gamma and blurring and blending better

at a low gamma. Curves and similar maneuver are more controllable in a high

gamma environment with respect to detail, but in a low gamma environment with

respect to color.

Dan,

Is this specific to how Adobe implements them in Photoshop or more general?

Lee

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Date: Mon, 24 Jan 2005 18:47:25 +0000

From: Shangara Singh

Subject: Re: ?It is better to work in programming in an linear or gamma corrected image?

It was 24/1/05 5:26 pm, when Paul D. DeRocco wrote:

Somewhere along the line, I managed to save the linear color space as an ICC

profile. Now, I can't figure out how I did it. I don't recall using a

separate program to extract it from an image. Do you know of a way of

getting PS to spit out a .icc file, based on a custom color space?

Paul

Possibly:

--Color Settings

--Working Spaces

--RGB popup menu: YourColorSpace

--Custom RGB from popup menu

--Input custom settings Custom RGB

--Custom name

--OK

--RGB popup menu: Customized workspace

--Choosing Save RGB...

Shangara Singh.

________________________________________________

http://www.photoshopace.com || http://www.examaids.com

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Date: Mon, 24 Jan 2005 11:04:37 -0700

From: Andrew Rodney

Subject: Re: ?It is better to work in programming in an linear or gamma corrected image?

On 1/24/05 10:26 AM, "Paul D. DeRocco" wrote:

Somewhere along the line, I managed to save the linear color space as an ICC

profile. Now, I can't figure out how I did it. I don't recall using a

separate program to extract it from an image. Do you know of a way of

getting PS to spit out a .icc file, based on a custom color space?

Go into Custom RGB, futz around with the settings, then select 3Save RGB.2 You get a simple Matrix ICC profile.

BTW, this term being used in this thread (Gamma) is technically incorrect. The proper term is Tone Response Curve. Gamma is a very simple formula that plots a specific value of which very, very few devices and even color spaces follow. Just to be clear. The correct statement would be "the TRC of Adobe RGB is Gamma 2.2" NOT "Adobe RGB has a Gamma of 2.2"

The second statement uses Gamma to mean TRC which it does not.

"Linear Gamma" is also incorrect

"Linear TRC" or "a TRC of Gamma 1.0" would both be correct.

Andrew Rodney

http://digitaldog.net/

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Date: Mon, 24 Jan 2005 13:17:32 EST

From: Dan Margulis

Subject: Re: ?It is better to work in programmi ng in an linear or gamma corrected image?

Paul DeRocco writes,

Somewhere along the line, I managed to save the linear color space as an ICC

profile. Now, I can't figure out how I did it. I don't recall using a

separate program to extract it from an image. Do you know of a way of

getting PS to spit out a .icc file, based on a custom color space?

Yes. Assume that what you want is an RGB that uses the same primaries as Wide Gamut RGB, but has a gamma of 1.0.

1. Open Photoshop: Color Settings and be sure that "Advanced Mode" is checked.

2. In the Color Settings dialog, change Working Spaces: RGB to Wide Gamut RGB. Do NOT click OK.

3. Change Working Spaces: RGB to Custom RGB. The Wide Gamut RGB numbers come up by default in the ensuing Custom RGB dialog.

4. In the Custom RGB dialog, change gamma to 1.0. The Name will now default to "Custom RGB". Change it to something comprehensible, like "Wide Gamut 1.0 gamma."

5. Click OK to return to the basic Color Settings menu. "Wide Gamut 1.0 gamma" should now read as your RGB working space.

6. Change Working Spaces: RGB to Save RGB. This prompts you to save out the new profile. The profile is NOT saved with the Save button in the Color Settings dialog.

7. Having saved out the profile, click Cancel in the Color Settings dialog to restore your original settings.

Dan Margulis

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Date: Mon, 24 Jan 2005 13:14:29 -0800

From: Paul D. DeRocco

Subject: RE: ?It is better to work in programmi ng in an linear or gamma corrected image?

Oh, I see it now. I was looking for a Save button somewhere, rather than a Save RGB entry in the drop-down list. Peculiar interface design.

Ciao, Paul D. DeRocco

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Date: Mon, 24 Jan 2005 17:15:44 EST

From: Dan Margulis

Subject: Re: ¿It is better to work in programmi ng in an linear or gamma corrected image?

Lee Clawson writes,

Is this specific to how Adobe implements them in Photoshop or more general?

For practical purposes it's general. Technically it's general for detail and Photoshop-specific for color, but doing the color in a more reliable way (e.g. converting to LAB internally and doing the mixing there) would be quite compute intensive. I suspect it would be possible today but it certainly wasn't a couple of years ago. So I doubt that any other program does it better. Most don't even offer the limited option Photoshop has of doing certain blends at 1.0 gamma.

Dan Margulis

Adobe Photoshop training classes are taught in the US by Sterling Ledet & Associates, Inc.