Photoshop Color Management: Slope Limiting

Photoshop Color Management has features called "slope-limiting" in its various stages and operations. There has been a lot of questions about them, what they are and how they affect.

For these evaluations you need to download the bp.gif shown below (in a steep gamma-space it appears rather black, but evaluations are to be made in Photoshop in linear space). The bp.gif has a gray swatch(es) with gray levels 1... 24 along with dithered comparison swatch(es).

Simple Slope Limiting Test Chart

Slope-limiting in the on-sceen compensation

1. have linear setting in Photoshop RGB setup dialog, that is gamma = 1.0

2. Open the bp.gif into Photoshop and convert it to RGB.

3. Set gamma in AdobeGamma utility to 1.0 and save the profile as Adobe Monitor Settings.icm (this profile name allows gamma space changes on the fly).

4. Evaluate the gamma match. If your system is well calibrated the match will be very good.

5. Set gamma in AdobeGamma to 2.2 or 2.5 and again save the profile as Adobe Monitor Settings.icm

6. Evaluate the gamma match again, you will notice that there are error in the first two patches above the black.

Slope Limiting in the ICC Profiles

1. Open the bp.gif into Photoshop and convert it to RGB.

2. Set gamma in the RGB setup to 1.0.

3. Save as Photoshop PSD.

4. Close the bp.psd.

5. Set gamma in the RGB setup to 2.2 or 2.5.

6. Open the bp.psd and allow Photoshop to convert colors, so it converts the gamma space also.

7. Now compare the appearance against the gamma 1.0 space appearance, some what difficult. So convert the bp.psd into gamma 2.2 or 2.5 space using Corel Photo-Paint, Paint-Shop Pro or any other sw that provides the gamma control (except ImageReady that also has the slope limiting) and then compare this against Photoshop Profile converted version using difference mode in Photoshop. There will be errors in the deep shadows.

Slope limiting in the middle input box in the Levels dialog

The middle input-box in the Levels dialog is sometimes called as "gamma" but it has a very strong slope-limiting, I have the demo about this here. Below is an other experiment, set RGB gamma to 1.0 for this experiment:

1. Open the bp.gif into Photoshop and convert it to RGB.

2. Go to Levels dialog, enter 2.5 into the middle input-box and press OK. This expands the shadows.

3. Go to Levels dialog again, enter 0.4 (that is 1/2.5) into the middle input-box and press OK. This should bring the shadows back as they were !

4. Compare against the original, the damage is very large.

5. Repeat this using with any other image processing program than Photoshop or ImageReady and you will see that there is absolutely no damage. (Note that e.g. in PaintShopPro the gamma convention is the other way around, you need to first to convert by 1/2.5 = 0.40 and then back by 2.50).

How to avoid Photoshop Slope-Limiting

1. Do not allow Photoshop to convert gamma space by profiles (this is happening when the info bar tells that Photoshop is converting colors right after an image is opened)

2. Always keep AdobeGamma in the same gamma space that Photoshop is.

3. Do not use the middle-input box in Levels dialog for gamma space change. You can change the image gamma accurately with the accurate gamma maps in the Curves dialog.