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Friday, June 29, 2007

Contrast Masks

Take control of the contrast in your images with precision using these Photoshop controls

Adjusting Contrast Masks

Since a mask is comprised of shades of gray, its density and contrast can be adjusted. Often, contrast masks can be improved by having their contrast adjusted. Use Curves for the greatest precision in contrast adjustment. Here are a few strategies.

Darken the mask and it allows less of a correction through. Lighten the mask and it allows more of an effect through. In special cases, you may want to raise the black point when applying Curves to a mask; this allows some adjustment to be applied to the deepest values with increasing intensity in the highlights.

Conversely, you can lower the white point to reduce the effect in highlights or you can simply reduce the opacity of the adjustment layer. Increase the contrast of a mask, and its lightest areas allow more of an effect to show through while its darkest areas allow less of an effect to show through.

Would you like to target the shadows instead of the highlights? Invert the mask to reverse the relationship between the highlights and shadows; black becomes white and white becomes black.

Would you like to quickly define a complex contour in an image? If a contour contains a reasonable degree of contrast from its surroundings, you can accentuate the contrast of a contrast mask to such a degree that all values except black and white are eliminated, leaving only the contour. It's likely you'll also need to paint away additional white values in the mask, either in surrounding areas or in specific areas of a contour or both, but you won't have to define a majority of the complex contours by hand, saving you hours of work.

With so many choices before you, which channel should you choose? Choose the channel that's light in the areas you want to adjust and dark in the areas you don't want affected. When in doubt, load the RGB master channel, as this gives you a selection based on luminance. If you choose an individual channel (R, G or B), related hues become lighter than others and complementary hues darker than others—for example, in the red channel, reds are very light while cyans are very dark.

Question whether you need a contrast mask when making image adjustments with Curves, which provides the ability to target specific ranges of tones without the use of a mask. If the adjustment you want to make to a specific region of tones in an image is extreme and you want to substantially reduce the effect on adjacent tones, then and only then consider using a contrast mask in combination with Curves. Guard against introducing posterization when doing this.

This technique can be used to compensate for over-inking and preserve deep shadow detail in inkjet prints. If you see that shadows are printing too dark (the file has more detail than the print), lighten them before making subsequent prints. Using a contrast mask to target this correction to the deepest values only eliminates any adverse effects this type of correction might have on other tonal values in the image.

Classically, photographers use a contrast mask when darkening very bright highlights or lightening very dark shadows. For these types of corrections, where industrial-strength methods are required, consider using the Blend modes Multiply (for highlights) or Screen (for shadows) in combination with a contrast mask to reduce the effect on the opposite end of the tonal scale and prevent loss of detail.

Enhancing Color Images With Contrast Masks

Today, you have additional opportunities to enhance color images using contrast masks. In addition to affecting the lightness (luminosity) of highlights or shadows, you can affect their hue and/or saturation. Compare these three solutions on pages 32 and 33.

All three versions appear more luminous or filled with light, but the qualities of light in each one differ. After you've tried this, try making opposite moves in the shadows. Once you've done these exercises, you'll begin seeing new potential in every color image.

Never have we had so much control over the quality of light within our images. Imagine the possibilities. Better yet, experience them. A world of possibilities for image enhancement will unfold before you.  
                                 
Author of Adobe Photoshop Master Class, John Paul Caponigro is an internationally acclaimed fine artist. A Canon Explorer of Light and an Epson Stylus Pro, his clients include Adobe, Apple, Gretag MacBeth, Lowepro, Imacon and Kodak. A passionate teacher, he offers an array of workshops in his studio throughout the year. See a live demonstration of this material at the currently touring Epson Print Academy. Receive a free subscription to his e-news Insights by e-mailing This e-mail address is being protected from spambots. You need JavaScript enabled to view it and learn more techniques.