Friday, June 29, 2007

Local Correction

Strategies for selectively lightening and darkening an image

Using Blend Modes For Image Adjustment

A more precise way to achieve the desired effect is to employ Blend modes. Found in the Layers palette, Blend modes determine how values of layers affect the values of layers below them. Blend modes are extremely useful for adjusting images. You can use them to make standard adjustments more precise or to make adjustments to an image.

The Blend modes menu is logically organized. Gray lines separate six sections. The first section contains two Blend modes, including the default Normal. The second section contains four modes, starting with Darken, and the rest, including Multiply, have similar effects. The third section also contains four Blend modes, starting with Lighten, and the rest, including Screen, also have similar effects. The fourth section starts with Overlay; this and the rest affect contrast, and can both lighten and darken an image. The fifth contains two modes for special effects. The sixth and final section contains four modes that affect color—Hue, Saturation, Color and Luminosity. Several Blend modes are particularly useful for making adjustments to images—Multiply, Screen and Overlay.

The Blend mode Multiply darkens an image, similar to registering two identical transparencies on a light table. The Blend mode Screen lightens an image, similar to projecting two transparencies in the same location. Overlay, essentially a combination of Multiply and Screen, both lightens and darkens an image.

You can duplicate a layer and change its Blend mode to see these effects. You can also create an adjustment layer and change its Blend mode to achieve the same effect; this is a more memory-efficient method, as adjustment layers don't contain pixels. Note that with these three Blend modes (Multiply, Screen and Overlay), the image will be affected whether or not you make a correction with the adjustment layer; the correction and the Blend mode are cumulative.

You can quickly scan very dark or very light areas of an image to see if they contain detail that can be improved. First, make a small selection in an area of interest using the Rectangular Marquee tool; then make an adjustment layer with a Blend mode of either Multiply (to darken) or Screen (to lighten). You can use the Move tool to move the resulting layer mask to other areas of an image to scan them for detail as well. If you like the effect, you can use a white brush to refine the mask for the selected area and to brush the effect into other areas of an image. Lower the opacity of the brush to reduce the intensity of the effect as desired.

The effects of these Blend modes are strong. While they can be used at very low opacities to adjust normal images, they're particularly useful for making industrial-strength corrections to images in need of a more aggressive approach. Screen is excellent for lightening very dark shadow detail, while Multiply is excellent for darkening very light highlights. Both Blend modes also have a strong effect on the opposite end of the tonal scale—Screen tends to make highlights too bright, while Multiply tends to make shadows too dark, so both are best used selectively by adding a layer mask.

A contrast mask can be added to an adjustment layer set to one of these Blend modes to constrain the effect to one end of the tonal scale or the other. To make a contrast mask, drag a channel in the Channels palette to the Load Channel as Selection icon (a circular dotted line). Fill the resulting selection with black to mask the highlights. Or, inverse the selection (Select > Inverse) and fill the resulting selection with black to mask the shadows. You can adjust the density and contrast of a mask by applying Curves to it (Image > Adjustments > Curves). This will help constrain the effect to a more precise range of tones and control transitions into surrounding values.

Both Blend modes have side effects; Multiply increases saturation, while Screen decreases saturation. You can quickly cure these side effects—Command click on the layer mask to load it as a selection; then create a Hue/Saturation adjustment layer to increase or decrease saturation.


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