Photoshop offers 19 brush modes when you use the pencil, paintbrush, airbrush, or any of the other tools shown along the left side of Figure 5-35. (An additional mode, Threshold, is an alternative to Normal in certain color modes.) To show you what these brush modes look like when applied to an image, Color Plates 5-2, 5-3, and 5-4 illustrate 18 modes, minus only Threshold. In each case, I used the paintbrush tool to apply a bit of green graffiti to a work of fourteenth-century religious iconography. Who among us hasn't been tempted with the primal urge to paint "Kilroy" on something old and priceless? Now, thanks to the miracle of digital imagery, you need resist this temptation no longer.
Just as you can cycle from one brush mode to the next from the keyboard, you can jump directly to a specific brush mode as well. Just press Shift+Alt and a letter key. For example, Shift+Alt+N selects the Normal mode, Shift+Alt+C selects the Color mode. I list the letter key for each brush mode in parentheses along with its description:
* Normal (N): Choose this mode to paint or edit an image normally. A paint tool coats the image with the foreground color, and an edit tool manipulates the existing colors in an image according to the Opacity or Pressure value.
* Threshold (L): Two color modes prevent Photoshop from rendering soft or translucent edges. The black-and-white and indexed modes (Image ^ Mode ^ Bitmap and Image ^ Mode ^ Indexed Color) simply don't have enough colors to go around. When painting in such a low-color image, Photoshop replaces
Note the Normal brush mode with Threshold, which results in harsh, jagged edges, just like a stroke painted with the pencil tool. You can alternatively dither the soft edges by selecting the Dissolve mode, as described next.
♦ Dissolve (I): This mode and the six that follow are not applicable to the edit tools (though I wonder why — the Dissolve mode would be especially useful with the smudge tool). Dissolve scatters colors along the edge of a brushstroke randomly throughout the course of your drag. The Dissolve mode produces the most pronounced effects when used with soft brushes and the airbrush tool.
♦ Behind (Q): This one is applicable exclusively to layers with transparent backgrounds. When Behind is selected, the paint tool applies color behind the image on the active layer, showing through only in the transparent and translucent areas. In Color Plate 5-2, for example, I painted over the Madonna's head, and yet the brushstroke appears behind her head because she is positioned on an independent layer. When you're working on an image without layers or on the background layer of a multilayered image, the Behind mode is dimmed.
♦ Multiply (M): The Multiply mode combines the foreground color with an existing color in an image to create a third color, darker than the other two. Using the multiply analogy, red times white is red, red times yellow is orange, red times green is brown, red times blue is violet, and so on. As discussed in Chapter 4, this is subtractive (CMYK) color theory at work. The effect is almost exactly like drawing with felt-tipped markers, except the colors don't bleed. Check out the first Kilroy in Color Plate 5-3 to see the Multiply mode in action.
The multiply mode has no effect on the paintbrush when it's set to Wet Edges; the Wet Edges brush setting already multiplies.
♦ Screen (S): The inverse of the Multiply mode, Screen combines the foreground color with each colored pixel you paint over to create a third color, lighter than the other two. Red on white is white, red on yellow is off-white, red on green is yellow, and red on blue is pink. The Screen mode uses additive (RGB) color theory. If the effect has a traditional counterpart, it's like some impossibly bright, radioactive Uranium-238 highlighter, hitherto used only by G-men to mark the pant cuffs of Communist sympathizers.
Because the Wet Edges option always multiplies, combining it with the Screen mode must render the brush invisible. If the paintbrush tool isn't working, this could be your problem.
♦ Overlay (O): Overlay, Soft Light, and Hard Light are cousins. Each mode multiplies the dark pixels in an image and screens the light pixels as you lay down color with a paint tool. But although related, the three modes are not variations on an identical theme. In other words, you can't emulate the Soft Light mode by simply applying the Hard Light mode at 70 percent or some similar opacity.
Of the three modes, Overlay is the kindest. Overlay always enhances contrast and boosts the saturation of colors in an image. In fact, Overlay works rather like a colored version of the sponge tool set to Saturate. It mixes the colors in the image with the foreground color to come up with a vivid blend that is almost always visually pleasing. Overlay may be the most interesting and downright useful brush mode of the bunch.
* Soft Light (F): This mode applies a subtle glazing of color to an image. In fact, Soft Light is remarkably similar to painting a diluted acrylic wash to a canvas. Soft Light never completely covers the underlying detail — even black or white applied at 100 percent Opacity does no more than darken or lighten the image — but it does slightly diminish contrast.
* Hard Light (H): This mode might better be named Obfuscate. It's as if you were applying a thicker, more opaque wash to the image. You might think of Hard Light as Normal with a whisper of underlying detail mixed in.
For examples of Overlay, Soft Light, and Hard Light, check out the middle brushstrokes in Color Plate 5-3.
* Color Dodge (D): This brush mode lightens the pixels in an image according to the lightness or darkness of the foreground color. Color Dodge produces a harsher, chalkier effect than the Screen mode and is designed to act like a dodge tool that also adds color. At 100 percent Opacity, even painting with black has a lightening effect.
* Color Burn (B): If Color Dodge is like drawing with chalk, Color Burn is like drawing with coal. It darkens pixels according to the lightness or darkness of the foreground color and is designed to simulate a colored version of the burn tool. For examples of Color Dodge and Color Burn, look to the last two Kilroys in Color Plate 5-3.
* Darken (K): Ah, back to the old familiars. If you choose the Darken mode, Photoshop applies a new color to a pixel only if that color is darker than the present color of the pixel. Otherwise, the pixel is left unchanged. The mode works on a channel-by-channel basis, so it might change a pixel in the green channel, for example, without changing the pixel in the red or blue channel.
I used this mode to create the first brushstroke of Color Plate 5-4.
* Lighten (G): The opposite of the preceding mode, Lighten ensures that Photoshop applies a new color to a pixel only if the color is lighter than the present color of the pixel. Otherwise, the pixel is left unchanged. On or off — either you see the color or you don't.
* Difference (E): When a paint tool is set to the Difference mode, Photoshop subtracts the brightness value of the foreground color from the brightness value of the pixels in the image. If the result is a negative number, Photoshop simply makes it positive. The result of this complex-sounding operation is an inverted effect. Black has no effect on an image; white inverts it completely. Colors in between create psychedelic effects. For instance, in the third example of Color Plate 5-4, the Difference mode inverts the green paint to create a red brushstroke.
Tip Because the Difference mode inverts an image, it results in an outline around the brushstroke. You can make this outline thicker by using a softer brush shape. For a really trippy effect, select the paintbrush tool, turn on Wet Edges, and apply the Difference mode with a soft brush shape.
♦ Exclusion (X): When I first asked Mark Hamburg, lead programmer for Photoshop, for his definition of Exclusion, he kindly explained, "Exclusion applies a probabilistic, fuzzy-set-theoretic, symmetric difference to each channel." Don't think about it too long — your frontal lobe will turn to boiled squash. After Mark remembered he was communicating with a lower life form, he told me (very slowly) that Exclusion inverts an image in much the same way as Difference, except colors in the middle of the spectrum mix to form medium gray. Exclusion typically results in high-contrast effects with less color saturation than Difference. My suggestion is to try the Difference mode first. If you're looking for something a little different, press Ctrl+Z and try Exclusion instead. (Both Difference and Exclusion brushstrokes appear in Color Plate 5-4.)
♦ Hue (U): Understanding this and the next few modes requires a color theory recap. Remember how the HSL color model calls for three color channels? One is for hue, the value that explains the colors in an image; the second is for saturation, which represents the intensity of the colors; and the third is for luminosity, which explains the lightness and darkness of colors. If you choose the Hue brush mode, therefore, Photoshop applies the hue from the foreground color without changing any saturation or luminosity values in the existing image.
None of the HSL brush modes — Hue, Saturation, Color, or Luminosity — are available when painting within grayscale images.
♦ Saturation (T): If you choose this mode, Photoshop changes the intensity of the colors in an image without changing the colors themselves or the lightness and darkness of individual pixels. In Color Plate 5-4, Saturation has the effect of breathing new life into those ancient egg-tempura colors.
♦ Color (C): This mode might be more appropriately titled Hue and Saturation. Color enables you to change the colors in an image and the intensity of those colors without changing the lightness and darkness of individual pixels.
Tip The Color mode is most often used to colorize grayscale photographs. Open a grayscale image and then choose Image Mode RGB Color to convert the image to the RGB mode. Then select the colors you want to use and start paint ing. The Color mode ensures that details in the image remain completely intact.
♦ Luminosity (Y): The opposite of the Color mode, Luminosity changes the lightness and darkness of pixels, but leaves the hue and saturation values unaffected. Frankly, this mode is rarely useful. But its counterpart — the Luminosity blend mode — is exceptionally useful when applied to layers. Read Chapter 13 to find out more.
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