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Next: 5.9 Common Problems and Up: 5. Colorspaces and Blending Previous: 5.7 Opacity and Transparency
Subsections

- 5.8.1 Colorization
- 5.8.2 Realistic Shadows and Highlights

# 5.8 Practical Uses of Blending Modes

Blending modes are fascinating toys that are lots of fun and can stimulate creative play with color. Results of experimenting with the blending modes often produce surprising and very aesthetic results.

Blending modes, however, are a lot more than toys for playing with color; some very useful operations would be impossible without them. Examples of practical uses for blending modes can be found in Sections 7.2, 7.3, and 7.4. This section describes two great applications of blending modes that do not appear elsewhere in this book.

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5.8.1 Colorization

It is often desirable to completely change the color of a subject--the color of a car, a lip gloss, eyes, a dress. The problem with changing the color of any real world image is that there are always lots of variations in the shading of a color. This is due to natural color gradations, lighting conditions, and textures. Figure 5.26(a)

Figure 5.26: Image Whose Color Is to Be Changed

illustrates such an image. Notice the subtle variations in the kitten's fur. The gray color of her coat has highlights and shadows as well as natural variations due to the texture of her fur. Changing the color of her coat requires choosing the new color and applying it in such a way that these natural looking variations are preserved.

The `Color` blending mode is purr-fect for this type of operation. This mode combines the hue and saturation of the foreground with the lightness of the background. Thus, the natural dark and light areas of the kitten's coat are preserved and only the color changes. The following illustrates how the technique is applied.

The objective is to change the kitten into a tabby colored cat with blue-ish eyes and to make the T-shirt of the person holding her a khaki green. To do this, a separate selection is made of the kitten's body, her eyes, her nose and mouth, and of the T-shirt. Each of these image components are selected using any of the techniques from Chapters 3 or 4 (I used the `Bezier Path` tool). Each selection is stored as a channel mask, as shown in the Channels dialog displayed in Figure 5.26(b). The four channel masks are labeled Body, Eyes, T-shirt, and Nose+Mouth.

The next step is to create a transparent layer above the kitten image where the new colors can be applied. The new layer is created by clicking on the `New Layer` button in the Layers dialog. When the `New Layer Options` dialog appears, the Transparent option is used, and the resulting layer is labeled Colorization Layer. Changing the blending mode of this new layer to `Color`, the color surgery can begin.

The Body channel mask is used to recover the selection of the kitten's body by applying the function `Channel to Selection` from the Channels menu (see Section 4.1.5). The Marching Ants for this selection can be seen in Figure 5.27(a). Now, making sure that the Colorization layer is active by clicking on its thumbnail in the Layers dialog, the `Color Selection` tool, as shown in Figure 5.27(b), is used to pick the desired color, and the `Bucket Fill` tool is used to apply the color to the selected region. Figure 5.27(a)

Figure 5.27: Making the Kitten's Coat Tabby-Colored

shows the result of this process. Note that the lightness variations of the kitten's coat from the lower layer are applied to the color and saturation choices in the upper one. Also note that, in the Layers dialog, shown in Figure 5.27(c), the Colorization Layer shows the thumbnail of the `Bucket Fill` operation.

The color of the kitten's eyes are changed using a similar operation. After applying the selection from the Eyes channel mask and making the Colorization layer active, you can apply the desired color to the eyes. Figure 5.28(b)

Figure 5.28: Making the Kitten's Eyes Blue

shows the color selection, Figure 5.28(a) shows the result of the eye-coloring procedure, and Figure 5.28(c) shows the corresponding Layers dialog.

Before proceeding to the final operation, that of changing the T-shirt color, an important touch is needed to make the kitten's nose, mouth, and inner ear look natural. These parts are not the same color as her fur and require a little pink to make them look correct. The nose and mouth are colored by converting the Nose+Mouth mask to a selection and by using the `Bucket Fill` tool to apply an appropriately chosen pink color. As for the kitten's inner ear, the color is applied a little differently. The inside of her ear consists of both fur and skin, and, consequently, the `Airbrush` tool is more suitable than the `Bucket Fill` tool for locally applying the pink paint.

Making the T-shirt a khaki green is a little different from the previous colorization efforts. Because the T-shirt's color in the original image is so light, it is impossible to make it any darker using the `Color` blending mode, which uses the lightness of the lower layer. This problem can be solved by changing blending modes. Creating a new transparent layer and setting the mode to `Multiply` allows darker color to be applied to the T-shirt. The result of this is shown in Figure 5.29(a).

Figure 5.29: Making the T-shirt a Khaki Green Using `Multiply` Mode

Figure 5.29(b) shows the color chosen for the T-shirt, and Figure 5.29(c) shows the thumbnail of the new layer and the choice of the `Multiply` blending mode. Note that I set the Opacity slider to 50% for this layer, which adjusted the color to a value I liked.

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5.8.2 Realistic Shadows and Highlights

A strong specular reflector illuminated by a point source creates a strong highlight and well defined shadows. However, more diffuse lighting produces less well-defined effects. Under these circumstances, the `Multiply` and `Screen` blending modes can be used to create realistic shadows and highlights in an image. These modes can be used to darken and lighten parts of an image without the risk of blowing out the tonal range, as is the case for specular reflectors and point source illumination. The following example illustrates this application of these blending modes.

Figures 5.30(a) and (b)

Figure 5.30: Original Circle with Measured Colors Shown in Color Selection Dialogs

illustrate two selected colors that are used to make the red circle on the yellow background shown in Figure 5.30(c). The goal is to give the circle a 3D look, to produce the effect of depth and light by creating a natural, diffuse looking highlight and shadow. The idea is to use each color itself to create the shadow and highlight. Applying a color to itself in `Multiply` mode tends to slightly darken the spot where the paint is applied. In `Screen` mode, it tends to slightly lighten it. This darkening and lightening, then, appears visually as a diffuse shadow and highlight. Repeated applications increase the shadow/highlight intensity until the desired effect is achieved.

Using the `Color Picker` tool, the measured colors of the image are 221R 191G 37B for the yellow background and 206R 26G 26B for the red circle. Because the colors are not fully saturated, they can be used. However, if either color were fully saturated (that is, a pure red, green, blue, cyan, magenta, yellow, or white), it would be necessary to select a color that is slightly off pure for this technique to work. This is easily done, however, using the `Color Selection` tool.

To make the highlight shown in Figure 5.31(a), a large fuzzy brush is chosen from the `Brush Selection` tool, as shown in Figure 5.31(b).

Figure 5.31: Screening a Diffuse Highlight

Note that, in the `Brush Selection` dialog, the blending mode has been set to `Screen` and the opacity to 60%. These numbers were chosen with some experimentation to achieve results I liked. A light application of the `Paintbrush` tool to the side of the red circle where I imagine the light to be coming from produces the highlight shown in Figure 5.31(a). Note that the `Airbrush` tool is also an excellent device for this type of work.

The shadow shown in Figure 5.32(a) is made with the same brush; however, now the `Multiply` blending mode is used, as shown in Figure 5.32(b).

Figure 5.32: Multiplying in a Diffuse Shadow

Here, the opacity slider in the `Brush Selection` dialog is set to 100%. Again, the numbers depend on the aesthetic sensibilities of the individual. Light application of the `Paintbrush` tool to the opposite side of the highlight and below the circle produces a believable, diffuse shadow.

Next: 5.9 Common Problems and Up: 5. Colorspaces and Blending Previous: 5.7 Opacity and Transparency
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