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Grokking The Gimp
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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
Figure 5.30

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
Figure 5.31

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
Figure 5.32

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.




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Grokking The Gimp
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