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                                                          522
             CHAPTER 7                                                                           Transparency



NAME                   RESULT

HardLight              Multiplies or screens the colors, depending on the source color value. The effect is similar
                       to shining a harsh spotlight on the backdrop.

                                           ⎧ Multiply(c b, 2 × c s)          if c s ≤ 0.5
                          B ( c b, c s ) = ⎨
                                           ⎩ Screen(c b, 2 × c s – 1)        if c s > 0.5

SoftLight              Darkens or lightens the colors, depending on the source color value. The effect is similar
                       to shining a diffused spotlight on the backdrop.

                                        ⎧ cb – ( 1 – 2 × cs ) × cb × ( 1 – cb )             if c s ≤ 0.5
                          B(c b, c s) = ⎨
                                        ⎩ c b + ( 2 × c s – 1 ) × ( D(c b) – c b )          if c s > 0.5

                       where

                                 ⎧ ( ( 16 × x – 12 ) × x + 4 ) × x              if x ≤ 0.25
                          D(x) = ⎨
                                 ⎩ x                                            if x > 0.25

Difference             Subtracts the darker of the two constituent colors from the lighter color:
                          B ( cb , cs ) = cb – cs

                       Painting with white inverts the backdrop color; painting with black produces no change.

Exclusion              Produces an effect similar to that of the Difference mode but lower in contrast. Painting
                       with white inverts the backdrop color; painting with black produces no change.
                          B(c b, c s) = c b + c s – 2 × c b × c
                                                                s

             Table 7.3 lists the standard nonseparable blend modes. Since the nonseparable
             blend modes consider all color components in combination, their computation
             depends on the blending color space in which the components are interpreted.
             They may be applied to all multiple-component color spaces that are allowed as
             blending color spaces (see Section 7.2.3, “Blending Color Space”).

             All of these blend modes conceptually entail the following steps:

             1. Convert the backdrop and source colors from the blending color space to an
                intermediate HSL (hue-saturation-luminosity) representation.
             2. Create a new color from some combination of hue, saturation, and luminosity
                components selected from the backdrop and source colors.

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