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                                                             521
            SECTION 7.2                                                  Basic Compositing Computations



NAME                RESULT

Screen              Multiplies the complements of the backdrop and source color values, then complements
                    the result:
                       B ( cb , cs ) = 1 – [ ( 1 – cb ) × ( 1 – cs ) ]
                                      = cb + cs – ( cb × cs )

                    The result color is always at least as light as either of the two constituent colors. Screening
                    any color with white produces white; screening with black leaves the original color un-
                    changed. The effect is similar to projecting multiple photographic slides simultaneously
                    onto a single screen.

Overlay             Multiplies or screens the colors, depending on the backdrop color value. Source colors
                    overlay the backdrop while preserving its highlights and shadows. The backdrop color is
                    not replaced but is mixed with the source color to reflect the lightness or darkness of the
                    backdrop.
                       B(c b, c s) = HardLight(c s, c b)

Darken              Selects the darker of the backdrop and source colors:
                       B ( c b , c s ) = min ( c b , c s )

                    The backdrop is replaced with the source where the source is darker; otherwise, it is left
                    unchanged.

Lighten             Selects the lighter of the backdrop and source colors:
                       B ( c b , c s ) = max ( c b , c s )

                    The backdrop is replaced with the source where the source is lighter; otherwise, it is left
                    unchanged.

ColorDodge          Brightens the backdrop color to reflect the source color. Painting with black produces no
                    changes.

                                           ⎧ min(1, c b ⁄ ( 1 – c s ))     if c s < 1
                          B ( c b, c s ) = ⎨
                                           ⎩1                              if c s = 1

ColorBurn           Darkens the backdrop color to reflect the source color. Painting with white produces no
                    change.

                                        ⎧ 1 – min(1, ( 1 – c b ) ⁄ c s)        if c s > 0
                       B ( c b, c s ) = ⎨
                                        ⎩0                                     if c s = 0

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