Add tonal color support for shadows.

Bug: skia:
Change-Id: Ib9bd9083da1d8a9fa90ae7c710386e6903541fd5
Reviewed-on: https://skia-review.googlesource.com/18148
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>

1 files changed, 36 insertions, 0 deletions

diff --git a/include/utils/SkShadowUtils.h b/include/utils/SkShadowUtils.h
index 838a12bba8..0ac2f3708d 100644
--- a/include/utils/SkShadowUtils.h
+++ b/include/utils/SkShadowUtils.h
@@ -69,6 +69,42 @@ public:
         DrawShadow(canvas, path, zPlane, lightPos, lightRadius, ambientAlpha, spotAlpha,
                    color, flags);
     }
+
+    /**
+     * Helper routine to compute scale alpha values for one-pass tonal alpha.
+     *
+     * The final color we want to emulate is generated by rendering a color shadow (C_rgb) using an
+     * alpha computed from the color's luminance (C_a), and then a black shadow with alpha (S_a)
+     * which is an adjusted value of 'a'.  Assuming SrcOver, a background color of B_rgb, and
+     * ignoring edge falloff, this becomes
+     *
+     *     (C_a - S_a*C_a)*C_rgb + (1 - (S_a + C_a - S_a*C_a))*B_rgb
+     *
+     * Since we use premultiplied alpha, this means we can scale the color by (C_a - S_a*C_a) and
+     * set the alpha to (S_a + C_a - S_a*C_a).
+     *
+     * @param r  Red value of color
+     * @param g  Red value of color
+     * @param b  Red value of color
+     * @param a  Red value of color
+     * @param colorScale  Factor to scale color values by
+     * @param tonalAlpha  Value to set alpha to
+     */
+    static inline void ComputeTonalColorParams(SkScalar r, SkScalar g, SkScalar b, SkScalar a,
+                                               SkScalar* colorScale, SkScalar* tonalAlpha) {
+        SkScalar max = SkTMax(SkTMax(r, g), b);
+        SkScalar min = SkTMin(SkTMin(r, g), b);
+        SkScalar luminance = 0.5f*(max + min);
+
+        // We get best results with a luminance between 0.3 and 0.5, with smoothstep applied
+        SkScalar adjustedLuminance = (0.6f - 0.4f*luminance)*luminance*luminance + 0.3f;
+        // Similarly, we need to tone down the given greyscale alpha depending on how
+        // much color we're applying.
+        a -= (0.5f*adjustedLuminance - 0.15f);
+
+        *colorScale = adjustedLuminance*(SK_Scalar1 - a);
+        *tonalAlpha = *colorScale + a;
+    }
 };
 
 #endif