1
2/*
3 * Copyright 2017 Google Inc.
4 *
5 * Use of this source code is governed by a BSD-style license that can be
6 * found in the LICENSE file.
7 */
8#ifndef SkShadowUtils_DEFINED
9#define SkShadowUtils_DEFINED
10
11#include "SkColor.h"
12#include "SkScalar.h"
13#include "../private/SkShadowFlags.h"
14#include <functional>
15
16class SkCanvas;
17class SkPath;
18class SkResourceCache;
19
20class SkShadowUtils {
21public:
22    /**
23     * Draw an offset spot shadow and outlining ambient shadow for the given path using a disc
24     * light. The shadow may be cached, depending on the path type and canvas matrix. If the
25     * matrix is perspective or the path is volatile, it will not be cached.
26     *
27     * @param canvas  The canvas on which to draw the shadows.
28     * @param path  The occluder used to generate the shadows.
29     * @param zPlaneParams  Values for the plane function which returns the Z offset of the
30     *  occluder from the canvas based on local x and y values (the current matrix is not applied).
31     * @param lightPos  The 3D position of the light relative to the canvas plane. This is
32     *  independent of the canvas's current matrix.
33     * @param lightRadius  The radius of the disc light.
34     * @param ambientAlpha  The maximum alpha of the ambient shadow.
35     * @param spotAlpha  The maxium alpha of the spot shadow.
36     * @param color  The shadow color.
37     * @param flags  Options controlling opaque occluder optimizations and shadow appearance. See
38     *               SkShadowFlags.
39     */
40    static void DrawShadow(SkCanvas* canvas, const SkPath& path, const SkPoint3& zPlaneParams,
41                           const SkPoint3& lightPos, SkScalar lightRadius, SkScalar ambientAlpha,
42                           SkScalar spotAlpha, SkColor color,
43                           uint32_t flags = SkShadowFlags::kNone_ShadowFlag);
44
45    /**
46    * Draw an offset spot shadow and outlining ambient shadow for the given path using a disc
47    * light.
48    *
49    * Deprecated version with height value (to be removed when Flutter is updated).
50    *
51    * @param canvas  The canvas on which to draw the shadows.
52    * @param path  The occluder used to generate the shadows.
53    * @param occluderHeight  The vertical offset of the occluder from the canvas. This is
54    *  independent of the canvas's current matrix.
55    * @param lightPos  The 3D position of the light relative to the canvas plane. This is
56    *  independent of the canvas's current matrix.
57    * @param lightRadius  The radius of the disc light.
58    * @param ambientAlpha  The maximum alpha of the ambient shadow.
59    * @param spotAlpha  The maxium alpha of the spot shadow.
60    * @param color  The shadow color.
61    * @param flags  Options controlling opaque occluder optimizations and shadow appearance. See
62    *               SkShadowFlags.
63    */
64    static void DrawShadow(SkCanvas* canvas, const SkPath& path, SkScalar occluderHeight,
65                           const SkPoint3& lightPos, SkScalar lightRadius, SkScalar ambientAlpha,
66                           SkScalar spotAlpha, SkColor color,
67                           uint32_t flags = SkShadowFlags::kNone_ShadowFlag) {
68        SkPoint3 zPlane = SkPoint3::Make(0, 0, occluderHeight);
69        DrawShadow(canvas, path, zPlane, lightPos, lightRadius, ambientAlpha, spotAlpha,
70                   color, flags);
71    }
72
73    /**
74     * Helper routine to compute scale alpha values for one-pass tonal alpha.
75     *
76     * The final color we want to emulate is generated by rendering a color shadow (C_rgb) using an
77     * alpha computed from the color's luminance (C_a), and then a black shadow with alpha (S_a)
78     * which is an adjusted value of 'a'.  Assuming SrcOver, a background color of B_rgb, and
79     * ignoring edge falloff, this becomes
80     *
81     *     (C_a - S_a*C_a)*C_rgb + (1 - (S_a + C_a - S_a*C_a))*B_rgb
82     *
83     * Since we use premultiplied alpha, this means we can scale the color by (C_a - S_a*C_a) and
84     * set the alpha to (S_a + C_a - S_a*C_a).
85     *
86     * @param r  Red value of color
87     * @param g  Red value of color
88     * @param b  Red value of color
89     * @param a  Red value of color
90     * @param colorScale  Factor to scale color values by
91     * @param tonalAlpha  Value to set alpha to
92     */
93    static inline void ComputeTonalColorParams(SkScalar r, SkScalar g, SkScalar b, SkScalar a,
94                                               SkScalar* colorScale, SkScalar* tonalAlpha) {
95        SkScalar max = SkTMax(SkTMax(r, g), b);
96        SkScalar min = SkTMin(SkTMin(r, g), b);
97        SkScalar luminance = 0.5f*(max + min);
98
99        // We get best results with a luminance between 0.3 and 0.5, with smoothstep applied
100        SkScalar adjustedLuminance = (0.6f - 0.4f*luminance)*luminance*luminance + 0.3f;
101        // Similarly, we need to tone down the given greyscale alpha depending on how
102        // much color we're applying.
103        a -= (0.5f*adjustedLuminance - 0.15f);
104
105        *colorScale = adjustedLuminance*(SK_Scalar1 - a);
106        *tonalAlpha = *colorScale + a;
107    }
108};
109
110#endif
111