1 /*
2 * Copyright 2020 Google Inc.
3 *
4 * Use of this source code is governed by a BSD-style license that can be
5 * found in the LICENSE file.
6 */
7
8 #include "include/core/SkColorFilter.h"
9 #include "include/core/SkData.h"
10 #include "include/core/SkRefCnt.h"
11 #include "include/core/SkScalar.h"
12 #include "include/core/SkString.h"
13 #include "include/effects/SkRuntimeEffect.h"
14 #include "include/private/base/SkAssert.h"
15 #include "include/private/base/SkTPin.h"
16 #include "modules/skottie/src/Adapter.h"
17 #include "modules/skottie/src/SkottiePriv.h"
18 #include "modules/skottie/src/SkottieValue.h"
19 #include "modules/skottie/src/effects/Effects.h"
20 #include "modules/sksg/include/SkSGColorFilter.h"
21 #include "modules/sksg/include/SkSGRenderNode.h"
22
23 #include <algorithm>
24 #include <cmath>
25 #include <cstddef>
26 #include <utility>
27
28 namespace skjson {
29 class ArrayValue;
30 }
31
32 namespace skottie::internal {
33 namespace {
34
35 // The contrast effect transfer function can be approximated with the following
36 // 3rd degree polynomial:
37 //
38 // f(x) = -2πC/3 * x³ + πC * x² + (1 - πC/3) * x
39 //
40 // where C is the normalized contrast value [-1..1].
41 //
42 // Derivation:
43 //
44 // - start off with sampling the AE contrast effect for various contrast/input values [1]
45 //
46 // - apply cubic polynomial curve fitting to determine best-fit coefficients for given
47 // contrast values [2]
48 //
49 // - observations:
50 // * negative contrast appears clamped at -0.5 (-50)
51 // * a,b coefficients vary linearly vs. contrast
52 // * the b coefficient for max contrast (1.0) looks kinda familiar: 3.14757 - coincidence?
53 // probably not. let's run with it: b == πC
54 //
55 // - additionally, we expect the following to hold:
56 // * f(0 ) = 0 \ | d = 0
57 // * f(1 ) = 1 | => | a = -2b/3
58 // * f(0.5) = 0.5 / | c = 1 - b/3
59 //
60 // - this yields a pretty decent approximation: [3]
61 //
62 //
63 // Note (courtesy of mtklein, reed): [4] seems to yield a closer approximation, but requires
64 // a more expensive sin
65 //
66 // f(x) = x + a * sin(2πx)/2π
67 //
68 // [1] https://www.desmos.com/calculator/oksptqpo8z
69 // [2] https://www.desmos.com/calculator/oukrf6yahn
70 // [3] https://www.desmos.com/calculator/ehem0vy3ft
71 // [4] https://www.desmos.com/calculator/5t4xi10q4v
72 //
73
74 #ifndef SKOTTIE_ACCURATE_CONTRAST_APPROXIMATION
make_contrast_coeffs(float contrast)75 static sk_sp<SkData> make_contrast_coeffs(float contrast) {
76 struct { float a, b, c; } coeffs;
77
78 coeffs.b = SK_ScalarPI * contrast;
79 coeffs.a = -2 * coeffs.b / 3;
80 coeffs.c = 1 - coeffs.b / 3;
81
82 return SkData::MakeWithCopy(&coeffs, sizeof(coeffs));
83 }
84
85 static constexpr char CONTRAST_EFFECT[] =
86 "uniform half a;"
87 "uniform half b;"
88 "uniform half c;"
89
90 "half4 main(half4 color) {"
91 // C' = a*C^3 + b*C^2 + c*C
92 "color.rgb = ((a*color.rgb + b)*color.rgb + c)*color.rgb;"
93 "return color;"
94 "}"
95 ;
96 #else
97 // More accurate (but slower) approximation:
98 //
99 // f(x) = x + a * sin(2πx)
100 //
101 // a = -contrast/3π
102 //
103 static sk_sp<SkData> make_contrast_coeffs(float contrast) {
104 const auto coeff_a = -contrast / (3 * SK_ScalarPI);
105
106 return SkData::MakeWithCopy(&coeff_a, sizeof(coeff_a));
107 }
108
109 static constexpr char CONTRAST_EFFECT[] =
110 "uniform half a;"
111
112 "half4 main(half4 color) {"
113 "color.rgb += a * sin(color.rgb * 6.283185);"
114 "return color;"
115 "}"
116 ;
117
118 #endif
119
120 // Brightness transfer function approximation:
121 //
122 // f(x) = 1 - (1 - x)^(2^(1.8*B))
123 //
124 // where B is the normalized [-1..1] brightness value
125 //
126 // Visualization: https://www.desmos.com/calculator/wuyqa2wtol
127 //
make_brightness_coeffs(float brightness)128 static sk_sp<SkData> make_brightness_coeffs(float brightness) {
129 const float coeff_a = std::pow(2.0f, brightness * 1.8f);
130
131 return SkData::MakeWithCopy(&coeff_a, sizeof(coeff_a));
132 }
133
134 static constexpr char BRIGHTNESS_EFFECT[] =
135 "uniform half a;"
136
137 "half4 main(half4 color) {"
138 "color.rgb = 1 - pow(1 - color.rgb, half3(a));"
139 "return color;"
140 "}"
141 ;
142
143 class BrightnessContrastAdapter final : public DiscardableAdapterBase<BrightnessContrastAdapter,
144 sksg::ExternalColorFilter> {
145 public:
BrightnessContrastAdapter(const skjson::ArrayValue & jprops,const AnimationBuilder & abuilder,sk_sp<sksg::RenderNode> layer)146 BrightnessContrastAdapter(const skjson::ArrayValue& jprops,
147 const AnimationBuilder& abuilder,
148 sk_sp<sksg::RenderNode> layer)
149 : INHERITED(sksg::ExternalColorFilter::Make(std::move(layer)))
150 , fBrightnessEffect(SkRuntimeEffect::MakeForColorFilter(SkString(BRIGHTNESS_EFFECT)).effect)
151 , fContrastEffect(SkRuntimeEffect::MakeForColorFilter(SkString(CONTRAST_EFFECT)).effect) {
152 SkASSERT(fBrightnessEffect);
153 SkASSERT(fContrastEffect);
154
155 enum : size_t {
156 kBrightness_Index = 0,
157 kContrast_Index = 1,
158 kUseLegacy_Index = 2,
159 };
160
161 EffectBinder(jprops, abuilder, this)
162 .bind(kBrightness_Index, fBrightness)
163 .bind( kContrast_Index, fContrast )
164 .bind( kUseLegacy_Index, fUseLegacy );
165 }
166
167 private:
onSync()168 void onSync() override {
169 this->node()->setColorFilter(SkScalarRoundToInt(fUseLegacy)
170 ? this->makeLegacyCF()
171 : this->makeCF());
172 }
173
makeLegacyCF() const174 sk_sp<SkColorFilter> makeLegacyCF() const {
175 // In 'legacy' mode, brightness is
176 //
177 // - in the [-100..100] range
178 // - applied component-wise as a direct offset (255-based)
179 // - (neutral value: 0)
180 // - transfer function: https://www.desmos.com/calculator/zne0oqwwzb
181 //
182 // while contrast is
183 //
184 // - in the [-100..100] range
185 // - applied as a component-wise linear transformation (scale+offset), such that
186 //
187 // -100 always yields mid-gray: contrast(x, -100) == 0.5
188 // 0 is the neutral value: contrast(x, 0) == x
189 // 100 always yields white: contrast(x, 100) == 1
190 //
191 // - transfer function: https://www.desmos.com/calculator/x5rxzhowhs
192 //
193
194 // Normalize to [-1..1]
195 const auto brightness = SkTPin(fBrightness, -100.0f, 100.0f) / 255, // [-100/255 .. 100/255]
196 contrast = SkTPin(fContrast , -100.0f, 100.0f) / 100; // [ -1 .. 1]
197
198 // The component scale is derived from contrast:
199 //
200 // Contrast[-1 .. 0] -> Scale[0 .. 1]
201 // Contrast( 0 .. 1] -> Scale(1 .. +inf)
202 const auto S = contrast > 0
203 ? 1 / std::max(1 - contrast, SK_ScalarNearlyZero)
204 : 1 + contrast;
205
206 // The component offset is derived from both brightness and contrast:
207 //
208 // Brightness[-100/255 .. 100/255] -> Offset[-100/255 .. 100/255]
209 // Contrast [ -1 .. 0] -> Offset[ 0.5 .. 0]
210 // Contrast ( 0 .. 1] -> Offset( 0 .. -inf)
211 //
212 // Why do these pre/post compose depending on contrast scale, you ask?
213 // Because AE - that's why!
214 const auto B = 0.5f * (1 - S) + brightness * std::max(S, 1.0f);
215
216 const float cm[] = {
217 S, 0, 0, 0, B,
218 0, S, 0, 0, B,
219 0, 0, S, 0, B,
220 0, 0, 0, 1, 0,
221 };
222
223 return SkColorFilters::Matrix(cm);
224 }
225
makeCF() const226 sk_sp<SkColorFilter> makeCF() const {
227 const auto brightness = SkTPin(fBrightness, -150.0f, 150.0f) / 150, // [-1.0 .. 1]
228 contrast = SkTPin(fContrast , -50.0f, 100.0f) / 100; // [-0.5 .. 1]
229
230 auto b_eff = SkScalarNearlyZero(brightness)
231 ? nullptr
232 : fBrightnessEffect->makeColorFilter(make_brightness_coeffs(brightness)),
233 c_eff = SkScalarNearlyZero(fContrast)
234 ? nullptr
235 : fContrastEffect->makeColorFilter(make_contrast_coeffs(contrast));
236
237 return SkColorFilters::Compose(std::move(c_eff), std::move(b_eff));
238 }
239
240 const sk_sp<SkRuntimeEffect> fBrightnessEffect,
241 fContrastEffect;
242
243 ScalarValue fBrightness = 0,
244 fContrast = 0,
245 fUseLegacy = 0;
246
247 using INHERITED = DiscardableAdapterBase<BrightnessContrastAdapter, sksg::ExternalColorFilter>;
248 };
249
250 } // namespace
251
attachBrightnessContrastEffect(const skjson::ArrayValue & jprops,sk_sp<sksg::RenderNode> layer) const252 sk_sp<sksg::RenderNode> EffectBuilder::attachBrightnessContrastEffect(
253 const skjson::ArrayValue& jprops, sk_sp<sksg::RenderNode> layer) const {
254 return fBuilder->attachDiscardableAdapter<BrightnessContrastAdapter>(jprops,
255 *fBuilder,
256 std::move(layer));
257 }
258
259 } // namespace skottie::internal
260