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 "modules/skottie/src/effects/Effects.h"
9
10 #include "include/core/SkColorFilter.h"
11 #include "include/effects/SkRuntimeEffect.h"
12 #include "include/private/SkTPin.h"
13 #include "modules/skottie/src/Adapter.h"
14 #include "modules/skottie/src/SkottieJson.h"
15 #include "modules/skottie/src/SkottieValue.h"
16 #include "modules/sksg/include/SkSGColorFilter.h"
17
18 namespace skottie::internal {
19
20 #ifdef SK_ENABLE_SKSL
21
22 namespace {
23
24 // The contrast effect transfer function can be approximated with the following
25 // 3rd degree polynomial:
26 //
27 // f(x) = -2πC/3 * x³ + πC * x² + (1 - πC/3) * x
28 //
29 // where C is the normalized contrast value [-1..1].
30 //
31 // Derivation:
32 //
33 // - start off with sampling the AE contrast effect for various contrast/input values [1]
34 //
35 // - apply cubic polynomial curve fitting to determine best-fit coefficients for given
36 // contrast values [2]
37 //
38 // - observations:
39 // * negative contrast appears clamped at -0.5 (-50)
40 // * a,b coefficients vary linearly vs. contrast
41 // * the b coefficient for max contrast (1.0) looks kinda familiar: 3.14757 - coincidence?
42 // probably not. let's run with it: b == πC
43 //
44 // - additionally, we expect the following to hold:
45 // * f(0 ) = 0 \ | d = 0
46 // * f(1 ) = 1 | => | a = -2b/3
47 // * f(0.5) = 0.5 / | c = 1 - b/3
48 //
49 // - this yields a pretty decent approximation: [3]
50 //
51 //
52 // Note (courtesy of mtklein, reed): [4] seems to yield a closer approximation, but requires
53 // a more expensive sin
54 //
55 // f(x) = x + a * sin(2πx)/2π
56 //
57 // [1] https://www.desmos.com/calculator/oksptqpo8z
58 // [2] https://www.desmos.com/calculator/oukrf6yahn
59 // [3] https://www.desmos.com/calculator/ehem0vy3ft
60 // [4] https://www.desmos.com/calculator/5t4xi10q4v
61 //
62
63 #ifndef SKOTTIE_ACCURATE_CONTRAST_APPROXIMATION
make_contrast_coeffs(float contrast)64 static sk_sp<SkData> make_contrast_coeffs(float contrast) {
65 struct { float a, b, c; } coeffs;
66
67 coeffs.b = SK_ScalarPI * contrast;
68 coeffs.a = -2 * coeffs.b / 3;
69 coeffs.c = 1 - coeffs.b / 3;
70
71 return SkData::MakeWithCopy(&coeffs, sizeof(coeffs));
72 }
73
74 static constexpr char CONTRAST_EFFECT[] = R"(
75 uniform half a;
76 uniform half b;
77 uniform half c;
78
79 half4 main(half4 color) {
80 // C' = a*C^3 + b*C^2 + c*C
81 color.rgb = ((a*color.rgb + b)*color.rgb + c)*color.rgb;
82 return color;
83 }
84 )";
85 #else
86 // More accurate (but slower) approximation:
87 //
88 // f(x) = x + a * sin(2πx)
89 //
90 // a = -contrast/3π
91 //
92 static sk_sp<SkData> make_contrast_coeffs(float contrast) {
93 const auto coeff_a = -contrast / (3 * SK_ScalarPI);
94
95 return SkData::MakeWithCopy(&coeff_a, sizeof(coeff_a));
96 }
97
98 static constexpr char CONTRAST_EFFECT[] = R"(
99 uniform half a;
100
101 half4 main(half4 color) {
102 color.rgb += a * sin(color.rgb * 6.283185);
103 return color;
104 }
105 )";
106
107 #endif
108
109 // Brightness transfer function approximation:
110 //
111 // f(x) = 1 - (1 - x)^(2^(1.8*B))
112 //
113 // where B is the normalized [-1..1] brightness value
114 //
115 // Visualization: https://www.desmos.com/calculator/wuyqa2wtol
116 //
make_brightness_coeffs(float brightness)117 static sk_sp<SkData> make_brightness_coeffs(float brightness) {
118 const float coeff_a = std::pow(2.0f, brightness * 1.8f);
119
120 return SkData::MakeWithCopy(&coeff_a, sizeof(coeff_a));
121 }
122
123 static constexpr char BRIGHTNESS_EFFECT[] = R"(
124 uniform half a;
125
126 half4 main(half4 color) {
127 color.rgb = 1 - pow(1 - color.rgb, half3(a));
128 return color;
129 }
130 )";
131
132 class BrightnessContrastAdapter final : public DiscardableAdapterBase<BrightnessContrastAdapter,
133 sksg::ExternalColorFilter> {
134 public:
BrightnessContrastAdapter(const skjson::ArrayValue & jprops,const AnimationBuilder & abuilder,sk_sp<sksg::RenderNode> layer)135 BrightnessContrastAdapter(const skjson::ArrayValue& jprops,
136 const AnimationBuilder& abuilder,
137 sk_sp<sksg::RenderNode> layer)
138 : INHERITED(sksg::ExternalColorFilter::Make(std::move(layer)))
139 , fBrightnessEffect(SkRuntimeEffect::MakeForColorFilter(SkString(BRIGHTNESS_EFFECT)).effect)
140 , fContrastEffect(SkRuntimeEffect::MakeForColorFilter(SkString(CONTRAST_EFFECT)).effect) {
141 SkASSERT(fBrightnessEffect);
142 SkASSERT(fContrastEffect);
143
144 enum : size_t {
145 kBrightness_Index = 0,
146 kContrast_Index = 1,
147 kUseLegacy_Index = 2,
148 };
149
150 EffectBinder(jprops, abuilder, this)
151 .bind(kBrightness_Index, fBrightness)
152 .bind( kContrast_Index, fContrast )
153 .bind( kUseLegacy_Index, fUseLegacy );
154 }
155
156 private:
onSync()157 void onSync() override {
158 this->node()->setColorFilter(SkScalarRoundToInt(fUseLegacy)
159 ? this->makeLegacyCF()
160 : this->makeCF());
161 }
162
makeLegacyCF() const163 sk_sp<SkColorFilter> makeLegacyCF() const {
164 // In 'legacy' mode, brightness is
165 //
166 // - in the [-100..100] range
167 // - applied component-wise as a direct offset (255-based)
168 // - (neutral value: 0)
169 // - transfer function: https://www.desmos.com/calculator/zne0oqwwzb
170 //
171 // while contrast is
172 //
173 // - in the [-100..100] range
174 // - applied as a component-wise linear transformation (scale+offset), such that
175 //
176 // -100 always yields mid-gray: contrast(x, -100) == 0.5
177 // 0 is the neutral value: contrast(x, 0) == x
178 // 100 always yields white: contrast(x, 100) == 1
179 //
180 // - transfer function: https://www.desmos.com/calculator/x5rxzhowhs
181 //
182
183 // Normalize to [-1..1]
184 const auto brightness = SkTPin(fBrightness, -100.0f, 100.0f) / 255, // [-100/255 .. 100/255]
185 contrast = SkTPin(fContrast , -100.0f, 100.0f) / 100; // [ -1 .. 1]
186
187 // The component scale is derived from contrast:
188 //
189 // Contrast[-1 .. 0] -> Scale[0 .. 1]
190 // Contrast( 0 .. 1] -> Scale(1 .. +inf)
191 const auto S = contrast > 0
192 ? 1 / std::max(1 - contrast, SK_ScalarNearlyZero)
193 : 1 + contrast;
194
195 // The component offset is derived from both brightness and contrast:
196 //
197 // Brightness[-100/255 .. 100/255] -> Offset[-100/255 .. 100/255]
198 // Contrast [ -1 .. 0] -> Offset[ 0.5 .. 0]
199 // Contrast ( 0 .. 1] -> Offset( 0 .. -inf)
200 //
201 // Why do these pre/post compose depending on contrast scale, you ask?
202 // Because AE - that's why!
203 const auto B = 0.5f * (1 - S) + brightness * std::max(S, 1.0f);
204
205 const float cm[] = {
206 S, 0, 0, 0, B,
207 0, S, 0, 0, B,
208 0, 0, S, 0, B,
209 0, 0, 0, 1, 0,
210 };
211
212 return SkColorFilters::Matrix(cm);
213 }
214
makeCF() const215 sk_sp<SkColorFilter> makeCF() const {
216 const auto brightness = SkTPin(fBrightness, -150.0f, 150.0f) / 150, // [-1.0 .. 1]
217 contrast = SkTPin(fContrast , -50.0f, 100.0f) / 100; // [-0.5 .. 1]
218
219 auto b_eff = SkScalarNearlyZero(brightness)
220 ? nullptr
221 : fBrightnessEffect->makeColorFilter(make_brightness_coeffs(brightness)),
222 c_eff = SkScalarNearlyZero(fContrast)
223 ? nullptr
224 : fContrastEffect->makeColorFilter(make_contrast_coeffs(contrast));
225
226 return SkColorFilters::Compose(std::move(c_eff), std::move(b_eff));
227 }
228
229 const sk_sp<SkRuntimeEffect> fBrightnessEffect,
230 fContrastEffect;
231
232 ScalarValue fBrightness = 0,
233 fContrast = 0,
234 fUseLegacy = 0;
235
236 using INHERITED = DiscardableAdapterBase<BrightnessContrastAdapter, sksg::ExternalColorFilter>;
237 };
238
239 } // namespace
240
241 #endif // SK_ENABLE_SKSL
242
attachBrightnessContrastEffect(const skjson::ArrayValue & jprops,sk_sp<sksg::RenderNode> layer) const243 sk_sp<sksg::RenderNode> EffectBuilder::attachBrightnessContrastEffect(
244 const skjson::ArrayValue& jprops, sk_sp<sksg::RenderNode> layer) const {
245 #ifdef SK_ENABLE_SKSL
246 return fBuilder->attachDiscardableAdapter<BrightnessContrastAdapter>(jprops,
247 *fBuilder,
248 std::move(layer));
249 #else
250 // TODO(skia:12197)
251 return layer;
252 #endif
253 }
254
255 } // namespace skottie::internal
256