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