[skottie] Improved Hue/Saturation effect

The current HueSaturation effect implementation relies on a simple
HSLA color matrix operation and assumes the controls are linear.

Turns out AE's saturation is more sophisticated, both in implementation
and in control mapping.

Updating the effect to use a chain of specialized color filters:

  - keep HSLAMatrix() for hue adjustments
  - introduce a custom runtime effect for saturation
    (following AE's semantics)
  - use a plain Matrix() CF for lightness adjustments

Change-Id: Iba6c9f7fd8c01dc33c1cd00822ea546867c057ac
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/452976
Commit-Queue: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

modules/skottie/src/effects/HueSaturationEffect.cpp

@@ -7,17 +7,71 @@
 
 #include "modules/skottie/src/effects/Effects.h"
 
+#include "include/effects/SkRuntimeEffect.h"
 #include "include/private/SkTPin.h"
 #include "modules/skottie/src/SkottieJson.h"
 #include "modules/skottie/src/SkottieValue.h"
 #include "modules/sksg/include/SkSGColorFilter.h"
 #include "src/utils/SkJSON.h"
 
-namespace skottie {
+namespace skottie::internal {
-namespace internal {
 
 namespace  {
 
+#ifdef SK_ENABLE_SKSL
+
+// AE Saturation semantics:
+//
+//   - saturation is applied as a component-wise scale (interpolation/extrapolation)
+//     relative to chroma mid point
+//   - the scale factor is clamped such that none of the components over/under saturates
+//     (e.g. below G/R and B are constrained to low_range and high_range, respectively)
+//   - the scale is also clammped to a maximum value of 126, empirically
+//   - the control is mapped linearly when desaturating, and non-linearly (1/1-S) when saturating
+//
+// 0               G    R                  B                                   1
+// |---------------+----+------------------+-----------------------------------|
+//                 |           |           |
+//                min         mid         max
+//                  <------- chroma ------>
+//  <------- low_range -------> <---------------- high_range ----------------->
+//
+// With some care, we can stay in premul for these calculations.
+static constexpr char gSaturateSkSL[] =
+"uniform half u_scale;"
+
+"half4 main(half4 c) {"
+    // component min/max
+    "half2 rg_srt = (c.r < c.g) ? c.rg : c.gr;"
+    "half c_min = min(rg_srt.x, c.b),"
+         "c_max = max(rg_srt.y, c.b),"
+
+    // chroma and mid-chroma (epsilon to avoid blowing up in the division below)
+    "ch     = max(c_max - c_min, 0.0001),"
+    "ch_mid = (c_min + c_max)*0.5,"
+
+    // clamp scale to the maximum value which doesn't over/under saturate individual components
+    "scale_max = min(ch_mid, c.a - ch_mid)/ch*2,"
+    "scale = min(u_scale, scale_max);"
+
+    // lerp
+    "c.rgb = ch_mid + (c.rgb - ch_mid)*scale;"
+
+    "return c;"
+"}";
+
+static sk_sp<SkColorFilter> make_saturate(float chroma_scale) {
+    static const auto* effect =
+            SkRuntimeEffect::MakeForColorFilter(SkString(gSaturateSkSL), {}).effect.release();
+    SkASSERT(effect);
+
+    return effect->makeColorFilter(SkData::MakeWithCopy(&chroma_scale, sizeof(chroma_scale)));
+}
+
+#else
+static sk_sp<SkColorFilter> make_saturate(float) { return nullptr; }
+#endif  // SK_ENABLE_SKSL
+
 class HueSaturationEffectAdapter final : public AnimatablePropertyContainer {
 public:
     static sk_sp<HueSaturationEffectAdapter> Make(const skjson::ArrayValue& jprops,
@@ -76,30 +130,57 @@ private:
             return nullptr;
         }
 
-        // AE semantics:
+        sk_sp<SkColorFilter> cf;
-        //
-        // master       hue [degrees]   => color.H offset
-        // master       sat [-100..100] => [-100..0) -> [0 .. color.S)
-        //                                 ( 0..100] -> (color.S .. 1]
-        // master lightness [-100..100] => [-100..0) -> [0 .. color.L]
-        //                                 ( 0..100] -> (color.L .. 1]
-        const auto h = fMasterHue / 360,
-                   s = SkTPin(fMasterSat   / 100, -1.0f, 1.0f),
-                   l = SkTPin(fMasterLight / 100, -1.0f, 1.0f),
-                   h_bias  = h,
-                   s_bias  = std::max(s, 0.0f),
-                   s_scale = 1 - std::abs(s),
-                   l_bias  = std::max(l, 0.0f),
-                   l_scale = 1 - std::abs(l);
 
-        const float hsl_cm[20] = {
+        if (!SkScalarNearlyZero(fMasterHue)) {
-            1,       0,       0, 0, h_bias,
+            // Linear control mapping hue(degrees) -> hue offset]
-            0, s_scale,       0, 0, s_bias,
+            const auto h = fMasterHue/360;
-            0,       0, l_scale, 0, l_bias,
-            0,       0,       0, 1,      0,
-        };
 
-        return SkColorFilters::HSLAMatrix(hsl_cm);
+            const float cm[20] = {
+                1, 0, 0, 0, h,
+                0, 1, 0, 0, 0,
+                0, 0, 1, 0, 0,
+                0, 0, 0, 1, 0,
+            };
+
+            cf = SkColorFilters::HSLAMatrix(cm);
+        }
+
+        if (!SkScalarNearlyZero(fMasterSat)) {
+            // AE clamps the max chroma scale to this value.
+            static constexpr auto kMaxScale = 126.0f;
+
+            // Control mapping:
+            //   * sat [-100 .. 0) -> scale [0 .. 1)   , linear
+            //   * sat  [0 .. 100] -> scale [1 .. max] , nonlinear: 100/(100 - sat)
+            const auto s            = SkTPin(fMasterSat/100, -1.0f, 1.0f),
+                       chroma_scale = s < 0 ? s + 1 : std::min(1/(1 - s), kMaxScale);
+
+            cf = SkColorFilters::Compose(std::move(cf), make_saturate(chroma_scale));
+        }
+
+        if (!SkScalarNearlyZero(fMasterLight)) {
+            // AE implements Lightness as a component-wise interpolation to 0 (for L < 0),
+            // or 1 (for L > 0).
+            //
+            // Control mapping:
+            //   * lightness [-100 .. 0) -> lerp[0 .. 1) from 0, linear
+            //   * lightness  [0 .. 100] -> lerp[1 .. 0] from 1, linear
+            const auto l  = SkTPin(fMasterLight/100, -1.0f, 1.0f),
+                       ls = 1 - std::abs(l),    // scale
+                       lo = l < 0 ? 0 : 1 - ls; // offset
+
+            const float cm[20] = {
+                ls,  0,  0, 0, lo,
+                 0, ls,  0, 0, lo,
+                 0,  0, ls, 0, lo,
+                 0,  0,  0, 1,  0,
+            };
+
+            cf = SkColorFilters::Compose(std::move(cf), SkColorFilters::Matrix(cm));
+        }
+
+        return cf;
     }
 
     const sk_sp<sksg::ExternalColorFilter> fColorFilter;
@@ -119,5 +200,4 @@ sk_sp<sksg::RenderNode> EffectBuilder::attachHueSaturationEffect(
                                                                           fBuilder);
 }
 
-} // namespace internal
+} // namespace skottie::internal
-} // namespace skottie