Convert Sk4fGradientInterval to (bias, factor) format

And rename some fileds:

 * p0, p1   -> t0, t1
 * fC0, fDc -> fCb, fCg

Change-Id: I94b520f83d8f21d62c9c24740b6d3e2dbc3a9ff7
Reviewed-on: https://skia-review.googlesource.com/11799
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>

3 files changed, 62 insertions, 57 deletions

diff --git a/src/effects/gradients/Sk4fGradientBase.cpp b/src/effects/gradients/Sk4fGradientBase.cpp
index b385d8cd23..81442df048 100644
--- a/src/effects/gradients/Sk4fGradientBase.cpp
+++ b/src/effects/gradients/Sk4fGradientBase.cpp
@@ -101,39 +101,41 @@ void addMirrorIntervals(const SkColor colors[],
                         bool premulColors, bool reverse,
                         Sk4fGradientIntervalBuffer::BufferType* buffer) {
     const IntervalIterator iter(colors, pos, count, reverse);
-    iter.iterate([&] (SkColor c0, SkColor c1, SkScalar p0, SkScalar p1) {
-        SkASSERT(buffer->empty() || buffer->back().fP1 == 2 - p0);
+    iter.iterate([&] (SkColor c0, SkColor c1, SkScalar t0, SkScalar t1) {
+        SkASSERT(buffer->empty() || buffer->back().fT1 == 2 - t0);
 
-        const auto mirror_p0 = 2 - p0;
-        const auto mirror_p1 = 2 - p1;
+        const auto mirror_t0 = 2 - t0;
+        const auto mirror_t1 = 2 - t1;
         // mirror_p1 & mirror_p1 may collapse for very small values - recheck to avoid
         // triggering Interval asserts.
-        if (mirror_p0 != mirror_p1) {
-            buffer->emplace_back(pack_color(c0, premulColors, componentScale), mirror_p0,
-                                 pack_color(c1, premulColors, componentScale), mirror_p1);
+        if (mirror_t0 != mirror_t1) {
+            buffer->emplace_back(pack_color(c0, premulColors, componentScale), mirror_t0,
+                                 pack_color(c1, premulColors, componentScale), mirror_t1);
         }
     });
 }
 
 } // anonymous namespace
 
-Sk4fGradientInterval::Sk4fGradientInterval(const Sk4f& c0, SkScalar p0,
-                                           const Sk4f& c1, SkScalar p1)
-    : fP0(p0)
-    , fP1(p1)
+Sk4fGradientInterval::Sk4fGradientInterval(const Sk4f& c0, SkScalar t0,
+                                           const Sk4f& c1, SkScalar t1)
+    : fT0(t0)
+    , fT1(t1)
     , fZeroRamp((c0 == c1).allTrue()) {
-    SkASSERT(p0 != p1);
+    SkASSERT(t0 != t1);
     // Either p0 or p1 can be (-)inf for synthetic clamp edge intervals.
-    SkASSERT(SkScalarIsFinite(p0) || SkScalarIsFinite(p1));
+    SkASSERT(SkScalarIsFinite(t0) || SkScalarIsFinite(t1));
 
-    const auto dp = p1 - p0;
+    const auto dt = t1 - t0;
 
     // Clamp edge intervals are always zero-ramp.
-    SkASSERT(SkScalarIsFinite(dp) || fZeroRamp);
-    const Sk4f dc = SkScalarIsFinite(dp) ? (c1 - c0) / dp : 0;
+    SkASSERT(SkScalarIsFinite(dt) || fZeroRamp);
+    SkASSERT(SkScalarIsFinite(t0) || fZeroRamp);
+    const Sk4f   dc = SkScalarIsFinite(dt) ? (c1 - c0) / dt : 0;
+    const Sk4f bias = c0 - (SkScalarIsFinite(t0) ? t0 * dc : 0);
 
-    c0.store(&fC0.fVec);
-    dc.store(&fDc.fVec);
+    bias.store(&fCb.fVec);
+    dc.store(&fCg.fVec);
 }
 
 void Sk4fGradientIntervalBuffer::init(const SkColor colors[], const SkScalar pos[], int count,
@@ -206,11 +208,11 @@ void Sk4fGradientIntervalBuffer::init(const SkColor colors[], const SkScalar pos
     }
 
     const IntervalIterator iter(colors, pos, count, reverse);
-    iter.iterate([&] (SkColor c0, SkColor c1, SkScalar p0, SkScalar p1) {
-        SkASSERT(fIntervals.empty() || fIntervals.back().fP1 == p0);
+    iter.iterate([&] (SkColor c0, SkColor c1, SkScalar t0, SkScalar t1) {
+        SkASSERT(fIntervals.empty() || fIntervals.back().fT1 == t0);
 
-        fIntervals.emplace_back(pack_color(c0, premulColors, componentScale), p0,
-                                pack_color(c1, premulColors, componentScale), p1);
+        fIntervals.emplace_back(pack_color(c0, premulColors, componentScale), t0,
+                                pack_color(c1, premulColors, componentScale), t1);
     });
 
     if (tileMode == SkShader::kClamp_TileMode) {
@@ -232,11 +234,11 @@ const Sk4fGradientInterval* Sk4fGradientIntervalBuffer::find(SkScalar t) const {
 
     while (i0 != i1) {
         SkASSERT(i0 < i1);
-        SkASSERT(t >= i0->fP0 && t <= i1->fP1);
+        SkASSERT(t >= i0->fT0 && t <= i1->fT1);
 
         const auto* i = i0 + ((i1 - i0) >> 1);
 
-        if (t > i->fP1) {
+        if (t > i->fT1) {
             i0 = i + 1;
         } else {
             i1 = i;
@@ -252,7 +254,7 @@ const Sk4fGradientInterval* Sk4fGradientIntervalBuffer::findNext(
 
     SkASSERT(!prev->contains(t));
     SkASSERT(prev >= fIntervals.begin() && prev < fIntervals.end());
-    SkASSERT(t >= fIntervals.front().fP0 && t <= fIntervals.back().fP1);
+    SkASSERT(t >= fIntervals.front().fT0 && t <= fIntervals.back().fT1);
 
     const auto* i = prev;
 
@@ -433,18 +435,18 @@ private:
 
     Sk4f lerp(SkScalar t) {
         SkASSERT(fInterval->contains(t));
-        return fCc + fDc * (t - fInterval->fP0);
+        return fCb + fCg * t;
     }
 
     void loadIntervalData(const Sk4fGradientInterval* i) {
-        fCc = DstTraits<dstType, premul>::load(i->fC0);
-        fDc = DstTraits<dstType, premul>::load(i->fDc);
+        fCb = DstTraits<dstType, premul>::load(i->fCb);
+        fCg = DstTraits<dstType, premul>::load(i->fCg);
     }
 
     const GradientShaderBase4fContext& fCtx;
     const Sk4fGradientInterval*        fInterval;
     SkScalar                           fPrevT;
     SkScalar                           fLargestIntervalValue;
-    Sk4f                               fCc;
-    Sk4f                               fDc;
+    Sk4f                               fCb;
+    Sk4f                               fCg;
 };
diff --git a/src/effects/gradients/Sk4fGradientBase.h b/src/effects/gradients/Sk4fGradientBase.h
index 2826f9e7b7..fcdcd9e8c7 100644
--- a/src/effects/gradients/Sk4fGradientBase.h
+++ b/src/effects/gradients/Sk4fGradientBase.h
@@ -18,18 +18,21 @@
 #include "SkTArray.h"
 
 struct Sk4fGradientInterval {
-    Sk4fGradientInterval(const Sk4f& c0, SkScalar p0,
-                         const Sk4f& c1, SkScalar p1);
+    Sk4fGradientInterval(const Sk4f& c0, SkScalar t0,
+                         const Sk4f& c1, SkScalar t1);
 
     bool contains(SkScalar t) const {
         // True if t is in [p0,p1].  Note: this helper assumes a
         // natural/increasing interval - so it's not usable in Sk4fLinearGradient.
-        SkASSERT(fP0 < fP1);
-        return t >= fP0 && t <= fP1;
+        SkASSERT(fT0 < fT1);
+        return t >= fT0 && t <= fT1;
     }
 
-    SkPM4f   fC0, fDc;
-    SkScalar fP0, fP1;
+    // Color bias and color gradient, such that for a t in this interval
+    //
+    //   C = fCb + t * fCg;
+    SkPM4f   fCb, fCg;
+    SkScalar fT0, fT1;
     bool     fZeroRamp;
 };
 
diff --git a/src/effects/gradients/Sk4fLinearGradient.cpp b/src/effects/gradients/Sk4fLinearGradient.cpp
index 1256b8f6da..fe900e8d03 100644
--- a/src/effects/gradients/Sk4fLinearGradient.cpp
+++ b/src/effects/gradients/Sk4fLinearGradient.cpp
@@ -138,14 +138,14 @@ LinearGradient4fContext::LinearGradient4fContext(const SkLinearGradient& shader,
 
 const Sk4fGradientInterval*
 SkLinearGradient::LinearGradient4fContext::findInterval(SkScalar fx) const {
-    SkASSERT(in_range(fx, fIntervals->front().fP0, fIntervals->back().fP1));
+    SkASSERT(in_range(fx, fIntervals->front().fT0, fIntervals->back().fT1));
 
     if (1) {
         // Linear search, using the last scanline interval as a starting point.
         SkASSERT(fCachedInterval >= fIntervals->begin());
         SkASSERT(fCachedInterval < fIntervals->end());
         const int search_dir = fDstToPos.getScaleX() >= 0 ? 1 : -1;
-        while (!in_range(fx, fCachedInterval->fP0, fCachedInterval->fP1)) {
+        while (!in_range(fx, fCachedInterval->fT0, fCachedInterval->fT1)) {
             fCachedInterval += search_dir;
             if (fCachedInterval >= fIntervals->end()) {
                 fCachedInterval = fIntervals->begin();
@@ -161,19 +161,19 @@ SkLinearGradient::LinearGradient4fContext::findInterval(SkScalar fx) const {
 
         while (i0 != i1) {
             SkASSERT(i0 < i1);
-            SkASSERT(in_range(fx, i0->fP0, i1->fP1));
+            SkASSERT(in_range(fx, i0->fT0, i1->fT1));
 
             const auto* i = i0 + ((i1 - i0) >> 1);
 
-            if (in_range(fx, i0->fP0, i->fP1)) {
+            if (in_range(fx, i0->fT0, i->fT1)) {
                 i1 = i;
             } else {
-                SkASSERT(in_range(fx, i->fP1, i1->fP1));
+                SkASSERT(in_range(fx, i->fT1, i1->fT1));
                 i0 = i + 1;
             }
         }
 
-        SkASSERT(in_range(fx, i0->fP0, i0->fP1));
+        SkASSERT(in_range(fx, i0->fT0, i0->fT1));
         return i0;
     }
 }
@@ -296,7 +296,7 @@ public:
                             SkScalar fx,
                             SkScalar dx,
                             bool is_vertical)
-        : fAdvX(is_vertical ? SK_ScalarInfinity : (i->fP1 - fx) / dx)
+        : fAdvX(is_vertical ? SK_ScalarInfinity : (i->fT1 - fx) / dx)
         , fFirstInterval(firstInterval)
         , fLastInterval(lastInterval)
         , fInterval(i)
@@ -305,10 +305,9 @@ public:
     {
         SkASSERT(fAdvX >= 0);
         SkASSERT(firstInterval <= lastInterval);
-        SkASSERT(in_range(fx, i->fP0, i->fP1));
 
         if (tileMode != kClamp_TileMode && !is_vertical) {
-            const auto spanX = (lastInterval->fP1 - firstInterval->fP0) / dx;
+            const auto spanX = (lastInterval->fT1 - firstInterval->fT0) / dx;
             SkASSERT(spanX >= 0);
 
             // If we're in a repeating tile mode and the whole gradient is compressed into a
@@ -322,12 +321,12 @@ public:
             }
         }
 
-        this->compute_interval_props(fx - i->fP0);
+        this->compute_interval_props(fx);
     }
 
     SkScalar currentAdvance() const {
         SkASSERT(fAdvX >= 0);
-        SkASSERT(fAdvX <= (fInterval->fP1 - fInterval->fP0) / fDx || !std::isfinite(fAdvX));
+        SkASSERT(fAdvX <= (fInterval->fT1 - fInterval->fT0) / fDx || !std::isfinite(fAdvX));
         return fAdvX;
     }
 
@@ -350,13 +349,15 @@ public:
 
 private:
     void compute_interval_props(SkScalar t) {
+        SkASSERT(in_range(t, fInterval->fT0, fInterval->fT1));
+
         fZeroRamp     = fIsVertical || fInterval->fZeroRamp;
-        fCc           = DstTraits<dstType, premul>::load(fInterval->fC0);
+        fCc           = DstTraits<dstType, premul>::load(fInterval->fCb);
 
         if (fInterval->fZeroRamp) {
             fDcDx = 0;
         } else {
-            const Sk4f dC = DstTraits<dstType, premul>::load(fInterval->fDc);
+            const Sk4f dC = DstTraits<dstType, premul>::load(fInterval->fCg);
             fCc           = fCc + dC * Sk4f(t);
             fDcDx         = dC * fDx;
         }
@@ -372,16 +373,15 @@ private:
         for (const auto* i = fFirstInterval; i <= fLastInterval; ++i) {
             // Each interval contributes its average color to the total/weighted average:
             //
-            //   C = (c0 + c1) / 2 = (c0 + c0 + dc * (p1 - p0)) / 2
+            //   C = (c0 + c1) / 2 = (Cb + Cg * t0 + Cb + Cg * t1) / 2 = Cb + Cg *(t0 + t1) / 2
             //
-            //   Avg += C * (p1 - p0)
+            //   Avg += C * (t1 - t0)
             //
-            const auto dp = i->fP1 - i->fP0;
-            auto c = DstTraits<dstType, premul>::load(i->fC0);
+            auto c = DstTraits<dstType, premul>::load(i->fCb);
             if (!i->fZeroRamp) {
-                c = c + DstTraits<dstType, premul>::load(i->fDc) * dp * 0.5f;
+                c = c + DstTraits<dstType, premul>::load(i->fCg) * (i->fT0 + i->fT1) * 0.5f;
             }
-            fCc = fCc + c * dp;
+            fCc = fCc + c * (i->fT1 - i->fT0);
         }
     }
 
@@ -404,11 +404,11 @@ private:
         do {
             advX -= fAdvX;
             fInterval = this->next_interval(fInterval);
-            fAdvX = (fInterval->fP1 - fInterval->fP0) / fDx;
+            fAdvX = (fInterval->fT1 - fInterval->fT0) / fDx;
             SkASSERT(fAdvX > 0);
         } while (advX >= fAdvX);
 
-        compute_interval_props(0);
+        compute_interval_props(fInterval->fT0);
 
         SkASSERT(advX >= 0);
         return advX;