Implement multi-color-stops in linear gradients using Sk4f

src/effects/gradients/SkLinearGradient.cpp

Index: src/effects/gradients/SkLinearGradient.cpp
diff --git a/src/effects/gradients/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index ada9d70b6a19af49520d1eaaebd9fad6678704f0..fdc016c0d2cf601714d556380f97b691e8610d88 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -7,6 +7,8 @@
 
 #include "SkLinearGradient.h"
 
+static const float kInv255Float = 1.0f / 255;
+
 static inline int repeat_bits(int x, const int bits) {
     return x & ((1 << bits) - 1);
 }
@@ -86,20 +88,86 @@ SkShader::Context* SkLinearGradient::onCreateContext(const ContextRec& rec, void
     return new (storage) LinearGradientContext(*this, rec);
 }
 
+// This swizzles SkColor into the same component order as SkPMColor, but does not actually
+// "pre" multiply the color components.
+//
+// This allows us to map directly to Sk4f, and eventually scale down to bytes to output a
+// SkPMColor from the floats, without having to swizzle each time.
+//
+static uint32_t SkSwizzle_Color_to_PMColor(SkColor c) {
+    return SkPackARGB32NoCheck(SkColorGetA(c), SkColorGetR(c), SkColorGetG(c), SkColorGetB(c));
+}
+
 SkLinearGradient::LinearGradientContext::LinearGradientContext(
-        const SkLinearGradient& shader, const ContextRec& rec)
-    : INHERITED(shader, rec)
+        const SkLinearGradient& shader, const ContextRec& ctx)
+    : INHERITED(shader, ctx)
 {
     unsigned mask = SkMatrix::kTranslate_Mask | SkMatrix::kScale_Mask;
     if ((fDstToIndex.getType() & ~mask) == 0) {
         // when we dither, we are (usually) not const-in-Y
-        if ((fFlags & SkShader::kHasSpan16_Flag) && !rec.fPaint->isDither()) {
+        if ((fFlags & SkShader::kHasSpan16_Flag) && !ctx.fPaint->isDither()) {
             // only claim this if we do have a 16bit mode (i.e. none of our
             // colors have alpha), and if we are not dithering (which obviously
             // is not const in Y).
             fFlags |= SkShader::kConstInY16_Flag;
         }
     }
+
+    // setup for Sk4f
+    int count = shader.fColorCount;
+    fRecs.setCount(count);
+    Rec* rec = fRecs.begin();
+    if (shader.fOrigPos) {
+        rec[0].fPos = 0;
+        SkDEBUGCODE(rec[0].fPosScale = SK_FloatNaN;)   // should never get used
+        for (int i = 1; i < count; ++i) {
+            rec[i].fPos = SkTPin(shader.fOrigPos[i], rec[i - 1].fPos, 1.0f);
+            rec[i].fPosScale = 1.0f / (rec[i].fPos - rec[i - 1].fPos);
+        }
+        rec[count - 1].fPos = 1;    // overwrite the last value just to be sure we end at 1.0
+    } else {
+        // no pos specified, so we compute evenly spaced values
+        const float scale = float(count - 1);
+        float invScale = 1.0f / scale;
+        for (int i = 0; i < count; ++i) {
+            rec[i].fPos = i * invScale;
+            rec[i].fPosScale = scale;
+        }
+    }
+
+    fApplyAlphaAfterInterp = true;
+    if ((shader.getGradFlags() & SkGradientShader::kInterpolateColorsInPremul_Flag) ||
+        shader.colorsAreOpaque())
+    {
+        fApplyAlphaAfterInterp = false;
+    }
+
+    if (fApplyAlphaAfterInterp) {
+        // Our fColor values are in PMColor order, but are still unpremultiplied, allowing us to
+        // interpolate in unpremultiplied space first, and then scale by alpha right before we
+        // convert to SkPMColor bytes.
+        const float paintAlpha = ctx.fPaint->getAlpha() * kInv255Float;
+        const Sk4f scale(1, 1, 1, paintAlpha);
+        for (int i = 0; i < count; ++i) {
+            uint32_t c = SkSwizzle_Color_to_PMColor(shader.fOrigColors[i]);
+            rec[i].fColor = Sk4f::FromBytes((const uint8_t*)&c) * scale;
+            if (i > 0) {
+                SkASSERT(rec[i - 1].fPos <= rec[i].fPos);
+            }
+        }
+    } else {
+        // Our fColor values are premultiplied, so converting to SkPMColor is just a matter
+        // of converting the floats down to bytes.
+        unsigned alphaScale = ctx.fPaint->getAlpha() + (ctx.fPaint->getAlpha() >> 7);
+        for (int i = 0; i < count; ++i) {
+            SkPMColor pmc = SkPreMultiplyColor(shader.fOrigColors[i]);
+            pmc = SkAlphaMulQ(pmc, alphaScale);
+            rec[i].fColor = Sk4f::FromBytes((const uint8_t*)&pmc);
+            if (i > 0) {
+                SkASSERT(rec[i - 1].fPos <= rec[i].fPos);
+            }
+        }
+    }
 }
 
 #define NO_CHECK_ITER               \
@@ -212,9 +280,17 @@ void shadeSpan_linear_repeat(TileProc proc, SkGradFixed dx, SkGradFixed fx,
 void SkLinearGradient::LinearGradientContext::shadeSpan(int x, int y, SkPMColor* SK_RESTRICT dstC,
                                                         int count) {
     SkASSERT(count > 0);
-
     const SkLinearGradient& linearGradient = static_cast<const SkLinearGradient&>(fShader);
 
+#ifndef SK_SUPPORT_LEGACY_LINEAR_GRADIENT_TABLE
+    if (SkShader::kClamp_TileMode == linearGradient.fTileMode &&
+        kLinear_MatrixClass == fDstToIndexClass)
+    {
+        this->shade4_clamp(x, y, dstC, count);
+        return;
+    }
+#endif
+
     SkPoint             srcPt;
     SkMatrix::MapXYProc dstProc = fDstToIndexProc;
     TileProc            proc = linearGradient.fTileProc;
@@ -576,3 +652,264 @@ void SkLinearGradient::toString(SkString* str) const {
     str->append(")");
 }
 #endif
+
+///////////////////////////////////////////////////////////////////////////////////////////////////
+
+#include "SkNx.h"
+
+static const SkLinearGradient::LinearGradientContext::Rec*
+find_forward(const SkLinearGradient::LinearGradientContext::Rec rec[], float tiledX) {
+    SkASSERT(tiledX >= 0 && tiledX <= 1);
+
+    SkASSERT(rec[0].fPos >= 0 && rec[0].fPos <= 1);
+    SkASSERT(rec[1].fPos >= 0 && rec[1].fPos <= 1);
+    SkASSERT(rec[0].fPos <= rec[1].fPos);
+    rec += 1;
+    while (rec->fPos < tiledX) {
+        SkASSERT(rec[0].fPos >= 0 && rec[0].fPos <= 1);
+        SkASSERT(rec[1].fPos >= 0 && rec[1].fPos <= 1);
+        SkASSERT(rec[0].fPos <= rec[1].fPos);
+        rec += 1;
+    }
+    return rec - 1;
+}
+
+static const SkLinearGradient::LinearGradientContext::Rec*
+find_backward(const SkLinearGradient::LinearGradientContext::Rec rec[], float tiledX) {
+    SkASSERT(tiledX >= 0 && tiledX <= 1);
+
+    SkASSERT(rec[0].fPos >= 0 && rec[0].fPos <= 1);
+    SkASSERT(rec[1].fPos >= 0 && rec[1].fPos <= 1);
+    SkASSERT(rec[0].fPos <= rec[1].fPos);
+    while (tiledX < rec->fPos) {
+        rec -= 1;
+        SkASSERT(rec[0].fPos >= 0 && rec[0].fPos <= 1);
+        SkASSERT(rec[1].fPos >= 0 && rec[1].fPos <= 1);
+        SkASSERT(rec[0].fPos <= rec[1].fPos);
+    }
+    return rec;
+}
+
+template <bool apply_alpha> SkPMColor trunc_from_255(const Sk4f& x) {
+    SkPMColor c;
+    x.toBytes((uint8_t*)&c);
+    if (apply_alpha) {
+        c = SkPreMultiplyARGB(SkGetPackedA32(c), SkGetPackedR32(c),
+                              SkGetPackedG32(c), SkGetPackedB32(c));
+    }
+    return c;
+}
+
+template <bool apply_alpha> void fill(SkPMColor dst[], int count,
+                                      const Sk4f& c4, const Sk4f& c4other) {
+    sk_memset32_dither(dst, trunc_from_255<apply_alpha>(c4),
+                       trunc_from_255<apply_alpha>(c4other), count);
+}
+
+template <bool apply_alpha> void fill(SkPMColor dst[], int count, const Sk4f& c4) {
+    // Assumes that c4 does not need to be dithered.
+    sk_memset32(dst, trunc_from_255<apply_alpha>(c4), count);
+}
+
+/*
+ *  TODOs
+ *
+ *  - tilemodes
+ *  - interp before or after premul
+ *  - perspective
+ *  - optimizations
+ *      - use fixed (32bit or 16bit) instead of floats?
+ */
+
+static Sk4f lerp_color(float fx, const SkLinearGradient::LinearGradientContext::Rec* rec) {
+    const float p0 = rec[0].fPos;
+    const Sk4f c0 = rec[0].fColor;
+    const Sk4f c1 = rec[1].fColor;
+    const Sk4f diffc = c1 - c0;
+    const float scale = rec[1].fPosScale;
+    const float t = (fx - p0) * scale;
+    return c0 + Sk4f(t) * diffc;
+}
+
+template <bool apply_alpha> void ramp(SkPMColor dstC[], int n, const Sk4f& c, const Sk4f& dc,
+                                      const Sk4f& dither0, const Sk4f& dither1) {
+    Sk4f dc2 = dc + dc;
+    Sk4f dc4 = dc2 + dc2;
+    Sk4f cd0 = c + dither0;
+    Sk4f cd1 = c + dc + dither1;
+    Sk4f cd2 = cd0 + dc2;
+    Sk4f cd3 = cd1 + dc2;
+    while (n >= 4) {
+        *dstC++ = trunc_from_255<apply_alpha>(cd0);
+        *dstC++ = trunc_from_255<apply_alpha>(cd1);
+        *dstC++ = trunc_from_255<apply_alpha>(cd2);
+        *dstC++ = trunc_from_255<apply_alpha>(cd3);
+        cd0 = cd0 + dc4;
+        cd1 = cd1 + dc4;
+        cd2 = cd2 + dc4;
+        cd3 = cd3 + dc4;
+        n -= 4;
+    }
+    if (n & 2) {
+        *dstC++ = trunc_from_255<apply_alpha>(cd0);
+        *dstC++ = trunc_from_255<apply_alpha>(cd1);
+        cd0 = cd0 + dc2;
+    }
+    if (n & 1) {
+        *dstC++ = trunc_from_255<apply_alpha>(cd0);
+    }
+}
+
+template <bool apply_alpha, bool dx_is_pos>
+void SkLinearGradient::LinearGradientContext::shade4_dx_clamp(SkPMColor dstC[], int count,
+                                                              float fx, float dx, float invDx,
+                                                              const float dither[2]) {
+    Sk4f dither0(dither[0]);
+    Sk4f dither1(dither[1]);
+    const Rec* rec = fRecs.begin();
+
+    const Sk4f dx4 = Sk4f(dx);
+    SkDEBUGCODE(SkPMColor* endDstC = dstC + count;)
+
+    if (dx_is_pos) {
+        if (fx < 0) {
+            int n = SkTMin(SkFloatToIntFloor(-fx * invDx) + 1, count);
+            fill<apply_alpha>(dstC, n, rec[0].fColor);
+            count -= n;
+            dstC += n;
+            fx += n * dx;
+            SkASSERT(0 == count || fx >= 0);
+            if (n & 1) {
+                SkTSwap(dither0, dither1);
+            }
+        }
+    } else { // dx < 0
+        if (fx > 1) {
+            int n = SkTMin(SkFloatToIntFloor((1 - fx) * invDx) + 1, count);
+            fill<apply_alpha>(dstC, n, rec[fRecs.count() - 1].fColor);
+            count -= n;
+            dstC += n;
+            fx += n * dx;
+            SkASSERT(0 == count || fx <= 1);
+            if (n & 1) {
+                SkTSwap(dither0, dither1);
+            }
+        }
+    }
+    SkASSERT(count >= 0);
+
+    const Rec* r;
+    if (dx_is_pos) {
+        r = fRecs.begin();                      // start at the beginning
+    } else {
+        r = fRecs.begin() + fRecs.count() - 2;  // start at the end
+    }
+
+    while (count > 0) {
+        if (dx_is_pos) {
+            if (fx >= 1) {
+                fill<apply_alpha>(dstC, count, rec[fRecs.count() - 1].fColor);
+                return;
+            }
+        } else {    // dx < 0
+            if (fx <= 0) {
+                fill<apply_alpha>(dstC, count, rec[0].fColor);
+                return;
+            }
+        }
+
+        if (dx_is_pos) {
+            r = find_forward(r, fx);
+        } else {
+            r = find_backward(r, fx);
+        }
+        SkASSERT(r >= fRecs.begin() && r < fRecs.begin() + fRecs.count() - 1);
+
+        const float p0 = r[0].fPos;
+        const Sk4f c0 = r[0].fColor;
+        const float p1 = r[1].fPos;
+        const Sk4f diffc = Sk4f(r[1].fColor) - c0;
+        const float scale = r[1].fPosScale;
+        const float t = (fx - p0) * scale;
+        const Sk4f c = c0 + Sk4f(t) * diffc;
+        const Sk4f dc = diffc * dx4 * Sk4f(scale);
+
+        int n;
+        if (dx_is_pos) {
+            n = SkTMin((int)((p1 - fx) * invDx) + 1, count);
+        } else {
+            n = SkTMin((int)((p0 - fx) * invDx) + 1, count);
+        }
+
+        fx += n * dx;
+        count -= n;
+        SkASSERT(count >= 0);
+        if (dx_is_pos) {
+            SkASSERT(0 == count || fx >= p1);
+        } else {
+            SkASSERT(0 == count || fx <= p0);
+        }
+
+        ramp<apply_alpha>(dstC, n, c, dc, dither0, dither1);
+        dstC += n;
+        SkASSERT(dstC <= endDstC);
+        
+        if (n & 1) {
+            SkTSwap(dither0, dither1);
+        }
+    }
+}
+
+void SkLinearGradient::LinearGradientContext::shade4_clamp(int x, int y, SkPMColor dstC[],
+                                                           int count) {
+    SkASSERT(count > 0);
+    SkASSERT(kLinear_MatrixClass == fDstToIndexClass);
+
+    SkPoint srcPt;
+    fDstToIndexProc(fDstToIndex, x + SK_ScalarHalf, y + SK_ScalarHalf, &srcPt);
+    float fx = srcPt.x();
+    const float dx = fDstToIndex.getScaleX();
+
+    // Default our dither bias values to 1/2, (rounding), which is no dithering
+    float dither0 = 0.5f;
+    float dither1 = 0.5f;
+    if (fDither) {
+        const float ditherCell[] = {
+            1/8.0f,   5/8.0f,
+            7/8.0f,   3/8.0f,
+        };
+        const int rowIndex = (y & 1) << 1;
+        dither0 = ditherCell[rowIndex];
+        dither1 = ditherCell[rowIndex + 1];
+        if (x & 1) {
+            SkTSwap(dither0, dither1);
+        }
+    }
+    const float dither[2] = { dither0, dither1 };
+    const float invDx = 1 / dx;
+
+    if (!SkScalarIsFinite(invDx)) { // dx is effectively zero, gradient is vertical
+        Sk4f c = lerp_color(fx, find_forward(fRecs.begin(), SkTPin(fx, 0.0f, 1.0f)));
+        if (fApplyAlphaAfterInterp) {
+            fill<true>(dstC, count, c + dither0, c + dither1);
+        } else {
+            fill<false>(dstC, count, c + dither0, c + dither1);
+        }
+        return;
+    }
+
+    if (dx > 0) {
+        if (fApplyAlphaAfterInterp) {
+            this->shade4_dx_clamp<true, true>(dstC, count, fx, dx, invDx, dither);
+        } else {
+            this->shade4_dx_clamp<false, true>(dstC, count, fx, dx, invDx, dither);
+        }
+    } else {
+        if (fApplyAlphaAfterInterp) {
+            this->shade4_dx_clamp<true, false>(dstC, count, fx, dx, invDx, dither);
+        } else {
+            this->shade4_dx_clamp<false, false>(dstC, count, fx, dx, invDx, dither);
+        }
+    }
+}
+