YUV and color xforms

src/core/SkColorSpaceXform.cpp

Index: src/core/SkColorSpaceXform.cpp
diff --git a/src/core/SkColorSpaceXform.cpp b/src/core/SkColorSpaceXform.cpp
index 599adc90f959609062c6d9955ec01b0560428321..12c3801e47567b7a7125b46c2a9deb25da2094e8 100644
--- a/src/core/SkColorSpaceXform.cpp
+++ b/src/core/SkColorSpaceXform.cpp
@@ -1329,6 +1329,173 @@ const
     }
 }
 
+static inline Sk4f srgb_to_linear(const Sk4f& x) {
+    // Approximation of the sRGB gamma curve (within 1 when scaled to 8-bit pixels).
+    // For 0.00000f <= x <  0.00349f,    12.92 * x
+    // For 0.00349f <= x <= 1.00000f,    0.679*(x.^0.5) + 0.423*x.^(0.25) - 0.101
+    // Note that 0.00349 was selected because it is a point where both functions produce the
+    // same pixel value when rounded.
+    auto y = (x + 0.055f) * (1.0f / 1.055f);
+    auto twodotfive = y * y * y.rsqrt().invert();
+    auto lo = ((1.0f / 12.92f)) * x;
+    auto hi = twodotfive;
+    auto out = (x < 0.04045f).thenElse(lo, hi);
+    return out;
+}
+
+template <SrcGamma kSrc, DstGamma kDst, ColorSpaceMatch kCSM>
+void SkColorSpaceXform_Base<kSrc, kDst, kCSM>
+::applyYUV(void* dst, const uint8_t* srcY, const uint8_t* srcU, const uint8_t* srcV, int len)
+const
+{
+    Sk4f rXgXbX, rYgYbY, rZgZbZ, rTgTbT;
+    load_matrix(fSrcToDst, rXgXbX, rYgYbY, rZgZbZ, rTgTbT);
+
+    if (len >= 4) {
+        // Naively this would be a loop of load-transform-store, but we found it faster to
+        // move the N+1th load ahead of the Nth store.  We don't bother doing this for N<4.
+        Sk4f r, g, b, a;
+
+        Sk4f y = (1.0f / 255.0f) * SkNx_cast<float, uint8_t>(Sk4b::Load(srcY));
+        Sk4f u = (1.0f / 255.0f) * (SkNx_cast<float, uint8_t>(Sk4b::Load(srcU)) - 128.0f);
+        Sk4f v = (1.0f / 255.0f) * (SkNx_cast<float, uint8_t>(Sk4b::Load(srcV)) - 128.0f);
+
+        r = (y + 1.402f * v);
+        g = (y - 0.34414f * u - 0.71414f * v);
+        b = (y + 1.772f * u);
+
+        r = srgb_to_linear(r);
+        g = srgb_to_linear(g);
+        b = srgb_to_linear(b);
+
+        srcY += 4;
+        srcU += 4;
+        srcV += 4;
+        len -= 4;
+
+        Sk4f dr, dg, db, da;
+        while (len >= 4) {
+            transform_gamut(r, g, b, a, rXgXbX, rYgYbY, rZgZbZ, dr, dg, db, da);
+            translate_gamut(rTgTbT, dr, dg, db);
+
+            y = (1.0f / 255.0f) * SkNx_cast<float, uint8_t>(Sk4b::Load(srcY));
+            u = (1.0f / 255.0f) * (SkNx_cast<float, uint8_t>(Sk4b::Load(srcU)) - 128.0f);
+            v = (1.0f / 255.0f) * (SkNx_cast<float, uint8_t>(Sk4b::Load(srcV)) - 128.0f);
+
+            r = (y + 1.402f * v);
+            g = (y - 0.34414f * u - 0.71414f * v);
+            b = (y + 1.772f * u);
+
+            r = srgb_to_linear(r);
+            g = srgb_to_linear(g);
+            b = srgb_to_linear(b);
+
+            srcY += 4;
+            srcU += 4;
+            srcV += 4;
+            len -= 4;
+
+            dr = sk_linear_to_srgb_needs_trunc(dr);
+            dg = sk_linear_to_srgb_needs_trunc(dg);
+            db = sk_linear_to_srgb_needs_trunc(db);
+
+            dr = sk_clamp_0_255(dr);
+            dg = sk_clamp_0_255(dg);
+            db = sk_clamp_0_255(db);
+
+            Sk4i rgba = (SkNx_cast<int>(dr) << 16)
+                      | (SkNx_cast<int>(dg) <<  8)
+                      | (SkNx_cast<int>(db) <<  0)
+                      | (0xFF << 24              );
+            rgba.store(dst);
+            dst = SkTAddOffset<void>(dst, 4 * 4);
+        }
+
+        transform_gamut(r, g, b, a, rXgXbX, rYgYbY, rZgZbZ, dr, dg, db, da);
+        translate_gamut(rTgTbT, dr, dg, db);
+
+        dr = sk_linear_to_srgb_needs_trunc(dr);
+        dg = sk_linear_to_srgb_needs_trunc(dg);
+        db = sk_linear_to_srgb_needs_trunc(db);
+
+        dr = sk_clamp_0_255(dr);
+        dg = sk_clamp_0_255(dg);
+        db = sk_clamp_0_255(db);
+
+        Sk4i rgba = (SkNx_cast<int>(dr) << 16)
+                  | (SkNx_cast<int>(dg) <<  8)
+                  | (SkNx_cast<int>(db) <<  0)
+                  | (0xFF << 24              );
+        rgba.store(dst);
+        dst = SkTAddOffset<void>(dst, 4 * 4);
+    }
+}
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 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
 std::unique_ptr<SkColorSpaceXform> SlowIdentityXform(const sk_sp<SkColorSpace>& space) {