Move sRGB <-> linear conversion components to their own files.

src/core/SkSRGB.h

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkSRGB_DEFINED
+#define SkSRGB_DEFINED
+
+#include "SkNx.h"
+
+/** Components for building our canonical sRGB -> linear and linear -> sRGB transformations.
+ *
+ *  Current best practices:
+ *      - for sRGB -> linear, lookup R,G,B in sk_linear_from_srgb;
+ *      - for linear -> sRGB, call sk_linear_to_srgb() for R,G,B, clamp to 255, and round;
+ *      - the alpha channel is linear in both formats, needing at most *(1/255.0f) or *255.0f.
+ *
+ *  sk_linear_to_srgb()'s output requires rounding; it does not round for you.
+ *
+ *  Given inputs in [0,1], sk_linear_to_srgb() will not underflow 0 but may overflow 255.
+ *  The overflow is small enough that you can safely either clamp then round or round then clamp.
+ *  (If you don't trust the inputs are in [0,1], you'd better clamp both sides immediately.)
+ *
+ *  sk_linear_to_srgb() will run a little faster than usual when compiled with SSE4.1+.
+ */
+
+extern const float sk_linear_from_srgb[256];
+
+static inline Sk4f sk_linear_to_srgb(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 rsqrt = x.rsqrt(),
+         sqrt  = rsqrt.invert(),
+         ftrt  = rsqrt.rsqrt();
+
+    auto lo = (12.92f * 255.0f) * x;
+
+    auto hi = (-0.101115084998961f * 255.0f) +
+              (+0.678513029959381f * 255.0f) * sqrt +
+              (+0.422602055039580f * 255.0f) * ftrt;
+
+    return (x < 0.00349f).thenElse(lo, hi);
+}
+
+#endif//SkSRGB_DEFINED