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

This makes them a little easier to use outside SkColorXform code.

I've added some notes about how best to use them and their eccentricities, and added a test.

Ultimately any software sRGB <-> linear conversion should funnel somehow through here.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2128893002
CQ_EXTRA_TRYBOTS=client.skia.android:Test-Android-GCC-Nexus5-CPU-NEON-Arm7-Release-Trybot;client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Committed: https://skia.googlesource.com/skia/+/45e58c8807179638980aae8503573b950b844e4c
Review-Url: https://codereview.chromium.org/2128893002

1 files changed, 50 insertions, 0 deletions

diff --git a/src/core/SkSRGB.h b/src/core/SkSRGB.h
new file mode 100644
index 0000000000..de80596de4
--- /dev/null
+++ b/src/core/SkSRGB.h
@@ -0,0 +1,50 @@
+/*
+ * 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