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, 41 insertions, 0 deletions

diff --git a/tests/SRGBTest.cpp b/tests/SRGBTest.cpp
new file mode 100644
index 0000000000..95d2866fdc
--- /dev/null
+++ b/tests/SRGBTest.cpp
@@ -0,0 +1,41 @@
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkSRGB.h"
+#include "SkTypes.h"
+#include "Test.h"
+#include <math.h>
+
+static uint8_t linear_to_srgb(float l) {
+    return (uint8_t)roundf(sk_linear_to_srgb(Sk4f{l})[0]);
+}
+
+DEF_TEST(sk_linear_to_srgb, r) {
+    // Should map 0 -> 0 and 1 -> 1.
+    REPORTER_ASSERT(r,   0 == linear_to_srgb(0.0f));
+    REPORTER_ASSERT(r, 255 == linear_to_srgb(1.0f));
+
+    // Should be monotonic between 0 and 1.
+    // We don't bother checking denorm values.
+    int tolerated_regressions = 0;
+#if defined(SK_ARM_HAS_NEON)
+    // Values around 0.166016 are usually 72 but drop briefly (41 floats) down to 71.
+    tolerated_regressions = 1;
+#endif
+    uint8_t prev = 0;
+    for (float f = FLT_MIN; f <= 1.0f; ) {
+        uint8_t srgb = linear_to_srgb(f);
+
+        REPORTER_ASSERT(r, srgb >= prev || tolerated_regressions > 0);
+        if (srgb < prev) { tolerated_regressions--; }
+        prev = srgb;
+
+        union { float flt; uint32_t bits; } pun = { f };
+        pun.bits++;
+        f = pun.flt;
+    }
+}