Add SkColorSpaceTransferFn to SkColorSpace

src/core/SkColorSpacePriv.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.
  */
 
 #define SkColorSpacePrintf(...)
 
-inline bool color_space_almost_equal(float a, float b) {
+static inline bool color_space_almost_equal(float a, float b) {
     return SkTAbs(a - b) < 0.01f;
 }
+
+static inline bool is_zero_to_one(float v) {
+    return (0.0f <= v) && (v <= 1.0f);
+}
+
+static inline bool is_valid_transfer_fn(const SkColorSpaceTransferFn& coeffs) {
+    if (SkScalarIsNaN(coeffs.fA) || SkScalarIsNaN(coeffs.fB) ||
+        SkScalarIsNaN(coeffs.fC) || SkScalarIsNaN(coeffs.fD) ||
+        SkScalarIsNaN(coeffs.fE) || SkScalarIsNaN(coeffs.fF) ||
+        SkScalarIsNaN(coeffs.fG))
+    {
+        return false;
+    }
+
+    if (!is_zero_to_one(coeffs.fD)) {
+        return false;
+    }
+
+    if (coeffs.fD == 0.0f) {
+        // Y = (aX + b)^g + c  for always
+        if (0.0f == coeffs.fA || 0.0f == coeffs.fG) {
+            SkColorSpacePrintf("A or G is zero, constant transfer function "
+                               "is nonsense");
+            return false;
+        }
+    }
+
+    if (coeffs.fD == 1.0f) {
+        // Y = eX + f          for always
+        if (0.0f == coeffs.fE) {
+            SkColorSpacePrintf("E is zero, constant transfer function is "
+                               "nonsense");
+            return false;
+        }
+    }
+
+    if ((0.0f == coeffs.fA || 0.0f == coeffs.fG) && 0.0f == coeffs.fE) {
+        SkColorSpacePrintf("A or G, and E are zero, constant transfer function "
+                           "is nonsense");
+        return false;
+    }
+
+    if (coeffs.fE < 0.0f) {
+        SkColorSpacePrintf("Transfer function must be increasing");
+        return false;
+    }
+
+    if (coeffs.fA < 0.0f || coeffs.fG < 0.0f) {
+        SkColorSpacePrintf("Transfer function must be positive or increasing");
+        return false;
+    }
+
+    return true;
+}
+
+static inline bool is_almost_srgb(const SkColorSpaceTransferFn& coeffs) {
+    return color_space_almost_equal(0.9479f, coeffs.fA) &&
+           color_space_almost_equal(0.0521f, coeffs.fB) &&
+           color_space_almost_equal(0.0000f, coeffs.fC) &&
+           color_space_almost_equal(0.0405f, coeffs.fD) &&
+           color_space_almost_equal(0.0774f, coeffs.fE) &&
+           color_space_almost_equal(0.0000f, coeffs.fF) &&
+           color_space_almost_equal(2.4000f, coeffs.fG);
+}
+
+static inline bool is_almost_2dot2(const SkColorSpaceTransferFn& coeffs) {
+    return color_space_almost_equal(1.0f, coeffs.fA) &&
+           color_space_almost_equal(0.0f, coeffs.fB) &&
+           color_space_almost_equal(0.0f, coeffs.fC) &&
+           color_space_almost_equal(0.0f, coeffs.fD) &&
+           color_space_almost_equal(0.0f, coeffs.fE) &&
+           color_space_almost_equal(0.0f, coeffs.fF) &&
+           color_space_almost_equal(2.2f, coeffs.fG);
+}