Correct sRGB <-> linear everywhere.

src/core/SkPM4fPriv.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 SkPM4fPriv_DEFINED
 #define SkPM4fPriv_DEFINED
 
 #include "SkColorPriv.h"
 #include "SkPM4f.h"
+#include "SkSRGB.h"
 
-static inline float get_alpha(const Sk4f& f4) {
-    return f4[SkPM4f::A];
+static inline Sk4f set_alpha(const Sk4f& px, float alpha) {
+    return { px[0], px[1], px[2], alpha };
 }
 
-static inline Sk4f set_alpha(const Sk4f& f4, float alpha) {
-    static_assert(3 == SkPM4f::A, "");
-    return Sk4f(f4[0], f4[1], f4[2], alpha);
+static inline float get_alpha(const Sk4f& px) {
+    return px[3];
 }
 
-static inline uint32_t to_4b(const Sk4f& f4) {
-    uint32_t b4;
-    SkNx_cast<uint8_t>(f4).store((uint8_t*)&b4);
-    return b4;
+
+static inline Sk4f Sk4f_fromL32(uint32_t px) {
+    return SkNx_cast<float>(Sk4b::Load(&px)) * (1/255.0f);
 }
 
-static inline Sk4f to_4f(uint32_t b4) {
-    return SkNx_cast<float>(Sk4b::Load((const uint8_t*)&b4));
+static inline Sk4f Sk4f_fromS32(uint32_t px) {
+    return { sk_linear_from_srgb[(px >>  0) & 0xff],
+             sk_linear_from_srgb[(px >>  8) & 0xff],
+             sk_linear_from_srgb[(px >> 16) & 0xff],
+                    (1/255.0f) * (px >> 24)          };
 }
 
-static inline Sk4f to_4f_rgba(uint32_t b4) {
-    return swizzle_rb_if_bgra(to_4f(b4));
+static inline uint32_t Sk4f_toL32(const Sk4f& px) {
+    uint32_t l32;
+    SkNx_cast<uint8_t>(Sk4f_round(px * 255.0f)).store(&l32);
+    return l32;
 }
 
-static inline Sk4f srgb_to_linear(const Sk4f& s4) {
-    return set_alpha(s4 * s4, get_alpha(s4));
+static inline uint32_t Sk4f_toS32(const Sk4f& px) {
+    Sk4i  rgb = sk_linear_to_srgb(px),
+         srgb = { rgb[0], rgb[1], rgb[2], (int)(255.0f * px[3] + 0.5f) };
+
+    uint32_t s32;
+    SkNx_cast<uint8_t>(srgb).store(&s32);
+    return s32;
 }
 
-static inline Sk4f linear_to_srgb(const Sk4f& l4) {
-    return set_alpha(l4.rsqrt().invert(), get_alpha(l4));
-}
 
-static inline float srgb_to_linear(float x) {
-    return x * x;
-}
-
-static inline float linear_to_srgb(float x) {
-    return sqrtf(x);
-}
-
-static void assert_unit(float x) {
-    SkASSERT(x >= 0 && x <= 1);
-}
-
-static inline float exact_srgb_to_linear(float x) {
-    assert_unit(x);
-    float linear;
-    if (x <= 0.04045) {
-        linear = x / 12.92f;
-    } else {
-        linear = powf((x + 0.055f) / 1.055f, 2.4f);
-    }
-    assert_unit(linear);
-    return linear;
-}
-
-static inline float exact_linear_to_srgb(float x) {
-    assert_unit(x);
-    float srgb;
-    if (x <= 0.0031308f) {
-        srgb = x * 12.92f;
-    } else {
-        srgb = 1.055f * powf(x, 0.41666667f) - 0.055f;
-    }
-    assert_unit(srgb);
-    return srgb;
-}
-
-static inline Sk4f exact_srgb_to_linear(const Sk4f& x) {
-    Sk4f linear(exact_srgb_to_linear(x[0]),
-                exact_srgb_to_linear(x[1]),
-                exact_srgb_to_linear(x[2]), 1);
-    return set_alpha(linear, get_alpha(x));
-}
-
-static inline Sk4f exact_linear_to_srgb(const Sk4f& x) {
-    Sk4f srgb(exact_linear_to_srgb(x[0]),
-              exact_linear_to_srgb(x[1]),
-              exact_linear_to_srgb(x[2]), 1);
-    return set_alpha(srgb, get_alpha(x));
-}
-
-///////////////////////////////////////////////////////////////////////////////////////////////////
-
-static inline Sk4f Sk4f_fromL32(uint32_t src) {
-    return to_4f(src) * Sk4f(1.0f/255);
-}
-
-static inline Sk4f Sk4f_fromS32(uint32_t src) {
-    return srgb_to_linear(to_4f(src) * Sk4f(1.0f/255));
-}
-
-// Color handling:
+// SkColor handling:
 //   SkColor has an ordering of (b, g, r, a) if cast to an Sk4f, so the code swizzles r and b to
 // produce the needed (r, g, b, a) ordering.
 static inline Sk4f Sk4f_from_SkColor(SkColor color) {
     return swizzle_rb(Sk4f_fromS32(color));
 }
 
-static inline uint32_t Sk4f_toL32(const Sk4f& x4) {
-    return to_4b(x4 * Sk4f(255) + Sk4f(0.5f));
+static inline void assert_unit(float x) {
+    SkASSERT(0 <= x && x <= 1);
 }
 
-static inline uint32_t Sk4f_toS32(const Sk4f& x4) {
-    return to_4b(linear_to_srgb(x4) * Sk4f(255) + Sk4f(0.5f));
-}
-
-static inline Sk4f exact_Sk4f_fromS32(uint32_t src) {
-    return exact_srgb_to_linear(to_4f(src) * Sk4f(1.0f/255));
-}
-static inline uint32_t exact_Sk4f_toS32(const Sk4f& x4) {
-    return to_4b(exact_linear_to_srgb(x4) * Sk4f(255) + Sk4f(0.5f));
-}
-
-////////////////////////////////////////////////////////////////////////////////////////////////////
-// An implementation of SrcOver from bytes to bytes in linear space that takes advantage of the
-// observation that the 255's cancel.
-//    invA = 1 - (As / 255);
-//
-//    R = 255 * sqrt((Rs/255)^2 + (Rd/255)^2 * invA)
-// => R = 255 * sqrt((Rs^2 + Rd^2 * invA)/255^2)
-// => R = sqrt(Rs^2 + Rd^2 * invA)
-// Note: src is assumed to be linear.
-static inline void srcover_blend_srgb8888_srgb_1(uint32_t* dst, const Sk4f& src) {
-    Sk4f d = srgb_to_linear(to_4f(*dst));
-    Sk4f invAlpha = 1.0f - Sk4f{src[SkPM4f::A]} * (1.0f / 255.0f);
-    Sk4f r = linear_to_srgb(src + d * invAlpha) + 0.5f;
-    *dst = to_4b(r);
-}
-
-static inline void srcover_srgb8888_srgb_1(uint32_t* dst, const uint32_t pixel) {
-    if ((~pixel & 0xFF000000) == 0) {
-        *dst = pixel;
-    } else if ((pixel & 0xFF000000) != 0) {
-        srcover_blend_srgb8888_srgb_1(dst, srgb_to_linear(to_4f(pixel)));
+static inline float exact_srgb_to_linear(float srgb) {
+    assert_unit(srgb);
+    float linear;
+    if (srgb <= 0.04045) {
+        linear = srgb / 12.92f;
+    } else {
+        linear = powf((srgb + 0.055f) / 1.055f, 2.4f);
     }
+    assert_unit(linear);
+    return linear;
 }
 
 #endif