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

src/opts/SkColorXform_opts.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 SkColorXform_opts_DEFINED
 #define SkColorXform_opts_DEFINED
 
 #include "SkNx.h"
 #include "SkColorPriv.h"
+#include "SkSRGB.h"
 
-extern const float sk_linear_from_srgb[256];
 extern const float sk_linear_from_2dot2[256];
 
 namespace SK_OPTS_NS {
 
 static Sk4f linear_to_2dot2(const Sk4f& x) {
     // x^(29/64) is a very good approximation of the true value, x^(1/2.2).
     auto x2  = x.rsqrt(),                            // x^(-1/2)
          x32 = x2.rsqrt().rsqrt().rsqrt().rsqrt(),   // x^(-1/32)
          x64 = x32.rsqrt();                          // x^(+1/64)
 
     // 29 = 32 - 2 - 1
     return 255.0f * x2.invert() * x32 * x64.invert();
 }
 
-static Sk4f linear_to_srgb(const Sk4f& x) {

[msarett, 2016/07/07 17:05:34]

FWIW, I'm thinking about/working on refactoring this function signature to be
more flexible...

I like this being interchangeable with other "linear to curve" functions.  It's
a bad match when we want to use a table to go "linear to curve" because that
must be written as Sk4f -> Sk4i (when planar) or Sk4f -> uint32_t (when
interleaved).

-    // 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 hi = (-0.101115084998961f * 255.0f) +
-              (+0.678513029959381f * 255.0f) * sqrt +
-              (+0.422602055039580f * 255.0f) * ftrt;
-
-    auto lo = (12.92f * 255.0f) * x;
-
-    auto mask = (x < 0.00349f);
-    return mask.thenElse(lo, hi);
-}
-
 static Sk4f clamp_0_to_255(const Sk4f& x) {
     // The order of the arguments is important here.  We want to make sure that NaN
     // clamps to zero.  Note that max(NaN, 0) = 0, while max(0, NaN) = NaN.
     return Sk4f::Min(Sk4f::Max(x, 0.0f), 255.0f);
 }
 
 template <const float (&linear_from_curve)[256], Sk4f (*linear_to_curve)(const Sk4f&)>
 static void color_xform_RGB1(uint32_t* dst, const uint32_t* src, int len,
                              const float matrix[16]) {
     Sk4f rXgXbX = Sk4f::Load(matrix + 0),
@@ skipping 88 matching lines @@
     color_xform_RGB1<sk_linear_from_srgb, linear_to_2dot2>(dst, src, len, matrix);
 }
 
 static void color_xform_RGB1_2dot2_to_2dot2(uint32_t* dst, const uint32_t* src, int len,
                                            const float matrix[16]) {
     color_xform_RGB1<sk_linear_from_2dot2, linear_to_2dot2>(dst, src, len, matrix);
 }
 
 static void color_xform_RGB1_srgb_to_srgb(uint32_t* dst, const uint32_t* src, int len,
                                            const float matrix[16]) {
-    color_xform_RGB1<sk_linear_from_srgb, linear_to_srgb>(dst, src, len, matrix);
+    color_xform_RGB1<sk_linear_from_srgb, sk_linear_to_srgb>(dst, src, len, matrix);
 }
 
 static void color_xform_RGB1_2dot2_to_srgb(uint32_t* dst, const uint32_t* src, int len,
                                            const float matrix[16]) {
-    color_xform_RGB1<sk_linear_from_2dot2, linear_to_srgb>(dst, src, len, matrix);
+    color_xform_RGB1<sk_linear_from_2dot2, sk_linear_to_srgb>(dst, src, len, matrix);
 }
 
 }  // namespace SK_OPTS_NS
 
 #endif // SkColorXform_opts_DEFINED