Revert of Move sRGB <-> linear conversion components to their own files. (patchset #5 id:80001 of https://codereview.chromium.org/2128893002/ )

Reason for revert:
Monotonicity assert is failing on ARM.  (Different rsqrt() and invert() precision?)  Will investigate a bit tomorrow... might reland with the test TODO.

Original issue's description:
> 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:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot
>
> Committed: https://skia.googlesource.com/skia/+/45e58c8807179638980aae8503573b950b844e4c

TBR=reed@google.com,msarett@google.com,mtklein@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review-Url: https://codereview.chromium.org/2131793002

1 files changed, 23 insertions, 3 deletions

diff --git a/src/opts/SkColorXform_opts.h b/src/opts/SkColorXform_opts.h
index 9904b3ef93..74aa53ce7a 100644
--- a/src/opts/SkColorXform_opts.h
+++ b/src/opts/SkColorXform_opts.h
@@ -10,8 +10,8 @@
 
 #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 {
@@ -26,6 +26,26 @@ static Sk4f linear_to_2dot2(const Sk4f& x) {
     return 255.0f * x2.invert() * x32 * x64.invert();
 }
 
+static Sk4f 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 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.
@@ -134,12 +154,12 @@ static void color_xform_RGB1_2dot2_to_2dot2(uint32_t* dst, const uint32_t* src,
 
 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, sk_linear_to_srgb>(dst, src, len, matrix);
+    color_xform_RGB1<sk_linear_from_srgb, 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, sk_linear_to_srgb>(dst, src, len, matrix);
+    color_xform_RGB1<sk_linear_from_2dot2, linear_to_srgb>(dst, src, len, matrix);
 }
 
 }  // namespace SK_OPTS_NS