Move HalfFloatMaker to media

media/video/half_float_maker.cc

+// Copyright (c) 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "media/video/half_float_maker.h"
+#include "third_party/libyuv/include/libyuv.h"
+
+namespace media {
+
+// By OR-ing with 0x3800, 10-bit numbers become half-floats in the
+// range [0.5..1) and 9-bit numbers get the range [0.5..0.75).
+//
+// Half-floats are evaluated as:
+// float value = pow(2.0, exponent - 25) * (0x400 + fraction);
+//
+// In our case the exponent is 14 (since we or with 0x3800) and
+// pow(2.0, 14-25) * 0x400 evaluates to 0.5 (our offset) and
+// pow(2.0, 14-25) * fraction is [0..0.49951171875] for 10-bit and
+// [0..0.24951171875] for 9-bit.
+//
+// https://en.wikipedia.org/wiki/Half-precision_floating-point_format
+class HalfFloatMaker_xor : public HalfFloatMaker {
+ public:
+  explicit HalfFloatMaker_xor(int bits_per_channel)
+      : bits_per_channel_(bits_per_channel) {}
+  float Offset() const override { return 0.5; }
+  float Multiplier() const override {
+    int max_input_value = (1 << bits_per_channel_) - 1;
+    // 2 << 11 = 2048 would be 1.0 with our exponent.
+    return 2048.0 / max_input_value;
+  }
+  void MakeHalfFloats(const uint16_t* src, size_t num, uint16_t* dst) override {
+    // Micro-benchmarking indicates that the compiler does
+    // a good enough job of optimizing this loop that trying
+    // to manually operate on one uint64 at a time is not
+    // actually helpful.
+    // Note to future optimizers: Benchmark your optimizations!
+    for (size_t i = 0; i < num; i++)
+      dst[i] = src[i] | 0x3800;
+  }
+
+ private:
+  int bits_per_channel_;
+};
+
+// Convert plane of 16 bit shorts to half floats using libyuv.
+class HalfFloatMaker_libyuv : public HalfFloatMaker {
+ public:
+  explicit HalfFloatMaker_libyuv(int bits_per_channel) {
+    int max_value = (1 << bits_per_channel) - 1;
+    // For less than 15 bits, we can give libyuv a multiplier of
+    // 1.0, which is faster on some platforms. If bits is 16 or larger,
+    // a multiplier of 1.0 would cause overflows. However, a multiplier
+    // of 1/max_value would cause subnormal floats, which perform
+    // very poorly on some platforms.
+    if (bits_per_channel <= 15) {
+      libyuv_multiplier_ = 1.0f;
+    } else {
+      // This multiplier makes sure that we avoid subnormal values.
+      libyuv_multiplier_ = 1.0f / 4096.0f;
+    }
+    resource_multiplier_ = 1.0f / libyuv_multiplier_ / max_value;
+  }
+  float Offset() const override { return 0.0f; }
+  float Multiplier() const override { return resource_multiplier_; }
+  void MakeHalfFloats(const uint16_t* src, size_t num, uint16_t* dst) override {
+    // Source and dest stride can be zero since we're only copying
+    // one row at a time.
+    int stride = 0;
+    int rows = 1;
+    libyuv::HalfFloatPlane(src, stride, dst, stride, libyuv_multiplier_, num,
+                           rows);
+  }
+
+ private:
+  float libyuv_multiplier_;
+  float resource_multiplier_;
+};
+
+std::unique_ptr<HalfFloatMaker> HalfFloatMaker::NewHalfFloatMaker(
+    int bits_per_channel) {
+  if (bits_per_channel < 11) {
+    return std::unique_ptr<HalfFloatMaker>(
+        new HalfFloatMaker_xor(bits_per_channel));
+  } else {
+    return std::unique_ptr<HalfFloatMaker>(
+        new HalfFloatMaker_libyuv(bits_per_channel));
+  }
+}
+
+}  // namespace media