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- // 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 {
- HalfFloatMaker::~HalfFloatMaker() = default;
- // 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
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