1 /* 2 * Copyright 2016 Google Inc. 3 * 4 * Use of this source code is governed by a BSD-style license that can be 5 * found in the LICENSE file. 6 */ 7 8 #ifndef SkFixed15_DEFINED 9 #define SkFixed15_DEFINED 10 11 #include "SkTypes.h" 12 13 // SkFixed15 is a fixed point value that represents values in [0,1] as [0x0000, 0x8000]. 14 // This mapping allows us to implement most operations in tightly packed 16-bit SIMD, 15 // most notably multiplying using Q15 multiplication instructions (and a little fixup). 16 17 class SkFixed15 { 18 public: 19 SkFixed15() = default; 20 21 SkFixed15(float val) : fVal(val * 32768) { SkASSERT(0.0f <= val && val <= 1.0f); } 22 explicit operator float() const { return fVal * (1/32768.0f); } 23 24 static SkFixed15 Load(uint16_t val) { 25 SkASSERT(val <= 32768); 26 return val; 27 } 28 uint16_t store() const { return fVal; } 29 30 static SkFixed15 FromU8(uint8_t val) { 31 return val*128 + (val>>1) // 32768/255 == 128.50196..., which is very close to 128 + 0.5. 32 + ((val+1)>>8); // All val but 255 are correct. +1 if val == 255 to get 32768. 33 } 34 35 uint8_t to_u8() const { 36 // FromU8() and to_u8() roundtrip all bytes. 37 // There is still much room to tweak this towards the ideal, a rounding scale by 255/32768. 38 return (fVal - (fVal>>8))>>7; 39 } 40 41 SkFixed15 operator +(SkFixed15 o) const { return fVal + o.fVal; } 42 SkFixed15 operator -(SkFixed15 o) const { return fVal - o.fVal; } 43 SkFixed15 operator *(SkFixed15 o) const { return (fVal * o.fVal + (1<<14)) >> 15; } 44 SkFixed15 operator<<(int bits) const { return fVal << bits; } 45 SkFixed15 operator>>(int bits) const { return fVal >> bits; } 46 47 SkFixed15& operator +=(SkFixed15 o) { return (*this = *this + o); } 48 SkFixed15& operator -=(SkFixed15 o) { return (*this = *this - o); } 49 SkFixed15& operator *=(SkFixed15 o) { return (*this = *this * o); } 50 SkFixed15& operator<<=(int bits) { return (*this = *this << bits); } 51 SkFixed15& operator>>=(int bits) { return (*this = *this >> bits); } 52 53 bool operator==(SkFixed15 o) const { return fVal == o.fVal; } 54 bool operator!=(SkFixed15 o) const { return fVal != o.fVal; } 55 bool operator<=(SkFixed15 o) const { return fVal <= o.fVal; } 56 bool operator>=(SkFixed15 o) const { return fVal >= o.fVal; } 57 bool operator< (SkFixed15 o) const { return fVal < o.fVal; } 58 bool operator> (SkFixed15 o) const { return fVal > o.fVal; } 59 60 private: 61 SkFixed15(int val) : fVal(val) {} 62 63 uint16_t fVal; 64 }; 65 66 // Notes 67 // - SSSE3+ multiply is _mm_abs_epi16(_mm_mulhrs_epi16(x, y)); 68 // - NEON multipy is vsraq_n_u16(vabsq_s16(vqrdmulhq_s16(x,y)), 69 // vandq_s16(x,y), 15); 70 // - Conversion to and from float can be done manually with bit masks and float add/subtract, 71 // rather than the naive version here involving int<->float conversion and float multiply. 72 // - On x86, conversion to float is _mm_sub_ps(_mm_unpacklo_epi16(x, _mm_set1_epi16(0x4380)), 73 // _mm_set1_ps(256.0f)). // 0x43800000 74 // - On ARM, we can use the vcvtq_n_f32_u32(vmovl_u16(x), 15) to convert to float, 75 // and vcvtq_n_u32_f32(..., 15) for the other way around. 76 77 #endif//SkFixed15_DEFINED 78
