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 
SkFixed15(float val)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 
Load(uint16_t val)24     static SkFixed15 Load(uint16_t val) {
25         SkASSERT(val <= 32768);
26         return val;
27     }
store()28     uint16_t store() const { return fVal; }
29 
FromU8(uint8_t val)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 
to_u8()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:
SkFixed15(int val)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