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/*
* Copyright 2016 Google Inc.
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#include "SkPngFilters.h"
// Functions in this file look at most 3 pixels (a,b,c) to predict the fourth (d).
// They're positioned like this:
// prev: c b
// row: a d
// The Sub filter predicts d=a, Avg d=(a+b)/2, and Paeth predicts d to be whichever
// of a, b, or c is closest to p=a+b-c. (Up also exists, predicting d=b.)
#if defined(__SSE2__)
template <int bpp>
static __m128i load(const void* p) {
static_assert(bpp <= 4, "");
uint32_t packed;
memcpy(&packed, p, bpp);
return _mm_cvtsi32_si128(packed);
}
template <int bpp>
static void store(void* p, __m128i v) {
static_assert(bpp <= 4, "");
uint32_t packed = _mm_cvtsi128_si32(v);
memcpy(p, &packed, bpp);
}
template <int bpp>
static void sk_sub_sse2(png_row_infop row_info, uint8_t* row, const uint8_t*) {
// The Sub filter predicts each pixel as the previous pixel, a.
// There is no pixel to the left of the first pixel. It's encoded directly.
// That works with our main loop if we just say that left pixel was zero.
__m128i a, d = _mm_setzero_si128();
int rb = row_info->rowbytes;
while (rb > 0) {
a = d; d = load<bpp>(row);
d = _mm_add_epi8(d, a);
store<bpp>(row, d);
row += bpp;
rb -= bpp;
}
}
template <int bpp>
void sk_avg_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
// The Avg filter predicts each pixel as the (truncated) average of a and b.
// There's no pixel to the left of the first pixel. Luckily, it's
// predicted to be half of the pixel above it. So again, this works
// perfectly with our loop if we make sure a starts at zero.
const __m128i zero = _mm_setzero_si128();
__m128i b;
__m128i a, d = zero;
int rb = row_info->rowbytes;
while (rb > 0) {
b = load<bpp>(prev);
a = d; d = load<bpp>(row );
// PNG requires a truncating average here, so sadly we can't just use _mm_avg_epu8...
__m128i avg = _mm_avg_epu8(a,b);
// ...but we can fix it up by subtracting off 1 if it rounded up.
avg = _mm_sub_epi8(avg, _mm_and_si128(_mm_xor_si128(a,b), _mm_set1_epi8(1)));
d = _mm_add_epi8(d, avg);
store<bpp>(row, d);
prev += bpp;
row += bpp;
rb -= bpp;
}
}
// Returns |x| for 16-bit lanes.
static __m128i abs_i16(__m128i x) {
#if defined(__SSSE3__)
return _mm_abs_epi16(x);
#else
// Read this all as, return x<0 ? -x : x.
// To negate two's complement, you flip all the bits then add 1.
__m128i is_negative = _mm_cmplt_epi16(x, _mm_setzero_si128());
x = _mm_xor_si128(x, is_negative); // Flip negative lanes.
x = _mm_add_epi16(x, _mm_srli_epi16(is_negative, 15)); // +1 to negative lanes, else +0.
return x;
#endif
}
// Bytewise c ? t : e.
static __m128i if_then_else(__m128i c, __m128i t, __m128i e) {
#if 0 && defined(__SSE4_1__) // Make sure we have a bot testing this before enabling.
return _mm_blendv_epi8(e,t,c);
#else
return _mm_or_si128(_mm_and_si128(c, t), _mm_andnot_si128(c, e));
#endif
}
template <int bpp>
void sk_paeth_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
// Paeth tries to predict pixel d using the pixel to the left of it, a,
// and two pixels from the previous row, b and c:
// prev: c b
// row: a d
// The Paeth function predicts d to be whichever of a, b, or c is nearest to p=a+b-c.
// The first pixel has no left context, and so uses an Up filter, p = b.
// This works naturally with our main loop's p = a+b-c if we force a and c to zero.
// Here we zero b and d, which become c and a respectively at the start of the loop.
const __m128i zero = _mm_setzero_si128();
__m128i c, b = zero,
a, d = zero;
int rb = row_info->rowbytes;
while (rb > 0) {
// It's easiest to do this math (particularly, deal with pc) with 16-bit intermediates.
c = b; b = _mm_unpacklo_epi8(load<bpp>(prev), zero);
a = d; d = _mm_unpacklo_epi8(load<bpp>(row ), zero);
__m128i pa = _mm_sub_epi16(b,c), // (p-a) == (a+b-c - a) == (b-c)
pb = _mm_sub_epi16(a,c), // (p-b) == (a+b-c - b) == (a-c)
pc = _mm_add_epi16(pa,pb); // (p-c) == (a+b-c - c) == (a+b-c-c) == (b-c)+(a-c)
pa = abs_i16(pa); // |p-a|
pb = abs_i16(pb); // |p-b|
pc = abs_i16(pc); // |p-c|
__m128i smallest = _mm_min_epi16(pc, _mm_min_epi16(pa, pb));
// Paeth breaks ties favoring a over b over c.
__m128i nearest = if_then_else(_mm_cmpeq_epi16(smallest, pa), a,
if_then_else(_mm_cmpeq_epi16(smallest, pb), b,
c));
d = _mm_add_epi8(d, nearest); // Note `_epi8`: we need addition to wrap modulo 255.
store<bpp>(row, _mm_packus_epi16(d,d));
prev += bpp;
row += bpp;
rb -= bpp;
}
}
void sk_sub3_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
sk_sub_sse2<3>(row_info, row, prev);
}
void sk_sub4_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
sk_sub_sse2<4>(row_info, row, prev);
}
void sk_avg3_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
sk_avg_sse2<3>(row_info, row, prev);
}
void sk_avg4_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
sk_avg_sse2<4>(row_info, row, prev);
}
void sk_paeth3_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
sk_paeth_sse2<3>(row_info, row, prev);
}
void sk_paeth4_sse2(png_row_infop row_info, uint8_t* row, const uint8_t* prev) {
sk_paeth_sse2<4>(row_info, row, prev);
}
#endif
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