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Diffstat (limited to 'src/codec/SkPngFilters.cpp')
| -rw-r--r-- | src/codec/SkPngFilters.cpp | 168 |
1 files changed, 168 insertions, 0 deletions
diff --git a/src/codec/SkPngFilters.cpp b/src/codec/SkPngFilters.cpp new file mode 100644 index 0000000000..115810ea7c --- /dev/null +++ b/src/codec/SkPngFilters.cpp @@ -0,0 +1,168 @@ +/* + * 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" +#include "SkTypes.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, png_bytep row, png_const_bytep) { + // 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, png_bytep row, png_const_bytep 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 bytewise |x-y|. + static __m128i absdiff_u8(__m128i x, __m128i y) { + // One of these two saturated subtractions will be the answer, the other zero. + return _mm_or_si128(_mm_subs_epu8(x,y), _mm_subs_epu8(y,x)); + } + + // Bytewise c ? t : e. + static __m128i if_then_else(__m128i c, __m128i t, __m128i e) { + // SSE 4.1+ would be: return _mm_blendv_epi8(e,t,c); + return _mm_or_si128(_mm_and_si128(c, t), _mm_andnot_si128(c, e)); + } + + template <int bpp> + void sk_paeth_sse2(png_row_infop row_info, png_bytep row, png_const_bytep 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. + __m128i c, b = _mm_setzero_si128(), + a, d = _mm_setzero_si128(); + + int rb = row_info->rowbytes; + while (rb > 0) { + c = b; b = load<bpp>(prev); + a = d; d = load<bpp>(row ); + + // We can't express p in 8 bits, but luckily we can use this faux p instead. + // (I have no deep insight here... I just proved this with brute force.) + __m128i min = _mm_min_epu8(a,b), + max = _mm_max_epu8(a,b), + faux_p = _mm_adds_epu8(min, _mm_subs_epu8(max, c)); + + // We could use faux_p for calculating all three of pa, pb, and pc, + // but it's a little quicker to calculate the correct pa and pb directly, + // and the predictor remains the same. (Again, brute force.) + __m128i pa = absdiff_u8(b,c), // |a+b-c - a| == |b-c| + pb = absdiff_u8(a,c), // |a+b-c - b| == |a-c| + faux_pc = absdiff_u8(faux_p, c); + + // From here, things are straightforward. Find the smallest distance to p... + __m128i smallest = _mm_min_epu8(_mm_min_epu8(pa, pb), faux_pc); + + // ... then the predictor is the input corresponding to that smallest distance, + // breaking ties in favor of a over b over c. + __m128i nearest = if_then_else(_mm_cmpeq_epi8(smallest, pa), a, + if_then_else(_mm_cmpeq_epi8(smallest, pb), b, + c)); + + // We've reconstructed d! Leave it for next round to become a, and write it out. + d = _mm_add_epi8(d, nearest); + store<bpp>(row, d); + + prev += bpp; + row += bpp; + rb -= bpp; + } + } + + void sk_sub3_sse2(png_row_infop row_info, png_bytep row, png_const_bytep prev) { + sk_sub_sse2<3>(row_info, row, prev); + } + void sk_sub4_sse2(png_row_infop row_info, png_bytep row, png_const_bytep prev) { + sk_sub_sse2<4>(row_info, row, prev); + } + + void sk_avg3_sse2(png_row_infop row_info, png_bytep row, png_const_bytep prev) { + sk_avg_sse2<3>(row_info, row, prev); + } + void sk_avg4_sse2(png_row_infop row_info, png_bytep row, png_const_bytep prev) { + sk_avg_sse2<4>(row_info, row, prev); + } + + void sk_paeth3_sse2(png_row_infop row_info, png_bytep row, png_const_bytep prev) { + sk_paeth_sse2<3>(row_info, row, prev); + } + void sk_paeth4_sse2(png_row_infop row_info, png_bytep row, png_const_bytep prev) { + sk_paeth_sse2<4>(row_info, row, prev); + } + +#endif |