commit 6029c8e0b5b3431f1982391c84187c9b8bd2caca
parent 020af692e6a0c2b28bf13cf8638402f46e806db1
Author: Sebastian Frysztak <sebfry@gmail.com>
Date: Fri, 11 Nov 2016 18:45:20 +0100
Clean up a bit.
Diffstat:
2 files changed, 25 insertions(+), 21 deletions(-)
diff --git a/blur.h b/blur.h
@@ -8,7 +8,7 @@ void blur_image_surface (cairo_surface_t *surface, int radius);
void blur_impl_naive(uint32_t* src, uint32_t* dst, int width, int height, int src_stride, int dst_stride, int radius);
void blur_impl_sse2(uint32_t* src, uint32_t* dst, int width, int height, float sigma);
-void blur_impl_horizontal_pass_sse2(uint32_t *src, uint32_t *dst, float *kernel, int width, int height);
+void blur_impl_horizontal_pass_sse2(uint32_t *src, uint32_t *dst, int width, int height);
#endif
diff --git a/blur_simd.c b/blur_simd.c
@@ -13,6 +13,7 @@
#define ALIGN16 __attribute__((aligned(16)))
#define KERNEL_SIZE 7
+#define SIGMA_AV 2
#define HALF_KERNEL KERNEL_SIZE / 2
// number of xmm registers needed to store
@@ -20,29 +21,31 @@
#define REGISTERS_CNT (KERNEL_SIZE + 4/2) / 4
void blur_impl_sse2(uint32_t *src, uint32_t *dst, int width, int height, float sigma) {
- // prepare kernel
- float kernel[KERNEL_SIZE];
- float coeff = 1.0 / sqrtf(2 * M_PI * sigma * sigma), sum = 0;
-
- for (int i = 0; i < KERNEL_SIZE; i++) {
- float x = HALF_KERNEL - i;
- kernel[i] = coeff * expf(-x * x / (2.0 * sigma * sigma));
- sum += kernel[i];
+ // according to a paper by Peter Kovesi [1], box filter of width w, equals to Gaussian blur of following sigma:
+ // σ_av = sqrt((w*w-1)/12)
+ // for our 7x7 filter we have σ_av = 2.0.
+ // applying the same Gaussian filter n times results in σ_n = sqrt(n*σ_av*σ_av) [2]
+ // after some trivial math, we arrive at n = ((σ_d)/(σ_av))^2
+ // since it's a box blur filter, n >= 3
+ //
+ // [1]: http://www.peterkovesi.com/papers/FastGaussianSmoothing.pdf
+ // [2]: https://en.wikipedia.org/wiki/Gaussian_blur#Mathematics
+
+ int n = lrintf((sigma*sigma)/(SIGMA_AV*SIGMA_AV));
+ if (n < 3) n = 3;
+
+ for (int i = 0; i < n; i++)
+ {
+ // horizontal pass includes image transposition:
+ // instead of writing pixel src[x] to dst[x],
+ // we write it to transposed location.
+ // (to be exact: dst[height * current_column + current_row])
+ blur_impl_horizontal_pass_sse2(src, dst, width, height);
+ blur_impl_horizontal_pass_sse2(dst, src, height, width);
}
-
- // normalize kernel
- for (int i = 0; i < KERNEL_SIZE; i++)
- kernel[i] /= sum;
-
- // horizontal pass includes image transposition:
- // instead of writing pixel src[x] to dst[x],
- // we write it to transposed location.
- // (to be exact: dst[height * current_column + current_row])
- blur_impl_horizontal_pass_sse2(src, dst, kernel, width, height);
- blur_impl_horizontal_pass_sse2(dst, src, kernel, height, width);
}
-void blur_impl_horizontal_pass_sse2(uint32_t *src, uint32_t *dst, float *kernel, int width, int height) {
+void blur_impl_horizontal_pass_sse2(uint32_t *src, uint32_t *dst, int width, int height) {
for (int row = 0; row < height; row++) {
for (int column = 0; column < width; column++, src++) {
__m128i rgbaIn[REGISTERS_CNT];
@@ -91,6 +94,7 @@ void blur_impl_horizontal_pass_sse2(uint32_t *src, uint32_t *dst, float *kernel,
acc = _mm_add_epi32(_mm_unpacklo_epi16(acc, zero),
_mm_unpackhi_epi16(acc, zero));
+ // multiplication is significantly faster than division
acc = _mm_cvtps_epi32(_mm_mul_ps(_mm_cvtepi32_ps(acc),
_mm_set1_ps(1/((float)KERNEL_SIZE))));
