【AI PC端算法优化】八，小半径中值滤波的SSE极速优化

矫情吗；* 2023-02-14 03:13 5阅读 0赞

# 1. 前言 #

大家应该经常碰到这种需求，那就是使用 3 × 3 3\\times 3 3×3或者 5 × 5 5\\times 5 5×5这种相对比较小的窗口进行中值滤波，而如果在图像的分辨率比较大的情况下这种操作也是比较耗时的。所以在这种固定场景下定制一个优化算法是有意义的。（这里针对PC端，而非Arm端）。

# 2. 普通的3\*3中值滤波实现 #

普通的实现没什么好说，就是直接在窗口区域内遍历寻找中位数即可，这里获取中值直接使用了c语言的qsort。代码实现如下：

int ComparisonFunction(const void *X, const void *Y) {
        
    	unsigned char Dx = *(unsigned char *)X;
    	unsigned char Dy = *(unsigned char *)Y;
    	if (Dx < Dy) return -1;
    	else if (Dx > Dy) return 1;
    	else return 0;
    }
    
    void MedianBlur3X3_Ori(unsigned char *Src, unsigned char *Dest, int Width, int Height, int Stride) {
        
    	int Channel = Stride / Width;
    	if (Channel == 1) {
        
    		unsigned char Array[9];
    		for (int Y = 1; Y < Height - 1; Y++) {
        
    			unsigned char *LineP0 = Src + (Y - 1) * Stride + 1;
    			unsigned char *LineP1 = LineP0 + Stride;
    			unsigned char *LineP2 = LineP1 + Stride;
    			unsigned char *LinePD = Dest + Y * Stride + 1;
    			for (int X = 1; X < Width - 1; X++) {
        
    				Array[0] = LineP0[X - 1];        Array[1] = LineP0[X];    Array[2] = LineP0[X + 1];
    				Array[3] = LineP1[X - 1];        Array[4] = LineP1[X];    Array[5] = LineP2[X + 1];
    				Array[6] = LineP2[X - 1];        Array[7] = LineP2[X];    Array[8] = LineP2[X + 1];
    				qsort(Array, 9, sizeof(unsigned char), &ComparisonFunction);
    				LinePD[X] = Array[4];
    			}
    		}
    	}
    	else {
        
    		unsigned char ArrayB[9], ArrayG[9], ArrayR[9];
    		for (int Y = 1; Y < Height - 1; Y++) {
        
    			unsigned char *LineP0 = Src + (Y - 1) * Stride + 3;
    			unsigned char *LineP1 = LineP0 + Stride;
    			unsigned char *LineP2 = LineP1 + Stride;
    			unsigned char *LinePD = Dest + Y * Stride + 3;
    			for (int X = 1; X < Width - 1; X++){
        
    				ArrayB[0] = LineP0[-3];       ArrayG[0] = LineP0[-2];       ArrayR[0] = LineP0[-1];
    				ArrayB[1] = LineP0[0];        ArrayG[1] = LineP0[1];        ArrayR[1] = LineP0[2];
    				ArrayB[2] = LineP0[3];        ArrayG[2] = LineP0[4];        ArrayR[2] = LineP0[5];
    
    				ArrayB[3] = LineP1[-3];       ArrayG[3] = LineP1[-2];       ArrayR[3] = LineP1[-1];
    				ArrayB[4] = LineP1[0];        ArrayG[4] = LineP1[1];        ArrayR[4] = LineP1[2];
    				ArrayB[5] = LineP1[3];        ArrayG[5] = LineP1[4];        ArrayR[5] = LineP1[5];
    
    				ArrayB[6] = LineP2[-3];       ArrayG[6] = LineP2[-2];       ArrayR[6] = LineP2[-1];
    				ArrayB[7] = LineP2[0];        ArrayG[7] = LineP2[1];        ArrayR[7] = LineP2[2];
    				ArrayB[8] = LineP2[3];        ArrayG[8] = LineP2[4];        ArrayR[8] = LineP2[5];
    
    				qsort(ArrayB, 9, sizeof(unsigned char), &ComparisonFunction);
    				qsort(ArrayG, 9, sizeof(unsigned char), &ComparisonFunction);
    				qsort(ArrayR, 9, sizeof(unsigned char), &ComparisonFunction);
    
    				LinePD[0] = ArrayB[4];
    				LinePD[1] = ArrayG[4];
    				LinePD[2] = ArrayR[4];
    
    				LineP0 += 3;
    				LineP1 += 3;
    				LineP2 += 3;
    				LinePD += 3;
    			}
    		}
    	}
    }

来测一把耗时情况：

# 3. 一个简单的改进 #

由于排序耗时是非常多的，而这里实际上就是在9个元素中找到中位数，这个其实不需要排序就可以办到，只要我们按照下面的方法进行比较就可以获得中位数。事实上只需要19次比较，就可以获得中位数，我们先看源码：

void Swap(int &X, int &Y) {
        
    	X ^= Y;
    	Y ^= X;
    	X ^= Y;
    }
    
    void MedianBlur3X3_Faster(unsigned char *Src, unsigned char *Dest, int Width, int Height, int Stride) {
        
    	int Channel = Stride / Width;
    	if (Channel == 1) {
        
    
    		for (int Y = 1; Y < Height - 1; Y++) {
        
    			unsigned char *LineP0 = Src + (Y - 1) * Stride + 1;
    			unsigned char *LineP1 = LineP0 + Stride;
    			unsigned char *LineP2 = LineP1 + Stride;
    			unsigned char *LinePD = Dest + Y * Stride + 1;
    			for (int X = 1; X < Width - 1; X++) {
        
    				int Gray0, Gray1, Gray2, Gray3, Gray4, Gray5, Gray6, Gray7, Gray8;
    				Gray0 = LineP0[X - 1];        Gray1 = LineP0[X];    Gray2 = LineP0[X + 1];
    				Gray3 = LineP1[X - 1];        Gray4 = LineP1[X];    Gray5 = LineP1[X + 1];
    				Gray6 = LineP2[X - 1];        Gray7 = LineP2[X];    Gray8 = LineP2[X + 1];
    
    				if (Gray1 > Gray2) Swap(Gray1, Gray2);
    				if (Gray4 > Gray5) Swap(Gray4, Gray5);
    				if (Gray7 > Gray8) Swap(Gray7, Gray8);
    				if (Gray0 > Gray1) Swap(Gray0, Gray1);
    				if (Gray3 > Gray4) Swap(Gray3, Gray4);
    				if (Gray6 > Gray7) Swap(Gray6, Gray7);
    				if (Gray1 > Gray2) Swap(Gray1, Gray2);
    				if (Gray4 > Gray5) Swap(Gray4, Gray5);
    				if (Gray7 > Gray8) Swap(Gray7, Gray8);
    				if (Gray0 > Gray3) Swap(Gray0, Gray3);
    				if (Gray5 > Gray8) Swap(Gray5, Gray8);
    				if (Gray4 > Gray7) Swap(Gray4, Gray7);
    				if (Gray3 > Gray6) Swap(Gray3, Gray6);
    				if (Gray1 > Gray4) Swap(Gray1, Gray4);
    				if (Gray2 > Gray5) Swap(Gray2, Gray5);
    				if (Gray4 > Gray7) Swap(Gray4, Gray7);
    				if (Gray4 > Gray2) Swap(Gray4, Gray2);
    				if (Gray6 > Gray4) Swap(Gray6, Gray4);
    				if (Gray4 > Gray2) Swap(Gray4, Gray2);
    
    				LinePD[X] = Gray4;
    			}
    		}
    
    	}
    	else {
        
    		for (int Y = 1; Y < Height - 1; Y++) {
        
    			unsigned char *LineP0 = Src + (Y - 1) * Stride + 3;
    			unsigned char *LineP1 = LineP0 + Stride;
    			unsigned char *LineP2 = LineP1 + Stride;
    			unsigned char *LinePD = Dest + Y * Stride + 3;
    			for (int X = 1; X < Width - 1; X++) {
        
    				int Blue0, Blue1, Blue2, Blue3, Blue4, Blue5, Blue6, Blue7, Blue8;
    				int Green0, Green1, Green2, Green3, Green4, Green5, Green6, Green7, Green8;
    				int Red0, Red1, Red2, Red3, Red4, Red5, Red6, Red7, Red8;
    				Blue0 = LineP0[-3];        Green0 = LineP0[-2];    Red0 = LineP0[-1];
    				Blue1 = LineP0[0];        Green1 = LineP0[1];        Red1 = LineP0[2];
    				Blue2 = LineP0[3];        Green2 = LineP0[4];        Red2 = LineP0[5];
    
    				Blue3 = LineP1[-3];        Green3 = LineP1[-2];    Red3 = LineP1[-1];
    				Blue4 = LineP1[0];        Green4 = LineP1[1];        Red4 = LineP1[2];
    				Blue5 = LineP1[3];        Green5 = LineP1[4];        Red5 = LineP1[5];
    
    				Blue6 = LineP2[-3];        Green6 = LineP2[-2];    Red6 = LineP2[-1];
    				Blue7 = LineP2[0];        Green7 = LineP2[1];        Red7 = LineP2[2];
    				Blue8 = LineP2[3];        Green8 = LineP2[4];        Red8 = LineP2[5];
    
    				if (Blue1 > Blue2) Swap(Blue1, Blue2);
    				if (Blue4 > Blue5) Swap(Blue4, Blue5);
    				if (Blue7 > Blue8) Swap(Blue7, Blue8);
    				if (Blue0 > Blue1) Swap(Blue0, Blue1);
    				if (Blue3 > Blue4) Swap(Blue3, Blue4);
    				if (Blue6 > Blue7) Swap(Blue6, Blue7);
    				if (Blue1 > Blue2) Swap(Blue1, Blue2);
    				if (Blue4 > Blue5) Swap(Blue4, Blue5);
    				if (Blue7 > Blue8) Swap(Blue7, Blue8);
    				if (Blue0 > Blue3) Swap(Blue0, Blue3);
    				if (Blue5 > Blue8) Swap(Blue5, Blue8);
    				if (Blue4 > Blue7) Swap(Blue4, Blue7);
    				if (Blue3 > Blue6) Swap(Blue3, Blue6);
    				if (Blue1 > Blue4) Swap(Blue1, Blue4);
    				if (Blue2 > Blue5) Swap(Blue2, Blue5);
    				if (Blue4 > Blue7) Swap(Blue4, Blue7);
    				if (Blue4 > Blue2) Swap(Blue4, Blue2);
    				if (Blue6 > Blue4) Swap(Blue6, Blue4);
    				if (Blue4 > Blue2) Swap(Blue4, Blue2);
    
    				if (Green1 > Green2) Swap(Green1, Green2);
    				if (Green4 > Green5) Swap(Green4, Green5);
    				if (Green7 > Green8) Swap(Green7, Green8);
    				if (Green0 > Green1) Swap(Green0, Green1);
    				if (Green3 > Green4) Swap(Green3, Green4);
    				if (Green6 > Green7) Swap(Green6, Green7);
    				if (Green1 > Green2) Swap(Green1, Green2);
    				if (Green4 > Green5) Swap(Green4, Green5);
    				if (Green7 > Green8) Swap(Green7, Green8);
    				if (Green0 > Green3) Swap(Green0, Green3);
    				if (Green5 > Green8) Swap(Green5, Green8);
    				if (Green4 > Green7) Swap(Green4, Green7);
    				if (Green3 > Green6) Swap(Green3, Green6);
    				if (Green1 > Green4) Swap(Green1, Green4);
    				if (Green2 > Green5) Swap(Green2, Green5);
    				if (Green4 > Green7) Swap(Green4, Green7);
    				if (Green4 > Green2) Swap(Green4, Green2);
    				if (Green6 > Green4) Swap(Green6, Green4);
    				if (Green4 > Green2) Swap(Green4, Green2);
    
    				if (Red1 > Red2) Swap(Red1, Red2);
    				if (Red4 > Red5) Swap(Red4, Red5);
    				if (Red7 > Red8) Swap(Red7, Red8);
    				if (Red0 > Red1) Swap(Red0, Red1);
    				if (Red3 > Red4) Swap(Red3, Red4);
    				if (Red6 > Red7) Swap(Red6, Red7);
    				if (Red1 > Red2) Swap(Red1, Red2);
    				if (Red4 > Red5) Swap(Red4, Red5);
    				if (Red7 > Red8) Swap(Red7, Red8);
    				if (Red0 > Red3) Swap(Red0, Red3);
    				if (Red5 > Red8) Swap(Red5, Red8);
    				if (Red4 > Red7) Swap(Red4, Red7);
    				if (Red3 > Red6) Swap(Red3, Red6);
    				if (Red1 > Red4) Swap(Red1, Red4);
    				if (Red2 > Red5) Swap(Red2, Red5);
    				if (Red4 > Red7) Swap(Red4, Red7);
    				if (Red4 > Red2) Swap(Red4, Red2);
    				if (Red6 > Red4) Swap(Red6, Red4);
    				if (Red4 > Red2) Swap(Red4, Red2);
    
    				LinePD[0] = Blue4;
    				LinePD[1] = Green4;
    				LinePD[2] = Red4;
    
    				LineP0 += 3;
    				LineP1 += 3;
    				LineP2 += 3;
    				LinePD += 3;
    			}
    		}
    	}
    }

其实上面的代码很好理解，我们将R,G,B三个通道分开看，每个通道执行的都是完全一样的操作，随着比较的不断执行，最后最小的4个数会排在前4个位置，最大的4个数会排在后4个位置，中位数恰好就在中间。这个算法的流水情况比第一个算法好多了，自然也会得到较大的速度提升。

同样来测试一下速度：

# 4. SSE优化 #

这里是本文的重点了，似乎这个算法看起来是不好做SSE优化的，因为窗口中像素的 9 9 9次比较不能直接用SIMD指令来做。根据ImageShop博主的提示，当我们看到下面这段代码`https://github.com/ARM-software/ComputeLibrary/blob/master/src/core/NEON/kernels/NEMedian3x3Kernel.cpp#L113`提示时，我们可以知道多个像素的比较是不相关的，（这个地方需要思考为什么不相关，因为我们比较的时候交换是使用临时变量，实际上是没有改变每个位置的像素的位置的）。所以SSE优化的思路就有了，现在可以一次性处理16个像素了。SSE优化的代码如下：

inline void _mm_sort_ab(__m128i &a, __m128i &b) {
        
    	const __m128i min = _mm_min_epu8(a, b);
    	const __m128i max = _mm_max_epu8(a, b);
    	a = min;
    	b = max;
    }
    
    void MedianBlur3X3_Fastest(unsigned char *Src, unsigned char *Dest, int Width, int Height, int Stride) {
        
    	int Channel = Stride / Width;
    	int BlockSize = 16, Block = ((Width - 2)* Channel) / BlockSize;
    	for (int Y = 1; Y < Height - 1; Y++) {
        
    		unsigned char *LineP0 = Src + (Y - 1) * Stride + Channel;
    		unsigned char *LineP1 = LineP0 + Stride;
    		unsigned char *LineP2 = LineP1 + Stride;
    		unsigned char *LinePD = Dest + Y * Stride + Channel;
    		for (int X = 0; X < Block * BlockSize; X += BlockSize, LineP0 += BlockSize, LineP1 += BlockSize, LineP2 += BlockSize, LinePD += BlockSize)
    		{
        
    			__m128i P0 = _mm_loadu_si128((__m128i *)(LineP0 - Channel));
    			__m128i P1 = _mm_loadu_si128((__m128i *)(LineP0 - 0));
    			__m128i P2 = _mm_loadu_si128((__m128i *)(LineP0 + Channel));
    			__m128i P3 = _mm_loadu_si128((__m128i *)(LineP1 - Channel));
    			__m128i P4 = _mm_loadu_si128((__m128i *)(LineP1 - 0));
    			__m128i P5 = _mm_loadu_si128((__m128i *)(LineP1 + Channel));
    			__m128i P6 = _mm_loadu_si128((__m128i *)(LineP2 - Channel));
    			__m128i P7 = _mm_loadu_si128((__m128i *)(LineP2 - 0));
    			__m128i P8 = _mm_loadu_si128((__m128i *)(LineP2 + Channel));
    
    			_mm_sort_ab(P1, P2);		_mm_sort_ab(P4, P5);		_mm_sort_ab(P7, P8);
    			_mm_sort_ab(P0, P1);		_mm_sort_ab(P3, P4);		_mm_sort_ab(P6, P7);
    			_mm_sort_ab(P1, P2);		_mm_sort_ab(P4, P5);		_mm_sort_ab(P7, P8);
    			_mm_sort_ab(P0, P3);		_mm_sort_ab(P5, P8);		_mm_sort_ab(P4, P7);
    			_mm_sort_ab(P3, P6);		_mm_sort_ab(P1, P4);		_mm_sort_ab(P2, P5);
    			_mm_sort_ab(P4, P7);		_mm_sort_ab(P4, P2);		_mm_sort_ab(P6, P4);
    			_mm_sort_ab(P4, P2);
    
    			_mm_storeu_si128((__m128i *)LinePD, P4);
    		}
    
    		for (int X = Block * BlockSize; X < (Width - 2) * Channel; X++, LinePD++) {
        
    			int Gray0, Gray1, Gray2, Gray3, Gray4, Gray5, Gray6, Gray7, Gray8;
    			Gray0 = LineP0[X - Block * BlockSize - Channel];        Gray1 = LineP0[X - Block * BlockSize];    Gray2 = LineP0[X - Block * BlockSize + Channel];
    			Gray3 = LineP1[X - Block * BlockSize - Channel];        Gray4 = LineP1[X - Block * BlockSize];    Gray5 = LineP1[X - Block * BlockSize + Channel];
    			Gray6 = LineP2[X - Block * BlockSize - Channel];        Gray7 = LineP2[X - Block * BlockSize];    Gray8 = LineP2[X - Block * BlockSize + Channel];
    
    			if (Gray1 > Gray2) Swap(Gray1, Gray2);
    			if (Gray4 > Gray5) Swap(Gray4, Gray5);
    			if (Gray7 > Gray8) Swap(Gray7, Gray8);
    			if (Gray0 > Gray1) Swap(Gray0, Gray1);
    			if (Gray3 > Gray4) Swap(Gray3, Gray4);
    			if (Gray6 > Gray7) Swap(Gray6, Gray7);
    			if (Gray1 > Gray2) Swap(Gray1, Gray2);
    			if (Gray4 > Gray5) Swap(Gray4, Gray5);
    			if (Gray7 > Gray8) Swap(Gray7, Gray8);
    			if (Gray0 > Gray3) Swap(Gray0, Gray3);
    			if (Gray5 > Gray8) Swap(Gray5, Gray8);
    			if (Gray4 > Gray7) Swap(Gray4, Gray7);
    			if (Gray3 > Gray6) Swap(Gray3, Gray6);
    			if (Gray1 > Gray4) Swap(Gray1, Gray4);
    			if (Gray2 > Gray5) Swap(Gray2, Gray5);
    			if (Gray4 > Gray7) Swap(Gray4, Gray7);
    			if (Gray4 > Gray2) Swap(Gray4, Gray2);
    			if (Gray6 > Gray4) Swap(Gray6, Gray4);
    			if (Gray4 > Gray2) Swap(Gray4, Gray2);
    
    			LinePD[X] = Gray4;
    			LineP0 += 1;
    			LineP1 += 1;
    			LineP2 += 1;
    		}
    	}
    }

来测一下速度：

<table> 
 <thead> 
  <tr> 
   <th>分辨率</th> 
   <th>算法优化</th> 
   <th>循环次数</th> 
   <th>速度</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>4032x3024</td> 
   <td>普通实现</td> 
   <td>100</td> 
   <td>8293.79ms</td> 
  </tr> 
  <tr> 
   <td>4032x3024</td> 
   <td>逻辑优化，更好的流水</td> 
   <td>100</td> 
   <td>83.75ms</td> 
  </tr> 
  <tr> 
   <td>4032x3024</td> 
   <td>SSE优化</td> 
   <td>100</td> 
   <td>11.93ms</td> 
  </tr> 
 </tbody> 
</table>

# 5. AVX优化 #

显然，我们可以将SSE版本稍加修改获得AVX指令优化的版本，这样我们就可以一次性处理32个元素了，代码如下：

inline void _mm_sort_AB(__m256i &a, __m256i &b) {
        
    	const __m256i min = _mm256_min_epu8(a, b);
    	const __m256i max = _mm256_max_epu8(a, b);
    	a = min;
    	b = max;
    }
    
    void MedianBlur3X3_Fastest_AVX(unsigned char *Src, unsigned char *Dest, int Width, int Height, int Stride) {
        
    	int Channel = Stride / Width;
    	int BlockSize = 32, Block = ((Width - 2)* Channel) / BlockSize;
    	for (int Y = 1; Y < Height - 1; Y++) {
        
    		unsigned char *LineP0 = Src + (Y - 1) * Stride + Channel;
    		unsigned char *LineP1 = LineP0 + Stride;
    		unsigned char *LineP2 = LineP1 + Stride;
    		unsigned char *LinePD = Dest + Y * Stride + Channel;
    		for (int X = 0; X < Block * BlockSize; X += BlockSize, LineP0 += BlockSize, LineP1 += BlockSize, LineP2 += BlockSize, LinePD += BlockSize)
    		{
        
    			__m256i P0 = _mm256_loadu_si256((const __m256i*)(LineP0 - Channel));
    			__m256i P1 = _mm256_loadu_si256((const __m256i*)(LineP0 - 0));
    			__m256i P2 = _mm256_loadu_si256((const __m256i*)(LineP0 + Channel));
    			__m256i P3 = _mm256_loadu_si256((const __m256i*)(LineP1 - Channel));
    			__m256i P4 = _mm256_loadu_si256((const __m256i*)(LineP1 - 0));
    			__m256i P5 = _mm256_loadu_si256((const __m256i*)(LineP1 + Channel));
    			__m256i P6 = _mm256_loadu_si256((const __m256i*)(LineP2 - Channel));
    			__m256i P7 = _mm256_loadu_si256((const __m256i*)(LineP2 - 0));
    			__m256i P8 = _mm256_loadu_si256((const __m256i*)(LineP2 + Channel));
    
    			_mm_sort_AB(P1, P2);		_mm_sort_AB(P4, P5);		_mm_sort_AB(P7, P8);
    			_mm_sort_AB(P0, P1);		_mm_sort_AB(P3, P4);		_mm_sort_AB(P6, P7);
    			_mm_sort_AB(P1, P2);		_mm_sort_AB(P4, P5);		_mm_sort_AB(P7, P8);
    			_mm_sort_AB(P0, P3);		_mm_sort_AB(P5, P8);		_mm_sort_AB(P4, P7);
    			_mm_sort_AB(P3, P6);		_mm_sort_AB(P1, P4);		_mm_sort_AB(P2, P5);
    			_mm_sort_AB(P4, P7);		_mm_sort_AB(P4, P2);		_mm_sort_AB(P6, P4);
    			_mm_sort_AB(P4, P2);
    
    			_mm256_storeu_si256((__m256i *)LinePD, P4);
    		}
    
    		for (int X = Block * BlockSize; X < (Width - 2) * Channel; X++, LinePD++) {
        
    			int Gray0, Gray1, Gray2, Gray3, Gray4, Gray5, Gray6, Gray7, Gray8;
    			Gray0 = LineP0[X - Block * BlockSize - Channel];        Gray1 = LineP0[X - Block * BlockSize];    Gray2 = LineP0[X - Block * BlockSize + Channel];
    			Gray3 = LineP1[X - Block * BlockSize - Channel];        Gray4 = LineP1[X - Block * BlockSize];    Gray5 = LineP1[X - Block * BlockSize + Channel];
    			Gray6 = LineP2[X - Block * BlockSize - Channel];        Gray7 = LineP2[X - Block * BlockSize];    Gray8 = LineP2[X - Block * BlockSize + Channel];
    
    			if (Gray1 > Gray2) Swap(Gray1, Gray2);
    			if (Gray4 > Gray5) Swap(Gray4, Gray5);
    			if (Gray7 > Gray8) Swap(Gray7, Gray8);
    			if (Gray0 > Gray1) Swap(Gray0, Gray1);
    			if (Gray3 > Gray4) Swap(Gray3, Gray4);
    			if (Gray6 > Gray7) Swap(Gray6, Gray7);
    			if (Gray1 > Gray2) Swap(Gray1, Gray2);
    			if (Gray4 > Gray5) Swap(Gray4, Gray5);
    			if (Gray7 > Gray8) Swap(Gray7, Gray8);
    			if (Gray0 > Gray3) Swap(Gray0, Gray3);
    			if (Gray5 > Gray8) Swap(Gray5, Gray8);
    			if (Gray4 > Gray7) Swap(Gray4, Gray7);
    			if (Gray3 > Gray6) Swap(Gray3, Gray6);
    			if (Gray1 > Gray4) Swap(Gray1, Gray4);
    			if (Gray2 > Gray5) Swap(Gray2, Gray5);
    			if (Gray4 > Gray7) Swap(Gray4, Gray7);
    			if (Gray4 > Gray2) Swap(Gray4, Gray2);
    			if (Gray6 > Gray4) Swap(Gray6, Gray4);
    			if (Gray4 > Gray2) Swap(Gray4, Gray2);
    
    			LinePD[X] = Gray4;
    			LineP0 += 1;
    			LineP1 += 1;
    			LineP2 += 1;
    		}
    	}
    }

同样，来测一下速度：

可以看到AVX虽然一次处理了32个像素，但速度的提升幅度并不是很大，只有2ms左右。

这里就不打算继续做AVX指令集的多线程优化和测速了，感兴趣的可以自行实验，基本上速度提升是很少了，

# 6. 总结 #

本文以一个 3 × 3 3\\times 3 3×3的中值滤波作为切入点，讨论了一下针对这个具体问题的优化思路，速度也从最开始普通实现的8293.79ms优化到了9.32ms，还是有一定的参考意义的。

--------------------

欢迎关注GiantPandaCV, 在这里你将看到独家的深度学习分享，坚持原创，每天分享我们学习到的新鲜知识。( • ̀ω•́ )✧

有对文章相关的问题，或者想要加入交流群，欢迎添加BBuf微信：

![二维码][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2p1c3Rfc29ydA_size_16_color_FFFFFF_t_70]  
为了方便读者获取资料以及我们公众号的作者发布一些Github工程的更新，我们成立了一个QQ群，二维码如下，感兴趣可以加入。

![公众号QQ交流群][QQ]

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2p1c3Rfc29ydA_size_16_color_FFFFFF_t_70]: https://img-blog.csdnimg.cn/20200110234905879.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2p1c3Rfc29ydA==,size_16,color_FFFFFF,t_70
[QQ]: https://img-blog.csdnimg.cn/20200517190745584.png#pic_center