本文将尝试使用MeanShift滤波来做磨皮算法;
MeanShift即均值漂移,最早由Fukunage在1975年提出,论文名字为:The Estimation of the Gradient of a density function.
MeanShift一般是指一个迭代的步骤,即先算出当前点的偏移均值,然后以此为新的起始点,继续移动,直到满足一定的结束条件;MeanShift广泛应用于图像聚类、平滑、分割和跟踪方面,本文主要讲的是图像的平滑滤波,尝试应用于人像的磨皮算法中;
我们使用一张图来讲解MeanShift的算法原理(此图来自网络):
Fig.1基本MeanShift算法示意图
我们假设起始位置的滤波半径为Radius,也就是图a中的蓝色圆形区域半径,图a为起始位置,假设红色点为目标像素,每个目标像素包含位置特征和像素RGB特征;
1,计算图a起始位置处,半径Radius内目标像素的位置特征和像素RGB特征的均值M,如图c所示;
2,将起始位置的初始特征(位置特征和RGB特征)更新为特征M;
3,计算M处半径Radius区域内,目标像素的均值特征M;
4,按照1-3的过程进行迭代,直到满足一定的迭代次数和限制条件;
5,图a中起始位置的RGB特征值即为迭代完成时M的RGB特征值,如图f所示;
整个过程也叫均值漂移,实际上不是位置从图a起始值漂移到了f图中的位置,而是图a和图f处的特征值归为了一类,当然这里指的是RGB像素值;
这里我们只讲最基本的MeanShift平滑滤波算法,对于改进的MeanShift算法不做讲解;
算法流程如下:
1,假设当前像素点P(i,j),滤波半径为R,迭代次数阈值为maxIter,像素差值阈值为threshold;
2,计算以P为中心,R为半径的圆形区域S内目标像素的均值特征,包含像素rgb的均值特征和位置的均值特征(质心),计算公式如下:
其中K为核函数,这里取得是|x-y|;
1,将P的特征值M更新为2中计算的新特征值;
2,按照2-3的步骤进行迭代,直到满足迭代次数阈值maxIter停止,P处的像素值即迭代终结时的rgb特征值;
上述即为MeanShift平滑滤波算法的流程,该算法最大缺点为速度慢,本文用它来尝试磨皮效果,采用YCbCr颜色空间,仅对Y通道处理,以此加速;
效果图如下所示:
完整C代码如下:
#include "string.h" #include "stdio.h" #include "stdlib.h" #include "math.h" #include"f_MeanShiftFilter.h" #include"TRGB2YCbCr.h" #define MIN2(a, b) ((a) < (b) ? (a) : (b)) #define MAX2(a, b) ((a) > (b) ? (a) : (b)) #define CLIP3(x, a, b) MIN2(MAX2(a,x), b) int MeanShiftOneChannel(unsigned char* srcData, int width ,int height, int radius, int threshold, int maxIter) { int len = sizeof(unsigned long) * width * height; int i, j; int gray = 0, sum = 0, srcGray = 0, count = 0; unsigned char* tempData = (unsigned char*) malloc(sizeof(unsigned char) * height * width); memcpy(tempData, srcData, sizeof(unsigned char) * height * width); for(j = 0; j < height; j++ ) { for(i = 0; i < width; i++) { len = i + j * width; int nIter = 0, cx = 0, cy = 0, sumx = 0, sumy = 0; srcGray = tempData[len]; cx = i; cy = j; while(nIter < maxIter) { sum = 0; sumx = 0; sumy = 0; count = 0; for(int y = cy - radius; y <= cy + radius; y++) { for(int x = cx - radius; x <= cx + radius; x++) { int px = CLIP3(x, 0, width - 1); int py = CLIP3(y, 0, height - 1); len = px + py * width; gray = tempData[len]; if(abs(gray - srcGray) < threshold) { count++; sum += gray; sumx += x; sumy += y; } } } if(count == 0) break; srcGray = sum / count; cx = sumx / count; cy = sumy / count; nIter++; } srcData[i + j * width] = srcGray; } } free(tempData); return 0; }; void f_MeanShiftFilter(unsigned char* srcData, int nWidth, int nHeight, int nStride, int radius, int threshold, int maxIter) { if (srcData == NULL) { return; } if(radius == 0 || threshold == 0) return; unsigned char* yData = (unsigned char*)malloc(sizeof(unsigned char) * nWidth * nHeight); unsigned char* cbData = (unsigned char*)malloc(sizeof(unsigned char) * nWidth * nHeight); unsigned char* crData = (unsigned char*)malloc(sizeof(unsigned char) * nWidth * nHeight); unsigned char* pSrc = srcData; int Y, CB, CR; unsigned char* pY = yData; unsigned char* pCb = cbData; unsigned char* pCr = crData; for(int j = 0; j < nHeight; j++) { for(int i = 0; i < nWidth; i++) { RGBToYCbCr(pSrc[2],pSrc[1],pSrc[0],&Y,&CB,&CR); *pY = Y; *pCb = CB; *pCr = CR; pY++; pCb++; pCr++; pSrc += 4; } } MeanShiftOneChannel(yData, nWidth, nHeight, radius, threshold, maxIter); pSrc = srcData; pY = yData; pCb = cbData; pCr = crData; int R, G, B; for(int j = 0; j < nHeight; j++) { for(int i = 0; i < nWidth; i++) { YCbCrToRGB(*pY, *pCb, *pCr, &R, &G, &B); pSrc[0] = B; pSrc[1] = G; pSrc[2] = R; pY++; pCb++; pCr++; pSrc += 4; } } free(yData); free(cbData); free(crData); }
代码已经贴出来了,这里就不给DEMO了,大家可以直接使用代码进行测试,代码中YCbCr转换函数前文博客中有,或者大家自己实现,都可以。
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