矩阵的掩模操作(锐化举例)

矩阵的掩模操作（通常也叫做卷积操作）非常简单。本文的中心思想是基于掩模矩阵（也称为内核或者卷积核）重新计算图像每个像素的值。此掩模矩阵（卷积核）的值定义了当前像素和相邻像素对新像素值进行影响的值。From a mathematical point of view we make a weighted average, with our specified values.(从数学的角度来看，基于掩模矩阵指定的值进行mask操作后再进行加权平均)。具体（实现锐化效果的）描述如下：

第一个是公式的形式; 第二个是基于掩模矩阵的版本。两个等价。

I(i,j)=5∗I(i,j)−[I(i−1,j)+I(i+1,j)+I(i,j−1)+I(i,j+1)]

等价于

I(i,j)∗M,其中M=0−10−15−10−10

实现锐化效果的例子代码如下：

#include <opencv2/imgcodecs.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/imgproc.hpp>
#include <iostream>

using namespace std;
using namespace cv;

static void help(char* progName)
{
    cout << endl
        <<  "This program shows how to filter images with mask: the write it yourself and the"
        << "filter2d way. " << endl
        <<  "Usage:"                                                                        << endl
        << progName << " [image_name -- default lena.jpg] [G -- grayscale] "        << endl << endl;
}


void Sharpen(const Mat& myImage,Mat& Result);

int main( int argc, char* argv[])
{
    help(argv[0]);
    const char* filename = argc >=2 ? argv[1] : "lena.jpg";

    Mat src, dst0, dst1;

    if (argc >= 3 && !strcmp("G", argv[2]))
        src = imread( filename, IMREAD_GRAYSCALE);
    else
        src = imread( filename, IMREAD_COLOR);

    if (src.empty())
    {
        cerr << "Can't open image ["  << filename << "]" << endl;
        return -1;
    }

    namedWindow("Input", WINDOW_AUTOSIZE);
    namedWindow("Output", WINDOW_AUTOSIZE);

    imshow( "Input", src );
    double t = (double)getTickCount();

    Sharpen( src, dst0 );

    t = ((double)getTickCount() - t)/getTickFrequency();
    cout << "Hand written function times passed in seconds: " << t << endl;

    imshow( "Output", dst0 );
    waitKey();

//掩模矩阵定义初始化，用于filter2D函数
    Mat kernel = (Mat_<char>(3,3) <<  0, -1,  0,
                                   -1,  5, -1,
                                    0, -1,  0);
    t = (double)getTickCount();
    filter2D( src, dst1, src.depth(), kernel );
    t = ((double)getTickCount() - t)/getTickFrequency();
    cout << "Built-in filter2D time passed in seconds:      " << t << endl;

    imshow( "Output", dst1 );

    waitKey();
    return 0;
}

 //基于公式计算的版本
void Sharpen(const Mat& myImage,Mat& Result)//公式计算方式
{
    CV_Assert(myImage.depth() == CV_8U);  // accept only uchar images

    const int nChannels = myImage.channels();
    Result.create(myImage.size(),myImage.type());

    for(int j = 1 ; j < myImage.rows-1; ++j)
    {
        const uchar* previous = myImage.ptr<uchar>(j - 1);
        const uchar* current  = myImage.ptr<uchar>(j    );
        const uchar* next     = myImage.ptr<uchar>(j + 1);

        uchar* output = Result.ptr<uchar>(j);

        for(int i= nChannels;i < nChannels*(myImage.cols-1); ++i)
        {
            *output++ = saturate_cast<uchar>(5*current[i]-current[i-nChannels] - current[i+nChannels] - previous[i] - next[i]);
        }
    }

    Result.row(0).setTo(Scalar(0));
    Result.row(Result.rows-1).setTo(Scalar(0));
    Result.col(0).setTo(Scalar(0));
    Result.col(Result.cols-1).setTo(Scalar(0));
}