convolution3x3.cpp

#include <iostream>
#include <vector>

using namespace std;
typedef vector< vector<float>> matrix;

void PrintMatrix(const matrix& mat)
{
    for (int i = 0, row = mat.size(); i < row; ++i)
    {
        for (int j = 0, col = mat[i].size(); j < col; ++j)
        {
            cout << mat[i][j] << "\t";
        }

        cout << endl;
    }
}

void ResizeMatrix(matrix& mat, int row, int col)
{
    mat.resize(row);

    for (int i = 0; i < row; ++i)
    {
        mat[i].resize(col);
    }
}

// 3*3, without padding
matrix convolution(const matrix& image, const matrix& kernel)
{
    matrix ret;
    int kernelSize = 3;
    int shrink = kernelSize / 2;

    // TODO - Error handle. Ex: Check image resolution
    int imageRow = image.size();
    int imageCol = image[0].size();
    ResizeMatrix(ret, imageRow - kernelSize + 1, imageCol - kernelSize + 1);

    for (int i = shrink; i < imageRow - shrink; ++i)
    {
        int retI = i - shrink;
        for (int j = shrink; j < imageCol - shrink; ++j)
        {
            int retJ = j - shrink;
            // TODO - Do not re-compute overlap region in each step
            for (int ki = 0; ki < kernelSize; ++ki)
            {
                for (int kj = 0; kj < kernelSize; ++kj)
                {
                    ret[retI][retJ] += kernel[ki][kj] * image[i - shrink + ki][j - shrink + kj];
                }
            }
        }
    }
    return move(ret);
}

int main()
{
    matrix image = { {1, 0, 0, 0}, 
                     {0, 1, 0, 0}, 
                     {0, 0, 1, 0}, 
                     {0, 0, 0, 1} };

    matrix kernel = { { 0.1f, 0.1f, 0.1f },
                      { 0.1f, 0.1f, 0.1f },
                      { 0.1f, 0.1f, 0.1f } };

    matrix result = convolution(image, kernel);
    
    cout << "image = " << endl;
    PrintMatrix(image);

    cout << endl << "kernel = " << endl;
    PrintMatrix(kernel);

    cout << endl << "result = " << endl;
    PrintMatrix(result);
}