C#图像处理OpenCV开发指南（CVStar，07）——通用滤波（Filter2D）的实例代码

1 函数定义

void Filter2D    (Mat src,
Mat dst,
int     ddepth,
InputArray     kernel,
Point     anchor = Point(-1,-1),
double     delta = 0,
int     borderType = BORDER_DEFAULT 
)        

1.1 原型

#include

Convolves an image with the kernel.

使用内核对图像进行卷积。

The function applies an arbitrary linear filter to an image. In-place operation is supported. When the aperture is partially outside the image, the function interpolates outlier pixel values according to the specified border mode.

该函数将任意线性滤波器应用于图像。支持就地操作。当光圈部分位于图像之外时，该函数会根据指定的边界模式对异常像素值进行插值。

The function does actually compute correlation, not the convolution:

That is, the kernel is not mirrored around the anchor point. If you need a real convolution, flip the kernel using flip and set the new anchor to (kernel.cols - anchor.x - 1, kernel.rows - anchor.y - 1).

The function uses the DFT-based algorithm in case of sufficiently large kernels (~11 x 11 or larger) and the direct algorithm for small kernels.

1.2 参数说明 Parameters

• src    input image. 输入图像。
• dst    output image of the same size and the same number of channels as src. 输出与src具有相同大小和相同通道数的图像。
• ddepth    desired depth of the destination image, see combinations 目标图像的所需深度，请参阅组合
• kernel    convolution kernel (or rather a correlation kernel), a single-channel floating point matrix; if you want to apply different kernels to different channels, split the image into separate color planes using split and process them individually. 核卷积核（或者更确切地说是相关核）、单通道浮点矩阵；如果要将不同的内核应用于不同的通道，请使用split将图像拆分为单独的颜色平面，然后分别进行处理。
• anchor    anchor of the kernel that indicates the relative position of a filtered point within the kernel; the anchor should lie within the kernel; default value (-1,-1) means that the anchor is at the kernel center. 所述内核的锚定锚，所述锚定锚指示所述内核内的滤波点的相对位置；锚应该位于内核内；默认值（-1，-1）表示锚点位于内核中心。
• delta    optional value added to the filtered pixels before storing them in dst. 可选值，在将滤波像素存储在dst中之前添加到滤波像素。
• borderType    pixel extrapolation method, see BorderTypes. BORDER_WRAP is not supported. 像素外推法，请参见BorderTypes。不支持BORDER_WRAP。

2 代码解释

2.1 核心代码

private void Filter2D(object? sender, EventArgs? e)
{
    Mat src = Cv2.ImRead(sourceImage);
    Mat dst = new Mat();
    // 自定义卷积核（通用过滤）
    InputArray arr = InputArray.Create(new float[3, 3] {
        { 0, -1, 0 },
        { -1, 5, -1 },
        { 0, -1, 0 }
    });
    Cv2.Filter2D(
        src: src,
        dst: dst,
        ddepth: -1,
        kernel: arr,
        anchor: new OpenCvSharp.Point(-1, -1),
        delta: 0,
        borderType: BorderTypes.Default);

    picResult.Image = CVUtility.Mat2Bitmap(dst);
    PicAutosize(picResult);
}

通过修改 arr 卷积核矩阵，可获得不同的效果。 

2.2 Form1.cs 完整源程序

using OpenCvSharp;

#pragma warning disable CS8602

namespace Legal.Truffer.CVStar
{
    public partial class Form1 : Form
    {
        string[] ImgExtentions = {
            "*.*|*.*",
            "JPEG|*.jpg;*.jpeg",
            "GIF|*.gif",
            "PNG|*.png",
            "TIF|*.tif;*.tiff",
            "BMP|*.bmp"
        };
        private int original_width { get; set; } = 0;
        private int original_height { get; set; } = 0;
        private string sourceImage { get; set; } = "";

        Panel? panelTop { get; set; } = null;
        Panel? panelBotton { get; set; } = null;
        PictureBox? picSource { get; set; } = null;
        PictureBox? picResult { get; set; } = null;
        Button? btnLoad { get; set; } = null;
        Button? btnSave { get; set; } = null;
        Button? btnFunction { get; set; } = null;

        public Form1()
        {
            InitializeComponent();

            this.Text = "OPENCV C#编程入手教程 POWERED BY 深度混淆（CSDN.NET）";
            this.StartPosition = FormStartPosition.CenterScreen;

            GUI();
            this.Resize += FormResize;
        }

        private void FormResize(object? sender, EventArgs? e)
        {
            if (this.Width < 200) { this.Width = 320; return; }
            if (this.Height < 200) { this.Height = 320; return; }
            GUI();
        }

        private void GUI()
        {
            if (panelTop == null) panelTop = new Panel();
            panelTop.Parent = this;
            panelTop.Top = 5;
            panelTop.Left = 5;
            panelTop.Width = this.Width - 26;
            panelTop.Height = 85;
            panelTop.BorderStyle = BorderStyle.FixedSingle;
            panelTop.BackColor = Color.FromArgb(200, 200, 255);

            if (panelBotton == null) panelBotton = new Panel();
            panelBotton.Parent = this;
            panelBotton.Top = panelTop.Top + panelTop.Height + 3;
            panelBotton.Left = 5;
            panelBotton.Width = panelTop.Width;
            panelBotton.Height = this.Height - panelBotton.Top - 55;
            panelBotton.BorderStyle = BorderStyle.FixedSingle;

            if (picSource == null) picSource = new PictureBox();
            picSource.Parent = panelBotton;
            picSource.Left = 5;
            picSource.Top = 5;
            picSource.Width = (panelBotton.Width - 10) / 2;
            picSource.Height = (panelBotton.Height - 10);
            picSource.BorderStyle = BorderStyle.FixedSingle;

            if (picResult == null) picResult = new PictureBox();
            picResult.Parent = panelBotton;
            picResult.Left = picSource.Left + picSource.Width + 5;
            picResult.Top = picSource.Top;
            picResult.Width = picSource.Width;
            picResult.Height = picSource.Height;
            picResult.BorderStyle = BorderStyle.FixedSingle;

            original_width = picSource.Width;
            original_height = picSource.Height;

            if (btnLoad == null) btnLoad = new Button();
            btnLoad.Parent = panelTop;
            btnLoad.Left = 5;
            btnLoad.Top = 5;
            btnLoad.Width = 90;
            btnLoad.Height = 38;
            btnLoad.Cursor = Cursors.Hand;
            btnLoad.Text = "Load";
            btnLoad.Click += Load_Image;
            btnLoad.BackColor = Color.LightCoral;

            if (btnSave == null) btnSave = new Button();
            btnSave.Parent = panelTop;
            btnSave.Left = panelTop.Width - btnSave.Width - 25;
            btnSave.Top = btnLoad.Top;
            btnSave.Width = 90;
            btnSave.Height = 38;
            btnSave.Cursor = Cursors.Hand;
            btnSave.Text = "Save";
            btnSave.Click += Save;
            btnSave.BackColor = Color.LightCoral;

            if (btnFunction == null) btnFunction = new Button();
            btnFunction.Parent = panelTop;
            btnFunction.Left = btnLoad.Left + btnLoad.Width + 5;
            btnFunction.Top = btnLoad.Top;
            btnFunction.Width = 90;
            btnFunction.Height = 38;
            btnFunction.Cursor = Cursors.Hand;
            btnFunction.Text = "Filter2D";
            btnFunction.Click += Filter2D;
            btnFunction.BackColor = Color.LightCoral;

            PicAutosize(picSource);
            PicAutosize(picResult);
        }

        private void Load_Image(object? sender, EventArgs? e)
        {
            OpenFileDialog openFileDialog = new OpenFileDialog();
            openFileDialog.Filter = String.Join("|", ImgExtentions);
            if (openFileDialog.ShowDialog() == DialogResult.OK)
            {
                sourceImage = openFileDialog.FileName;
                picSource.Image = Image.FromFile(sourceImage);
                picResult.Image = picSource.Image;
                PicAutosize(picSource);
                PicAutosize(picResult);
            }
        }

        private void PicAutosize(PictureBox pb)
        {
            if (pb == null) return;
            if (pb.Image == null) return;
            Image img = pb.Image;
            int w = original_width;
            int h = w * img.Height / img.Width;
            if (h > original_height)
            {
                h = original_height;
                w = h * img.Width / img.Height;
            }
            pb.SizeMode = PictureBoxSizeMode.Zoom;
            pb.Width = w;
            pb.Height = h;
            pb.Image = img;
            pb.Refresh();
        }

        private void Save(object? sender, EventArgs? e)
        {
            SaveFileDialog saveFileDialog = new SaveFileDialog();
            saveFileDialog.Filter = String.Join("|", ImgExtentions);
            if (saveFileDialog.ShowDialog() == DialogResult.OK)
            {
                picResult.Image.Save(saveFileDialog.FileName);
                MessageBox.Show("Image Save to " + saveFileDialog.FileName);
            }
        }


        private void Filter2D(object? sender, EventArgs? e)
        {
            Mat src = Cv2.ImRead(sourceImage);
            Mat dst = new Mat();
            // 自定义卷积核（通用过滤）
            InputArray arr = InputArray.Create(new float[3, 3] {
                { 0, -1, 0 },
                { -1, 5, -1 },
                { 0, -1, 0 }
            });
            Cv2.Filter2D(
                src: src,
                dst: dst,
                ddepth: -1,
                kernel: arr,
                anchor: new OpenCvSharp.Point(-1, -1),
                delta: 0,
                borderType: BorderTypes.Default);

            picResult.Image = CVUtility.Mat2Bitmap(dst);
            PicAutosize(picResult);
        }

    }
}

3 运行效果

3x3的卷积核显然太小了，效果不明显。