功能需求
- 在图片的给定位置上添加文字水印
- 水印可以旋转和设置透明度
先说说自己的实现思路：
- 先创建具有透明背景色的文字水印图像
- 将水印图像添加到原图像中
实现
首先创建一个接口，用于约束水印的创建方式：
public interface IWatermark
{
Bitmap CreateWatermark(string markText, Font font, Brush brush, Rectangle rectangle);
}
具体实现：
public class Watermark : IWatermark
{
//水印画布
protected virtual Rectangle WatermarkCanvas { set; get; }
protected Watermark(){}
public Watermark(string markText, Font font)
{
int width = (int)((markText.Length + 1) * font.Size);
int height = font.Height;
WatermarkCanvas = new Rectangle(0, 0, width, height);
}
///
/// 给图片添加水印，文字大小以像素(Pixel)为计量单位
///
/// 图片文件全名
public Bitmap Mark(string filename, string markText, Font font, Brush brush, float positionX, float positionY, int angle, int transparency)
{
return CreateMarkCore(filename, markText, font, brush, positionX, positionY, angle, transparency);
}
///
/// 绘制文字水印，文字大小以像素(Pixel)为计量单位
///
public virtual Bitmap CreateWatermark(string markText, Font font, Brush brush, Rectangle rectangle)
{
Bitmap watermark = new Bitmap(rectangle.Width, rectangle.Height);
Graphics graphics = Graphics.FromImage(watermark);
//消除锯齿
graphics.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias;
graphics.DrawString(markText, font, brush, rectangle);
graphics.Dispose();
return watermark;
}
///
/// 给图片添加水印，文字大小以像素(Pixel)为计量单位
///
/// 图片文件全名
protected virtual Bitmap CreateMarkCore(string filename, string markText, Font font, Brush brush, float positionX, float positionY, int angle, int transparency)
{
if (!File.Exists(filename))
{
throw new FileNotFoundException("文件不存在！");
}
Bitmap resultImg;
using (Bitmap rawImg = new Bitmap(filename))
{
using (Bitmap watermarkImg = CreateWatermark(markText, font, brush, WatermarkCanvas))
using (Bitmap rotateImg = Rotate(watermarkImg, angle))
{
using (Bitmap temp = SetAlpha(rotateImg, transparency))
{
resultImg = new Bitmap(rawImg.Width, rawImg.Height);
using (Graphics newGraphics = Graphics.FromImage(resultImg))
{
newGraphics.DrawImage(rawImg, 0, 0);
newGraphics.DrawImage(temp, positionX, positionY);
}
}
}
}
return resultImg;
}
}
水印图片透明度设置和旋转（下面这段代码和上面一段代码都位于Watermark类中，因为代码量较大，所以分开来展示）：
public class Watermark : IWatermark
{
protected Bitmap Rotate(Bitmap rawImg, int angle)
{
angle = angle % 360;
//弧度转换
double radian = TranslateAngleToRadian(angle);
//原图的宽和高
int width = rawImg.Width;
int height = rawImg.Height;
//旋转之后图像的宽和高
Rectangle rotateRec = RecalculateRectangleSize(width, height, angle);
int rotateWidth = rotateRec.Width;
int rotateHeight = rotateRec.Height;
//目标位图
Bitmap targetImg = new Bitmap(rotateWidth, rotateHeight);
Graphics targetGraphics = Graphics.FromImage(targetImg);
//计算偏
Point Offset = new Point((rotateWidth - width) / 2, (rotateHeight - height) / 2);
//构造图像显示区域：让图像的中心与窗口的中心点一致
Rectangle rect = new Rectangle(Offset.X, Offset.Y, width, height);
Point centerPoint = new Point(rect.X + rect.Width / 2, rect.Y + rect.Height / 2);
targetGraphics.TranslateTransform(centerPoint.X, centerPoint.Y);
targetGraphics.RotateTransform(angle);
//恢复图像在水平和垂直方向的平移
targetGraphics.TranslateTransform(-centerPoint.X, -centerPoint.Y);
targetGraphics.DrawImage(rawImg, rect);
//重至绘图的所有变换
targetGraphics.ResetTransform();
targetGraphics.Save();
targetGraphics.Dispose();
return targetImg;
}
///
/// 设置图像透明度，0：全透明，255：不透明
///
protected Bitmap SetAlpha(Bitmap rawImg, int alpha)
{
if (!(0 <= alpha) && alpha <= 255)
{
throw new ArgumentOutOfRangeException("alpha ranges from 0 to 255.");
}
//颜色矩阵
float[][] matrixItems =
{
new float[]{1,0,0,0,0},
new float[]{0,1,0,0,0},
new float[]{0,0,1,0,0},
new float[]{0,0,0,alpha/255f,0},
new float[]{0,0,0,0,1}
};
ColorMatrix colorMatrix = new ColorMatrix(matrixItems);
ImageAttributes imageAtt = new ImageAttributes();
imageAtt.SetColorMatrix(colorMatrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);
Bitmap resultImg = new Bitmap(rawImg.Width, rawImg.Height);
Graphics g = Graphics.FromImage(resultImg);
g.DrawImage(rawImg, new Rectangle(0, 0, rawImg.Width, rawImg.Height),
0, 0, rawImg.Width, rawImg.Height, GraphicsUnit.Pixel, imageAtt);
g.Dispose();
return resultImg;
}
protected double TranslateAngleToRadian(float angle)
{
return angle * Math.PI / 180;
}
protected Rectangle RecalculateRectangleSize(int width, int height, float angle)
{
double radian = TranslateAngleToRadian(angle);
double cos = Math.Cos(radian);
double sin = Math.Sin(radian);
double newWidth = (int)(Math.Max(Math.Abs(width * cos - height * sin), Math.Abs(width * cos + height * sin)));
double newHeight = (int)(Math.Max(Math.Abs(width * sin - height * cos), Math.Abs(width * sin + height * cos)));
return new Rectangle(0, 0, (int)newWidth, (int)newHeight);
}
}
Watermark类对外暴露了API：Bitmap Mark(string filename, string markText, Font font, Brush brush, float positionX, float positionY, int angle, int transparency)，向图片中添加水印只需创建Watermark实例，然后调用该方法即可。具体实现代码如下：
//.NET中，Font尺寸的默认单位是Point，这里统一使用Pixel作为计量单位
string path = @"C:\Users\chiwenjun\Desktop\1.PNG";
string markText = "字体：微软雅黑";
Font font = new Font("微软雅黑", 40, FontStyle.Bold, GraphicsUnit.Pixel);
Watermark watermark = new Watermark(markText, font);
Bitmap img = watermark.Mark(path, markText, font, new SolidBrush(Color.FromArgb(0, 0, 0)), 160, 535, 0, 180);
img.Save(path, ImageFormat.Png);
原图
添加水印效果图
水印顺时针旋转550效果
旋转前后，水印图像的宽和高会发生变化，如下图所示：
水印图片旋转前后宽高变化
扩展
上面的代码很好的实现了在图片上添加单行水印的效果，若要实现多行水印可以通过对Watermark类的扩展来实现。
创建类MultiLineWatermark继承自Watermark，然后覆写属性WatermarkCanvas来指定水印画布的大小；覆写方法CreateWatermark来实现多行水印效果。
public class MultiLineWatermark : Watermark
{
protected int _canvasWidth = 0;
protected int _canvasHeight = 0;
//每行水印所允许的最大字数
protected int _lineMaxLength = 0;
//水印所允许的最大字数
protected int _wordMaxLength = 0;
protected override Rectangle WatermarkCanvas
{
get
{
return new Rectangle(0, 0, this._canvasWidth, this._canvasHeight);
}
}
public MultiLineWatermark(int canvasWidth, int canvasHeight, int lineMaxLength, int wordMaxLength)
{
this._canvasWidth = canvasWidth;
this._canvasHeight = canvasHeight;
this._lineMaxLength = lineMaxLength;
this._wordMaxLength = wordMaxLength;
}
public override Bitmap CreateWatermark(string markText, Font font, Brush brush, Rectangle rectangle)
{
Bitmap watermark = new Bitmap(rectangle.Width, rectangle.Height);
Graphics graphics = Graphics.FromImage(watermark);
//消除锯齿
graphics.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias;
int lineHeight = _canvasHeight / (_wordMaxLength / _lineMaxLength);
if (markText.Contains('#'))
{
string[] textList = markText.Split('#');
int count = (int)Math.Min(textList.Length, Math.Ceiling(_wordMaxLength * 1.0 / _lineMaxLength));
for (int i = 0; i < count; i++)
{
if (textList[i].Length > _lineMaxLength)
{
textList[i] = textList[i].Substring(0, _lineMaxLength);
}
//文字居中
graphics.DrawString(textList[i], font, brush, (rectangle.Width - textList[i].Length * font.Size) / 2, i * lineHeight);
}
}
else
{
//文字居中
if (markText.Length <= _lineMaxLength)
{
graphics.DrawString(markText, font, brush, (rectangle.Width - markText.Length * font.Size) / 2, 0);
}
else
{
int count = (int)Math.Min(Math.Ceiling(_wordMaxLength * 1.0 / _lineMaxLength), Math.Ceiling(markText.Length * 1.0 / _lineMaxLength));
string[] temp = new string[count];
for (int i = 0; i < count; i++)
{
if (i * _lineMaxLength + _lineMaxLength <= markText.Length)
{
temp[i] = markText.Substring(i * _lineMaxLength, _lineMaxLength);
}
else
{
temp[i] = markText.Substring(i * _lineMaxLength, markText.Length - i * _lineMaxLength);
}
graphics.DrawString(temp[i], font, brush, (rectangle.Width - temp[i].Length * font.Size) / 2, i * lineHeight);
}
}
}
graphics.Dispose();
return watermark;
}
}
具体的使用方式和调用Watermark类似，代码如下：
string path = @"C:\Users\chiwenjun\Desktop\1.PNG";
//以#作为换行标记
string markText = "字体：#微软雅黑雅黑雅黑";
Font font = new Font("微软雅黑", 40, FontStyle.Bold, GraphicsUnit.Pixel);
//若字数超过每行所允许的最大值，超出部分被忽略
int lineMaxLength = 7;
//超出的字数会被忽略
int wordMaxLength = 14;
//行高，用于计算水印图像的高
int lineHeight = 55;
int width = (int)((lineMaxLength + 1) * font.Size);
int height = (int)(Math.Ceiling(wordMaxLength * 1.0 / lineMaxLength) * lineHeight);
Watermark watermark = new MultiLineWatermark(width, height, lineMaxLength, wordMaxLength);
Bitmap img = watermark.Mark(path, markText, font, new SolidBrush(Color.FromArgb(0, 0, 0)), 150, 535, 0, 180);
img.Save(path, ImageFormat.Png);
多行水印的文字是居中显示的：
多行水印效果图
若没有使用#标记换行，当一行字数超过指定的最大字数时，会自动换行。
自动换行效果
这篇文章是对自己项目中添加水印功能的记录，通篇以代码为主，看起来可能会感觉比较枯燥。
功能的实现没有太多难点，唯有一点感受较深，就是水印图像宽和高的计算。.NET(.NET Framework 4.5)中字体大小的度量单位默认是Point，而图像的度量单位是Pixel，单位的不同导致水印图像尺寸的计算出现偏差，这一点折磨我很久。
图像旋转和透明度设置的两个方法Rotate和SetAlpha是在网友代码基础上修改得到，非本人原创，代码原文已在参考文章中列出，在此对两位网友表示感谢。
参考文章：
C#图像旋转
设置图片透明度的四种方法