NET生成动态验证码的完整步骤

前言

验证码是图片上写上几个字，然后对这几个字做特殊处理，如扭曲、旋转、修改文字位置，然后加入一些线条，或加入一些特殊效果，使这些在人类能正常识别的同时，机器却很难识别出来，以达到防爬虫、防机器人的效果。

验证码通常用于网站中，是防爬虫、防机器人侵入的好方法。以往.NET中创建验证码，通常会使用System.Drawing创建“正常”的验证码。

在前一往篇博客.NET中生成水印更好的方法中，提到了如何给图片加水印。本文将基于上篇博客进一步探索，使用Direct2D创建验证码。

传统System.Drawing的做法

前置条件：引用System.Drawing，或者安装NuGet包：System.Drawing.Common：

<PackageReference Include="System.Drawing.Common" Version="4.5.1" />

首先创建一个有几个文字的图片（基本操作）：

byte[] GetImage(int width, int height, string text)
{
 using (var bitmap = new Bitmap(width, height))
 using (var g = Graphics.FromImage(bitmap))
 {
  var r = new Random();

  g.Clear(ColorFromHsl(r.NextDouble(), 1.0f, 0.8f, 0xff));

  var brush = new SolidBrush(Color.Black);
  var fontSize = width / text.Length;
  var font = new Font(FontFamily.GenericSerif, fontSize, FontStyle.Bold, GraphicsUnit.Pixel);
  for (var i = 0; i < text.Length; i++)
  {
   brush.Color = ColorFromHsl(r.NextDouble(), 1.0f, 0.3f, 0xff);
   float x = i * fontSize;
   float y = r.Next(0, height - fontSize);

   g.DrawString(text[i].ToString(), font, brush, x, y);
  }

  // 在这里面加入一些其它效果

  var ms = new MemoryStream();
  bitmap.Save(ms, ImageFormat.Png);
  return ms.ToArray();
 }
}

效果（Gif是由LINQPad生成多次截图而来，实际为静态图）：

然后再加入一些线条：

using (var pen = new Pen(brush, 3))
{
 for (var i = 0; i < 4; ++i)
 {
  pen.Color = ColorFromHsl(r.NextDouble(), 1.0f, 0.4f, 0xff);
  var p1 = new Point(r.Next(width), r.Next(height));
  var p2 = new Point(r.Next(width), r.Next(height));
  g.DrawLine(pen, p1, p2);
 }
}

效果（Gif是由LINQPad生成多次截图而来，实际为静态图）：

还能做什么？

很遗憾，还有很多可以做，即使是加入线条，机器依然能轻而易举地识别出来。

不过Edi.Wang在他的博客中也发布了一个生成验证码的NuGet包：Edi.Captcha，截止目前最新版是1.3.1：

<PackageReference Include="Edi.Captcha" Version="1.3.1" />

这个包基于System.Drawing，加入了扭曲效果，加入了一些随机的x坐标偏移，极大地增加了AI识别的难度。

使用方式：

CaptchaResult result = CaptchaImageGenerator.GetImage(200, 100, "HELLO");

其中CaptchaResult的定义如下：

public class CaptchaResult
{
 public string CaptchaCode { get; set; }

 public byte[] CaptchaByteData { get; set; }

 public string CaptchBase64Data => Convert.ToBase64String(CaptchaByteData);

 public DateTime Timestamp { get; set; }
}

生成的效果如下（Gif是由LINQPad生成多次截图而来，实际为静态图）：

Direct2D

在前一篇博客中，已经有了Direct2D的相关简介。这里将不再介绍。

首先从最简单的图片上写文字开始：

byte[] SaveD2DBitmap(int width, int height, string text)
{
 using var wic = new WIC.ImagingFactory2();
 using var d2d = new D2D.Factory();
 using var wicBitmap = new WIC.Bitmap(wic, width, height, WIC.PixelFormat.Format32bppPBGRA, WIC.BitmapCreateCacheOption.CacheOnDemand);
 using var target = new D2D.WicRenderTarget(d2d, wicBitmap, new D2D.RenderTargetProperties());
 using var dwriteFactory = new SharpDX.DirectWrite.Factory();
 using var brush = new SolidColorBrush(target, Color.Yellow);
 
 var r = new Random();
 
 target.BeginDraw();
 target.Clear(ColorFromHsl(r.NextFloat(0, 1), 1.0f, 0.3f));
 var textFormat = new DWrite.TextFormat(dwriteFactory, "Times New Roman", 
  DWrite.FontWeight.Bold, 
  DWrite.FontStyle.Normal, 
  width / text.Length);
 for (int charIndex = 0; charIndex < text.Length; ++charIndex)
 {
  using var layout = new DWrite.TextLayout(dwriteFactory, text[charIndex].ToString(), textFormat, float.MaxValue, float.MaxValue);
  var layoutSize = new Vector2(layout.Metrics.Width, layout.Metrics.Height);
  using var b2 = new LinearGradientBrush(target, new D2D.LinearGradientBrushProperties
  {
   StartPoint = Vector2.Zero, 
   EndPoint = layoutSize, 
  }, new GradientStopCollection(target, new[]
  {
   new GradientStop{ Position = 0.0f, Color = ColorFromHsl(r.NextFloat(0, 1), 1.0f, 0.8f) },
   new GradientStop{ Position = 1.0f, Color = ColorFromHsl(r.NextFloat(0, 1), 1.0f, 0.8f) },
  }));

  var position = new Vector2(charIndex * width / text.Length, r.NextFloat(0, height - layout.Metrics.Height));
  target.Transform = 
   Matrix3x2.Translation(-layoutSize / 2) * 
   // 文字旋转和扭曲效果，取消注释以下两行代码
   // Matrix3x2.Skew(r.NextFloat(0, 0.5f), r.NextFloat(0, 0.5f)) *
   // Matrix3x2.Rotation(r.NextFloat(0, MathF.PI * 2)) * 
   Matrix3x2.Translation(position + layoutSize / 2);
  target.DrawTextLayout(Vector2.Zero, layout, b2);
 }
 // 其它效果在这里插入

 target.EndDraw();

 using (var encoder = new WIC.BitmapEncoder(wic, WIC.ContainerFormatGuids.Png))
 using (var ms = new MemoryStream())
 {
  encoder.Initialize(ms);
  using (var frame = new WIC.BitmapFrameEncode(encoder))
  {
   frame.Initialize();
   frame.SetSize(wicBitmap.Size.Width, wicBitmap.Size.Height);

   var pixelFormat = wicBitmap.PixelFormat;
   frame.SetPixelFormat(ref pixelFormat);
   frame.WriteSource(wicBitmap);

   frame.Commit();
  }

  encoder.Commit();
  return ms.ToArray();
 }
}

使用方式：

byte[] captchaBytes = SaveD2DBitmap(200, 100, "Hello");

效果（Gif是由LINQPad生成多次截图而来，实际为静态图）：

可以注意到，Direct2D生成的文字没有System.Drawing那样的锯齿。

如果取消里面的两行注释，可以得到更加扭曲和旋转的效果（Gif是由LINQPad生成多次截图而来，实际为静态图）：

然后加入线条：

for (var i = 0; i < 4; ++i)
{
 target.Transform = Matrix3x2.Identity;
 brush.Color = ColorFromHsl(r.NextFloat(0,1), 1.0f, 0.3f);
 target.DrawLine(
  r.NextVector2(Vector2.Zero, new Vector2(width, height)),
  r.NextVector2(Vector2.Zero, new Vector2(width, height)),
  brush, 3.0f);
}

效果（Gif是由LINQPad生成多次截图而来，实际为静态图）：

Direct2D的骚操作

Direct2D中内置了许多特效，如阴影(Shadow)等，这里我们需要用到的是位移特效(Displacement)和水流特效(Turbulence)，为了实现特效，需要加入一个Bitmap层，整体代码如下：

byte[] SaveD2DBitmap(int width, int height, string text)
{
 using var wic = new WIC.ImagingFactory2();
 using var d2d = new D2D.Factory();
 using var wicBitmap = new WIC.Bitmap(wic, width, height, WIC.PixelFormat.Format32bppPBGRA, WIC.BitmapCreateCacheOption.CacheOnDemand);
 using var target = new D2D.WicRenderTarget(d2d, wicBitmap, new D2D.RenderTargetProperties());
 using var dwriteFactory = new SharpDX.DirectWrite.Factory();
 using var brush = new D2D.SolidColorBrush(target, Color.Yellow);
 using var encoder = new WIC.PngBitmapEncoder(wic); // PngBitmapEncoder
 
 using var ms = new MemoryStream();
 using var dc = target.QueryInterface<D2D.DeviceContext>();
 using var bmpLayer = new D2D.Bitmap1(dc, target.PixelSize,
  new D2D.BitmapProperties1(new D2D.PixelFormat(SharpDX.DXGI.Format.B8G8R8A8_UNorm, D2D.AlphaMode.Premultiplied),
  d2d.DesktopDpi.Width, d2d.DesktopDpi.Height,
  D2D.BitmapOptions.Target));

 var r = new Random();
 encoder.Initialize(ms);

 D2D.Image oldTarget = dc.Target;
 {
  dc.Target = bmpLayer;
  dc.BeginDraw();
  var textFormat = new DWrite.TextFormat(dwriteFactory, "Times New Roman",
   DWrite.FontWeight.Bold,
   DWrite.FontStyle.Normal,
   width / text.Length);
  for (int charIndex = 0; charIndex < text.Length; ++charIndex)
  {
   using var layout = new DWrite.TextLayout(dwriteFactory, text[charIndex].ToString(), textFormat, float.MaxValue, float.MaxValue);
   var layoutSize = new Vector2(layout.Metrics.Width, layout.Metrics.Height);
   using var b2 = new D2D.LinearGradientBrush(dc, new D2D.LinearGradientBrushProperties
   {
    StartPoint = Vector2.Zero,
    EndPoint = layoutSize,
   }, new D2D.GradientStopCollection(dc, new[]
   {
    new D2D.GradientStop{ Position = 0.0f, Color = ColorFromHsl(r.NextFloat(0, 1), 1.0f, 0.8f) },
    new D2D.GradientStop{ Position = 1.0f, Color = ColorFromHsl(r.NextFloat(0, 1), 1.0f, 0.8f) },
   }));

   var position = new Vector2(charIndex * width / text.Length, r.NextFloat(0, height - layout.Metrics.Height));
   dc.Transform =
    Matrix3x2.Translation(-layoutSize / 2) *
    Matrix3x2.Skew(r.NextFloat(0, 0.5f), r.NextFloat(0, 0.5f)) *
    //Matrix3x2.Rotation(r.NextFloat(0, MathF.PI * 2)) *
    Matrix3x2.Translation(position + layoutSize / 2);
   dc.DrawTextLayout(Vector2.Zero, layout, b2);
  }
  for (var i = 0; i < 4; ++i)
  {
   target.Transform = Matrix3x2.Identity;
   brush.Color = ColorFromHsl(r.NextFloat(0, 1), 1.0f, 0.3f);
   target.DrawLine(
    r.NextVector2(Vector2.Zero, new Vector2(width, height)),
    r.NextVector2(Vector2.Zero, new Vector2(width, height)),
    brush, 3.0f);
  }
  target.EndDraw();
 }
 
 Color background = ColorFromHsl(r.NextFloat(0, 1), 1.0f, 0.3f);
 // for (var frameId = -10; frameId < 10; ++frameId)
 {
  dc.Target = null;
  using var displacement = new D2D.Effects.DisplacementMap(dc);
  displacement.SetInput(0, bmpLayer, true);
  displacement.Scale = 100.0f; // Math.Abs(frameId) * 10.0f;
  
  var turbulence = new D2D.Effects.Turbulence(dc);
  displacement.SetInputEffect(1, turbulence);

  dc.Target = oldTarget;
  dc.BeginDraw();
  dc.Clear(background);
  dc.DrawImage(displacement);
  dc.EndDraw();

  using (var frame = new WIC.BitmapFrameEncode(encoder))
  {
   frame.Initialize();
   frame.SetSize(wicBitmap.Size.Width, wicBitmap.Size.Height);

   var pixelFormat = wicBitmap.PixelFormat;
   frame.SetPixelFormat(ref pixelFormat);
   frame.WriteSource(wicBitmap);

   frame.Commit();
  }
 }

 encoder.Commit();
 return ms.ToArray();
}

注意此代码使用了using var语句，是C# 8.0的using declaration功能，可以用using (var )语句代替。

效果如下（Gif是由LINQPad生成多次截图而来，实际为静态图）：