如何在.NET中生成动态验证码？相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。
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）：
在此基础上，（感谢Direct2D/WIC）经过较小的改动，即可生成一个动态的Gif图片。
只要略微修改以上代码：
将PngBitmapEncoder改成GifBitmapEncoder*
然后将下面的for循环取消注释
将displacement.Scale = 100.0f;改成displacement.Scale = Math.Abs(frameId) * 10.0f;
看完上述内容，你们掌握如何在.NET中生成动态验证码的方法了吗？如果还想学到更多技能或想了解更多相关内容，欢迎关注亿速云行业资讯频道，感谢各位的阅读！
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