验证码优化

src/CaptchaKit.AspNetCore/CaptchaKitService.cs

@@ -147,21 +147,35 @@ public sealed class SkiaTextCaptchaGenerator : ICaptchaGenerator
         using var bitmap = new SKBitmap(width, 54);
         using var canvas = new SKCanvas(bitmap);
         canvas.Clear(new SKColor(244, 248, 252));
-        DrawNoise(canvas, width, 54, 16);
-        using var paint = new SKPaint { IsAntialias = true };
-        using var font = new SKFont(SKTypeface.FromFamilyName("Arial", SKFontStyle.Bold), 30);
-        var spacing = Math.Max(22, (width - 24) / displayText.Length);
+        DrawNoise(canvas, width, 54, 20);
+        // Use Skia's portable default typeface instead of relying on a server-installed font such as Arial.
+        using var font = new SKFont(SKTypeface.Default, 32) { Embolden = true };
+        using var outlinePaint = new SKPaint
+        {
+            IsAntialias = true,
+            Color = new SKColor(255, 255, 255),
+            Style = SKPaintStyle.Stroke,
+            StrokeWidth = 3
+        };
+        using var textPaint = new SKPaint
+        {
+            IsAntialias = true,
+            Color = new SKColor(20, 48, 80)
+        };
+        var spacing = Math.Min(34, Math.Max(24, (width - 28) / displayText.Length));
+        var startX = width / 2f - (displayText.Length - 1) * spacing / 2f;
+        var metrics = font.Metrics;
+        var baseline = (54 - (metrics.Descent - metrics.Ascent)) / 2f - metrics.Ascent;
         for (var index = 0; index < displayText.Length; index++)
         {
-            var x = 12 + index * spacing + RandomNumberGenerator.GetInt32(-2, 3);
-            var y = 38 + RandomNumberGenerator.GetInt32(-4, 5);
-            paint.Color = RandomColor(35, 145);
+            var x = startX + index * spacing + RandomNumberGenerator.GetInt32(-2, 3);
+            var y = baseline + RandomNumberGenerator.GetInt32(-3, 4);
             canvas.Save();
             canvas.RotateDegrees(RandomNumberGenerator.GetInt32(-20, 21), x, y);
-            canvas.DrawText(displayText[index].ToString(), x, y, SKTextAlign.Left, font, paint);
+            canvas.DrawText(displayText[index].ToString(), x, y, SKTextAlign.Center, font, outlinePaint);
+            canvas.DrawText(displayText[index].ToString(), x, y, SKTextAlign.Center, font, textPaint);
             canvas.Restore();
         }
-        DrawNoise(canvas, width, 54, 10);
         using var image = SKImage.FromBitmap(bitmap);
         using var data = image.Encode(SKEncodedImageFormat.Png, 100);
         var challenge = new CaptchaChallenge(request.Id, type, request.ExpiresAt,
@@ -179,10 +193,6 @@ public sealed class SkiaTextCaptchaGenerator : ICaptchaGenerator
         }
     }
 
-    private static SKColor RandomColor(int min, int max) => new(
-        (byte)RandomNumberGenerator.GetInt32(min, max),
-        (byte)RandomNumberGenerator.GetInt32(min, max),
-        (byte)RandomNumberGenerator.GetInt32(min, max));
     private static byte RandomByte() => RandomNumberGenerator.GetBytes(1)[0];
 }