防止设置色差过大，导致循环次数过多，增加默认色差方法

hutool-captcha/src/test/java/cn/hutool/captcha/CaptchaUtilTest.java

@@ -26,17 +26,26 @@ public class CaptchaUtilTest {
 
 	@Test
 	@Ignore
-	public void drawStringColourfulTest() {
+	public void drawStringColourfulColorDistanceTest() {
 		for(int i = 0; i < 10; i++) {
-			AbstractCaptcha lineCaptcha = new TestLineCaptcha(200, 100, 5, 10);
+			AbstractCaptcha lineCaptcha = new TestLineCaptchaColorDistance(200, 100, 5, 10);
-			lineCaptcha.write("d:/captcha/line"+i+".png");
+			lineCaptcha.write("d:/captcha/line1-"+i+".png");
 		}
 	}
 
-	static class TestLineCaptcha extends AbstractCaptcha{
+	@Test
+	@Ignore
+	public void drawStringColourfulDefaultColorDistanceTest() {
+		for(int i = 0; i < 10; i++) {
+			AbstractCaptcha lineCaptcha = new TestLineCaptchaColorDistanceDefaultColorDistance(200, 100, 5, 10);
+			lineCaptcha.write("d:/captcha/line2-"+i+".png");
+		}
+	}
+
+	static class TestLineCaptchaColorDistance extends AbstractCaptcha{
 		private static final long serialVersionUID = -558846929114465692L;
 
-		public TestLineCaptcha(int width, int height, int codeCount, int interfereCount) {
+		public TestLineCaptchaColorDistance(int width, int height, int codeCount, int interfereCount) {
 			super(width, height, codeCount, interfereCount);
 		}
 
@@ -62,11 +71,12 @@ public class CaptchaUtilTest {
 		 * @param g {@link Graphics}画笔
 		 * @param code 验证码
 		 */
-		private void drawString(Graphics2D g, String code) {
+		protected void drawString(Graphics2D g, String code) {
 			// 指定透明度
 			if (null != this.textAlpha) {
 				g.setComposite(this.textAlpha);
 			}
+			// 自定义与背景颜色的色差值，200是基于Color.WHITE较为居中的值
 			GraphicsUtil.drawStringColourful(g, code, this.font, this.width, this.height,Color.WHITE,200);
 		}
 
@@ -89,4 +99,22 @@ public class CaptchaUtilTest {
 		}
 		// ----------------------------------------------------------------------------------------------------- Private method start
 	}
+
+	static class TestLineCaptchaColorDistanceDefaultColorDistance extends TestLineCaptchaColorDistance {
+
+
+		public TestLineCaptchaColorDistanceDefaultColorDistance(int width, int height, int codeCount, int interfereCount) {
+			super(width, height, codeCount, interfereCount);
+		}
+
+		@Override
+		protected void drawString(Graphics2D g, String code) {
+			// 指定透明度
+			if (null != this.textAlpha) {
+				g.setComposite(this.textAlpha);
+			}
+			// 使用默认色差设置
+			GraphicsUtil.drawStringColourful(g, code, this.font, this.width, this.height,Color.WHITE);
+		}
+	}
 }

hutool-core/src/main/java/cn/hutool/core/img/ColorUtil.java

@@ -1,6 +1,7 @@
 package cn.hutool.core.img;
 
 import cn.hutool.core.convert.Convert;
+import cn.hutool.core.lang.Assert;
 import cn.hutool.core.util.ArrayUtil;
 import cn.hutool.core.util.RandomUtil;
 import cn.hutool.core.util.StrUtil;
@@ -178,6 +179,9 @@ public class ColorUtil {
 	 * @since 5.8.30
 	 */
 	public static Color randomColor(Color compareColor,int minDistance) {
+		// 注意minDistance太大会增加循环次数，保证至少1/3的概率生成成功
+		Assert.isTrue(minDistance < maxDistance(compareColor) / 3 * 2,
+				"minDistance is too large, there are too few remaining colors!");
 		Color color = randomColor();
 		while (computeColorDistance(compareColor,color) < minDistance) {
 			color = randomColor();
@@ -195,6 +199,20 @@ public class ColorUtil {
 		return randomColor(null);
 	}
 
+	/**
+	 * 计算给定点与其他点之间的最大可能距离。
+	 *
+	 * @param color 指定颜色
+	 * @return 其余颜色与color的最大距离
+	 * @since 6.0.0-M16
+	 */
+	public static int maxDistance(final Color color) {
+		final int maxX = RGB_COLOR_BOUND - 2 * color.getRed();
+		final int maxY = RGB_COLOR_BOUND - 2 * color.getGreen();
+		final int maxZ = RGB_COLOR_BOUND - 2 * color.getBlue();
+		return (int)Math.sqrt(maxX * maxX + maxY * maxY + maxZ * maxZ);
+	}
+
 	/**
 	 * 计算两个颜色之间的色差，按三维坐标距离计算
 	 *

hutool-core/src/main/java/cn/hutool/core/img/GraphicsUtil.java

@@ -101,6 +101,23 @@ public class GraphicsUtil {
 		return drawString(g, str, font, null, width, height, compareColor, minColorDistance);
 	}
 
+	/**
+	 * 绘制字符串，使用随机颜色，并且与背景颜色保持一定色差，默认抗锯齿
+	 *
+	 * @param g      {@link Graphics}画笔
+	 * @param str    字符串
+	 * @param font   字体
+	 * @param width  字符串总宽度
+	 * @param height 字符串背景高度
+	 * @param backgroundColor 背景颜色
+	 * @return 画笔对象
+	 * @since 4.5.10
+	 */
+	public static Graphics drawStringColourful(Graphics g, String str, Font font, int width, int height, Color backgroundColor) {
+		// 默认色差为最大色差的1/2
+		return drawString(g, str, font, null, width, height, backgroundColor, ColorUtil.maxDistance(backgroundColor) / 2);
+	}
+
 	/**
 	 * 绘制字符串，默认抗锯齿
 	 *

hutool-core/src/main/java/cn/hutool/core/img/ImgUtil.java

@@ -2237,6 +2237,17 @@ public class ImgUtil {
 		return ColorUtil.randomColor(compareColor,minDistance);
 	}
 
+	/**
+	 * 生成随机颜色，与指定颜色有一定的区分度
+	 *
+	 * @param compareColor 比较颜色
+	 * @return 与指定颜色有一定的区分度的随机颜色，默认是最大可能的三维距离的一半
+	 * @since 5.8.30
+	 */
+	public static Color randomColor(Color compareColor) {
+		return ColorUtil.randomColor(compareColor,ColorUtil.maxDistance(compareColor) / 2);
+	}
+
 	/**
 	 * 生成随机颜色
 	 *