Java 图片像素碰撞检测

在 2D 游戏或图形应用中，检测两个对象是否“碰撞”是常见需求。大多数情况下，开发者会用矩形边界（bounding box）来判断是否接触，但这会带来误判——两个透明区域的图片可能会被判定为相交。

今天分享的 ImageIconCrack 类，就通过像素级碰撞检测来解决这个问题。

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功能介绍

ImageIconCrack.detection() 方法接收两张图片（File a、File b）及它们在画布上的位置 (a_X, a_Y) 和 (b_X, b_Y)，返回一个布尔值：

• true → 图片上非透明像素实际重叠

• false → 没有像素重叠

import javax.imageio.ImageIO;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

public class ImageIconCrack {


    public static boolean detection(File a, File b, int a_X, int a_Y, int b_X, int b_Y) throws IOException {
        BufferedImage a_BufferImage = ImageIO.read(a);
        BufferedImage b_BufferImage = ImageIO.read(b);
        int a_Width = a_BufferImage.getWidth();
        int a_Height = a_BufferImage.getHeight();
        int b_Width = b_BufferImage.getWidth();
        int b_Height = b_BufferImage.getHeight();
        int a_X_right = a_X + a_Width;
        int a_Y_downside = a_Y + a_Height;
        int b_X_right = b_X + b_Width;
        int b_Y_downside = b_Y + b_Height;

        if (!findIfMeets(a_X, a_Y, b_X, b_Y, a_X_right, a_Y_downside, b_X_right, b_Y_downside)) {
            return false;
        }

        int[] x1_y1_x2_y2 = findMiddles(a_X, a_Y, b_X, b_Y, a_X_right, a_Y_downside, b_X_right, b_Y_downside);

        int a_ShouldSearch_x1 = Math.max(0, x1_y1_x2_y2[0] - a_X);
        int a_ShouldSearch_y1 = Math.max(0, x1_y1_x2_y2[1] - a_Y);
        int a_ShouldSearch_x2 = Math.min(a_Width - 1, x1_y1_x2_y2[2] - a_X);
        int a_ShouldSearch_y2 = Math.min(a_Height - 1, x1_y1_x2_y2[3] - a_Y);

        int b_ShouldSearch_x1 = Math.max(0, x1_y1_x2_y2[0] - b_X);
        int b_ShouldSearch_y1 = Math.max(0, x1_y1_x2_y2[1] - b_Y);
        int b_ShouldSearch_x2 = Math.min(b_Width - 1, x1_y1_x2_y2[2] - b_X);
        int b_ShouldSearch_y2 = Math.min(b_Height - 1, x1_y1_x2_y2[3] - b_Y);

        if (a_ShouldSearch_x1 > a_ShouldSearch_x2 || a_ShouldSearch_y1 > a_ShouldSearch_y2 ||
                b_ShouldSearch_x1 > b_ShouldSearch_x2 || b_ShouldSearch_y1 > b_ShouldSearch_y2) {
            return false;
        }

        int searchTimesX = a_ShouldSearch_x2 - a_ShouldSearch_x1 + 1;
        int searchTimesY = a_ShouldSearch_y2 - a_ShouldSearch_y1 + 1;

        for (int x = 0; x < searchTimesX; x++) {
            for (int y = 0; y < searchTimesY; y++) {
                int a_x = a_ShouldSearch_x1 + x;
                int a_y = a_ShouldSearch_y1 + y;
                int b_x = b_ShouldSearch_x1 + x;
                int b_y = b_ShouldSearch_y1 + y;

                if (a_x >= a_Width || a_y >= a_Height || b_x >= b_Width || b_y >= b_Height) {
                    continue;
                }

                int pixel_a = a_BufferImage.getRGB(a_x, a_y);
                int pixel_b = b_BufferImage.getRGB(b_x, b_y);
                if (!isAnEmptyPixel(pixel_a) && !isAnEmptyPixel(pixel_b)) {
                    return true;
                }
            }
        }
        return false;

    }

    private static boolean isAnEmptyPixel(int pixel) {
        int alpha = (pixel >> 24) & 0xFF;
        return alpha == 0;
    }

    private static boolean findIfMeets(int a_X, int a_Y, int b_X, int b_Y, int a_X_right, int a_Y_downside, int b_X_right, int b_Y_downside) {
        return a_X < b_X_right && a_X_right > b_X && a_Y < b_Y_downside && a_Y_downside > b_Y;
    }

    private static int[] findMiddles(int a_X, int a_Y, int b_X, int b_Y, int a_X_right, int a_Y_downside, int b_X_right, int b_Y_downside) {
        int[] x_four = {a_X, b_X, a_X_right, b_X_right};
        int[] y_four = {a_Y, b_Y, a_Y_downside, b_Y_downside};
        fourNumbers(x_four);
        fourNumbers(y_four);
        return new int[]{x_four[1], y_four[1], x_four[2], y_four[2]};
    }

    private static void fourNumbers(int[] numbers) {
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 3 - i; j++) {
                if (numbers[j] > numbers[j + 1]) {
                    int temp = numbers[j];
                    numbers[j] = numbers[j + 1];
                    numbers[j + 1] = temp;
                }
            }
        }
    }

}