Android Studio+OpenCV触摸取色始终返回R(0)G(0)B(0)问题排查

我在Android Studio中用OpenCV实现了相机实时预览功能，现在想要获取触摸像素点(x,y)对应的RGB颜色值，但当前代码始终返回R(0)G(0)B(0)，代码如下：

import androidx.annotation.NonNonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.Manifest;
import android.annotation.SuppressLint;
import android.content.pm.PackageManager;
import android.graphics.Bitmap;
import android.graphics.Color;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.View;
import android.widget.TextView;
import android.widget.Toast;

import org.opencv.android.CameraActivity;
import org.opencv.android.CameraBridgeViewBase;
import org.opencv.android.OpenCVLoader;
import org.opencv.core.Mat;
import java.util.Collections;
import java.util.List;

public class MainActivity extends CameraActivity {

    CameraBridgeViewBase cameraBridgeViewBase;
    TextView textView2, textView1;
    Bitmap bitmap;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        getPermission();

        cameraBridgeViewBase = findViewById(R.id.cameraView);
        textView2 = findViewById(R.id.textView2);
        textView1 = findViewById(R.id.textView);


        cameraBridgeViewBase.setCvCameraViewListener(new CameraBridgeViewBase.CvCameraViewListener2() {
            @Override
            public void onCameraViewStarted(int width, int height) {

            }

            @Override
            public void onCameraViewStopped() {}

            @Override
            public Mat onCameraFrame(CameraBridgeViewBase.CvCameraViewFrame inputFrame) {
                return inputFrame.rgba();
            }
        });
        if(OpenCVLoader.initDebug()){
            cameraBridgeViewBase.enableView();
        }
        cameraBridgeViewBase.setDrawingCacheEnabled(true);
         cameraBridgeViewBase.buildDrawingCache(true);
        cameraBridgeViewBase.setOnTouchListener(new View.OnTouchListener() {
            
            @SuppressLint({"SetTextI18n", "ClickableViewAccessibility"})
            @Override
            public boolean onTouch(View view, MotionEvent motionEvent) {
                bitmap = cameraBridgeViewBase.getDrawingCache();

                int h =bitmap.getHeight();
                int x = (int) motionEvent.getX();
                int y = (int) motionEvent.getY();
               

                int pixel = bitmap.getPixel(x, y);
                int r = Color.red(pixel);
                int g = Color.green(pixel);
                int b = Color.blue(pixel);



                textView2.setText("R("+r+")"+"G("+g+")"+"B("+b+")");
                textView2.setTextColor(Color.WHITE);
                textView2.setBackgroundColor(Color.rgb(r,g,b));


                return true;
            }
        });
    }

    @Override
    protected void onResume() {
        super.onResume();
        cameraBridgeViewBase.enableView();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        cameraBridgeViewBase.disableView();
    }

    @Override
    protected void onPause() {
        super.onPause();
        cameraBridgeViewBase.disableView();
    }

    @Override
    protected List<? extends CameraBridgeViewBase> getCameraViewList() {
        return Collections.singletonList(cameraBridgeViewBase);
    }

    void getPermission(){
        if(checkSelfPermission(Manifest.permission.CAMERA)!=PackageManager.PERMISSION_GRANTED){
            requestPermissions(new String[]{Manifest.permission.CAMERA},101);
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if (grantResults.length > 0 && grantResults[0] != PackageManager.PERMISSION_GRANTED) {
            getPermission();
        }
    }
}

核心原因 #

你依赖CameraBridgeViewBase的DrawingCache获取像素，但OpenCV的相机预览是通过OpenGL渲染的，DrawingCache无法正确捕获实时相机帧数据；同时触摸坐标未与相机帧的实际尺寸、方向做映射，导致读取到空白区域的像素（全0）。

修复方案（推荐）：直接从OpenCV的Mat中读取像素 #

放弃使用DrawingCache，改为在onCameraFrame回调中保存当前帧的Mat，触摸时直接从Mat中读取对应坐标的像素值：

1. 添加全局变量保存当前帧：

private Mat currentFrame;

2. 在onCameraFrame中更新当前帧：

@Override
public Mat onCameraFrame(CameraBridgeViewBase.CvCameraViewFrame inputFrame) {
    currentFrame = inputFrame.rgba();
    return currentFrame;
}

3. 修改触摸事件处理逻辑，处理坐标映射并读取Mat像素：

cameraBridgeViewBase.setOnTouchListener(new View.OnTouchListener() {
    @SuppressLint({"SetTextI18n", "ClickableViewAccessibility"})
    @Override
    public boolean onTouch(View view, MotionEvent motionEvent) {
        if (currentFrame == null) {
            return true;
        }

        // 转换触摸坐标到相机帧坐标
        float viewWidth = view.getWidth();
        float viewHeight = view.getHeight();
        float frameWidth = currentFrame.cols();
        float frameHeight = currentFrame.rows();

        // 计算缩放比例，竖屏预览时反转Y坐标（根据实际相机方向调整）
        float scaleX = frameWidth / viewWidth;
        float scaleY = frameHeight / viewHeight;

        int frameX = (int) (motionEvent.getX() * scaleX);
        int frameY = (int) ((viewHeight - motionEvent.getY()) * scaleY);

        // 确保坐标在帧范围内
        if (frameX >= 0 && frameX < frameWidth && frameY >= 0 && frameY < frameHeight) {
            // RGBA格式Mat，像素数组顺序为[R, G, B, A]
            double[] pixel = currentFrame.get(frameY, frameX);
            int r = (int) pixel[0];
            int g = (int) pixel[1];
            int b = (int) pixel[2];

            textView2.setText("R("+r+")"+"G("+g+")"+"B("+b+")");
            textView2.setTextColor(Color.WHITE);
            textView2.setBackgroundColor(Color.rgb(r,g,b));
        } else {
            Toast.makeText(MainActivity.this, "触摸位置超出预览范围", Toast.LENGTH_SHORT).show();
        }
        return true;
    }
});

4. 移除无用的DrawingCache代码：
   删除以下两行：

cameraBridgeViewBase.setDrawingCacheEnabled(true);
cameraBridgeViewBase.buildDrawingCache(true);

额外注意事项 #

• 相机预览方向可能与屏幕方向不一致，需根据实际情况调整坐标映射逻辑（比如前置摄像头需镜像处理）。
• 在onCameraViewStopped中释放Mat避免内存泄漏：

@Override
public void onCameraViewStopped() {
    if (currentFrame != null) {
        currentFrame.release();
        currentFrame = null;
    }
}

内容的提问来源于stack exchange，提问作者tlmary