android - OpenCv Core.line 在预期颜色时绘制白色

Question

在 OpenCv4Android 环境中，当我创建一个 Mat 图像并使用 Core.line() 在图像上绘制时，它总是显示白色而不是我指定的颜色。

我看过a question related to gray scale , 但我的图像还没有被转换成灰色。

public class DrawingTest extends AppCompatActivity {
    public static final Scalar GREEN = new Scalar(0,255,0);
    private RelativeLayout mLayout;
    private ImageView imageView;

    static {
        System.loadLibrary("opencv_java");
    }

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

        mLayout = (RelativeLayout) findViewById(R.id.activity_drawing_test);
        mLayout.setDrawingCacheEnabled(true);

        imageView = (ImageView) this.findViewById(imageView_dt);

        //test.jpg is in the drawable-nodpi folder, is an normal color jpg image.
        int drawableResourceId = getResources().getIdentifier("test", "drawable", getPackageName());
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(), drawableResourceId);
        //Mat matImage = new Mat(); // Also white
        Mat matImage = new Mat(bitmap.getHeight(), bitmap.getWidth(), CV_8UC4);
        Utils.bitmapToMat(bitmap, matImage);


        // Attempt to draw a GREEN box, but it comes out white
        Core.line(matImage, new Point(new double[]{100,100}), new Point(new double[]{100, 200}), GREEN,4);
        Core.line(matImage, new Point(new double[]{100,200}), new Point(new double[]{200, 200}), GREEN,4);
        Core.line(matImage, new Point(new double[]{200,200}), new Point(new double[]{200, 100}), GREEN,4);
        Core.line(matImage, new Point(new double[]{200,100}), new Point(new double[]{100, 100}), GREEN,4);

        Bitmap bitmapToDisplay = Bitmap.createBitmap(matImage.cols(), matImage.rows(), Bitmap.Config.ARGB_8888);
        Utils.matToBitmap(matImage, bitmapToDisplay);
        imageView.setImageBitmap(bitmapToDisplay);
    }
}

Answer 1

问题出在你初始化的颜色上

public static final Scalar GREEN = new Scalar(0,255,0); 

根据这个声明

Mat matImage = new Mat(bitmap.getHeight(), bitmap.getWidth(), CV_8UC4);`

您正在创建一个 4 channel 垫，但仅使用 3 个组件初始化 GREEN 标量，因此是第 4 个组件，它定义了您的线的第四个 channel 的颜色，这是默认设置在你的情况下值 0。

因此，您所感知的白色实际上是透明的。您可以通过使用 CV_8UC3 创建 matImage 或将 GREEN 标量更改为 public static final Scalar GREEN = new Scalar( 0,255,0, 255);