使用Matrix.setPolyToPoly

Question

我在Android上使用位图，在使用4点选择位图上的区域时遇到问题。并非所有的4点都适合我。在某些情况下，结果只是一个空白位图而不是裁剪位图（如图中所示），并且logcat中没有任何错误（甚至是内存错误）。 这是我用来进行转换的基本代码。

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Matrix;
import android.widget.ImageView;

public class CropImageActivity extends Activity {
private ImageView mCroppedImageView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.crop);
    setupViews();
    doCropping();
}

private void doCropping() {
    Bitmap srcBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.sample);

    //target size
    int bitmapWidth = 400;
    int bitmapHeight = 400;

    Bitmap bitmap = Bitmap.createBitmap(bitmapWidth, bitmapHeight, Bitmap.Config.ARGB_8888);
    Canvas canvas = new Canvas(bitmap);

    //This is one of bad quadangle.
    points[0] = 0;    //top-left.x
    points[1] = 0;    //top-left.y
    points[2] = 230;  //top-right.x
    points[3] = 100;  //top-right.y
    points[4] = 350;  //bottom-right.x
    points[5] = 350;  //bottom-right.y
    points[6] = 0;    //bottom-left.x
    points[7] = 350;  //bottom-left.y

    float[] src = new float[]{
            points[0], points[1],
            points[2], points[3],
            points[4], points[5],
            points[6], points[7]
    };
    float[] dsc = new float[]{
            0, 0,
            bitmapWidth, 0,
            bitmapWidth, bitmapHeight,
            0, bitmapHeight
    };

    Matrix matrix = new Matrix();
    boolean transformResult = matrix.setPolyToPoly(src, 0, dsc, 0, 4);

    canvas.drawBitmap(srcBitmap, matrix, null);
    mCroppedImageView.setImageBitmap(bitmap);
}

private void setupViews() {
    mCroppedImageView = (ImageView) findViewById(R.id.croppedImageView);
}
}

那么，4点坐标会影响画布绘制还是矩阵变换？任何帮助表示赞赏。

谢谢

Answer 1

最后，我使用OpenCV解决了我的问题。感谢this question中的答案！似乎Matrix.setPolyToPoly并不适用于所有情况。

Answer 2

一切都更简单..

     @Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    Matrix matrix = new Matrix();

    matrix.setPolyToPoly(src, 0, dst, 0, 4);
    canvas.setMatrix(matrix);
    canvas.drawBitmap(bit,0,0,null );
}

Answer 3

您看到的错误不在Matrix.setPolyToPloy中，而在CPU栅格化中。要解决此问题，您可以简单地创建一个新的转换后的Bitmap而不使用Canvas，然后裁剪Bitmap以获得所需的结果。