Select an area on bitmap with 4 points using Matrix.setPolyToPoly

问题

I am playing with bitmap on Android and I am facing an issue when selecting an area on the bitmap using the 4 points. Not all the sets of 4 points work for me. In some cases, the result is just a blank bitmap instead of the cropped bitmap (like in the picture) and there is not any error in logcat(even memory error). Here is the basic code I used to do the transformation.

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);
}
}

So, does the 4 points coordinates affect the canvas drawing or the matrix transformation? Any help is appreciated.

Thank you

回答1:

Finally, I solve my issue using OpenCV. Thanks for the answer in this question! It seems that Matrix.setPolyToPoly does not work for all the cases.

回答2:

Everything is much simpler..

     @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 );
}

回答3:

The bug you see is not in Matrix.setPolyToPloy but in CPU rasterization. To workaround it, you can simply creating a new transformed Bitmap without using a Canvas, then crop the Bitmap to get desired result.

来源：https://stackoverflow.com/questions/34620327/select-an-area-on-bitmap-with-4-points-using-matrix-setpolytopoly

标签

android

matrix

bitmap

transformation