h2nghia h2nghia - 1 year ago 171
Android Question

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

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;

protected void onCreate(Bundle savedInstanceState) {

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);

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

enter image description here
This works

Answer Source

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.

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