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

i playing bitmap on android , facing issue when selecting area on bitmap using 4 points. not sets of 4 points work me. in cases, result blank bitmap instead of cropped bitmap (like in picture) , there not error in logcat(even memory error). here basic code used 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 1 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, 4 points coordinates affect canvas drawing or matrix transformation? appreciated.

thank you

finally, solve issue using opencv. answer in this question! seems matrix.setpolytopoly not work cases.