Digital Signature Clear issue in Android app

I am working on implementing digital signature functionality in android app using custom view.
I want user be able to clear and retry new signature without closing dialog or re-creating activity.When I clear,it successfully clears the signature but doesn't allow to draw a new signature and draws some black overlays along with previously drawn signature.

Here is the class which draws digital signature.
Signature.java

import android.content.Context;  
import android.graphics.Bitmap;  
import android.graphics.Canvas;  
import android.graphics.Color;  
import android.graphics.Paint;  
import android.graphics.Path;  
import android.graphics.PorterDuff;  
import android.provider.MediaStore;  
import android.util.AttributeSet;  
import android.util.Log;  
import android.view.MotionEvent;  
import android.view.View;  
import java.io.File;  
import java.io.FileOutputStream;  

public class Signature extends View  
{  
    public static final float STROCK_WIDTH = 5f;  
    public static final float HALF_STROKE_WIDTH = STROCK_WIDTH / 2;  
    private Paint paint = new Paint();  
    View mContent;  
    File mPath1;  
    private Paint mPaint;  
    private Bitmap mBitmap;  
    private Canvas mCanvas;  
    private Path mPath;  
    private Paint mBitmapPaint;  
    boolean clear = false;    

    public Signature(Context context)
    {
        super(context);
    }

    public Signature(Context context, AttributeSet attrs)
    {
        super(context, attrs);
        paint.setAntiAlias(true);
        paint.setColor(Color.BLACK);
        paint.setStyle(Paint.Style.STROKE);
        paint.setStrokeJoin(Paint.Join.ROUND);
        paint.setStrokeWidth(STROCK_WIDTH);
    }

    public Signature(Context context, AttributeSet attrs, View view, File pathToSave)
    {
        this(context, attrs);
        mContent = view;
        mPath1 = pathToSave;
        mPath = new Path();
        mBitmapPaint = new Paint(Paint.DITHER_FLAG);
        setPaint();
    }

    public void setPaint()
    {
        mPaint = new Paint();
        mPaint.setAntiAlias(true);
        mPaint.setDither(true);
        mPaint.setColor(0xFFFF0000);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeJoin(Paint.Join.ROUND);
        mPaint.setStrokeCap(Paint.Cap.ROUND);
        mPaint.setStrokeWidth(12);
    }

    public String save(View v)
    {
        String imagePath = null;
        Log.v("TAG", "Width :" + v.getWidth());
        Log.v("TAG", "Height :" + v.getHeight());
        if (mBitmap == null)
        {
            mBitmap = Bitmap.createBitmap(mContent.getWidth(), mContent.getHeight(), Bitmap.Config.RGB_565);
        }
        mBitmap = v.getDrawingCache();
        try
        {
            FileOutputStream mFileOutStream = new FileOutputStream(mPath1);
            mBitmap.compress(Bitmap.CompressFormat.PNG, 90, mFileOutStream);
            mFileOutStream.flush();
            mFileOutStream.close();
            imagePath = MediaStore.Images.Media.insertImage(getContext().getContentResolver(), mBitmap, "title", null);
            Log.v("log_tag", "url: " + imagePath + "::we are saving at :" + mPath1);
        }
        catch (Exception e)
        {
            Log.v("log_tag", e.toString());
        }
        return imagePath;
    }

    public void clear()
    {
        mPath.reset();  
        mPaint.reset();  
        clear = true;  
        mPath1.delete();  
        mBitmapPaint = new Paint(Paint.DITHER_FLAG);  
        postInvalidate();  
    }  

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh)
    {
        super.onSizeChanged(w, h, oldw, oldh);
        try
        {
            mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
            mCanvas = new Canvas(mBitmap);
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }

    @Override
    protected void onDraw(Canvas canvas)
    {
        if (clear)
        {
            clear = false;
        }
        else
        {
            canvas.drawColor(0xFFFFFFFF);
            canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
            canvas.drawPath(mPath, mPaint);
        }
    }

    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4;

    private void touch_start(float x, float y)
    {
        mPath.reset();
        mPath.moveTo(x, y);
        mX = x;
        mY = y;
    }

    private void touch_move(float x, float y)
    {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE)
        {
            mPath.quadTo(mX, mY, (x + mX) / 2, (y + mY) / 2);
            mX = x;
            mY = y;
        }
    }

    private void touch_up()
    {
        mPath.lineTo(mX, mY);
        // commit the path to our offscreen
        mCanvas.drawPath(mPath, mPaint);
        // kill this so we don't double draw
        mPath.reset();
    }

    @Override
    public boolean onTouchEvent(MotionEvent event)
    {
        float x = event.getX();
        float y = event.getY();

        switch (event.getAction())
        {
            case MotionEvent.ACTION_DOWN:
                touch_start(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_MOVE:
                touch_move(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
                touch_up();
                invalidate();
                break;
        }
        return true;
    }  
}  

Activity in which above class is used
MainActivity.java

import android.annotation.SuppressLint;
import android.app.Activity;
import android.app.Dialog;
import android.content.ContextWrapper;
import android.graphics.Bitmap;
import android.graphics.Color;
import android.graphics.drawable.BitmapDrawable;
import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.provider.MediaStore;
import android.support.annotation.Nullable;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.AppCompatActivity;
import android.util.Base64;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.view.Window;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.Toast;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.IOException;
import java.util.Calendar;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    Button btnOpen;
    String imagePath = null,str_signature = "";
    ImageView iv_sign;
    public static String tempDir;

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

        try {
            btnOpen= (Button) findViewById(R.id.btnOpen);
            btnOpen.setOnClickListener(this);

            iv_sign= (ImageView) findViewById(R.id.ivSign);
        }
        catch (Exception ex)
        {
            ex.printStackTrace();
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onClick(View v) {

        if(v==btnOpen)
        {
            SignatureFragment dialog = new SignatureFragment(onImageClicked);
            dialog.show(MainActivity.this.getSupportFragmentManager(), "NoticeDialogFragment");
        }
    }

    ImageSaved onImageClicked = new ImageSaved() {
        @Override
        public void onImageSaved(String path) {
            imagePath = path;
            Uri uri = Uri.parse(imagePath);
            Log.d("TAG", "We got image path :" + imagePath);
            try {
                Bitmap bitmap = MediaStore.Images.Media.getBitmap(getContentResolver(), uri);
                iv_sign.setImageBitmap(bitmap);

                ImageView img = iv_sign;
                BitmapDrawable mBitmapDrawable = (BitmapDrawable) img.getDrawable();
                Bitmap mBitmap = mBitmapDrawable.getBitmap();
                ByteArrayOutputStream baos = new ByteArrayOutputStream();
                mBitmap.compress(Bitmap.CompressFormat.JPEG, 100, baos);
                byte[] byteArrayImage = baos.toByteArray();
                str_signature = Base64.encodeToString(byteArrayImage,
                        Base64.DEFAULT);

            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    };

    public static class SignatureFragment extends android.support.v4.app.DialogFragment implements View.OnClickListener {
        LinearLayout ll_signature_view;
        Signature signature;
        Button btn_done, btn_clear,btnCancel;
        String EXTERNAL_DIR = "TTD";
        ImageSaved saveListner;
        File mypath;

        @SuppressLint("ValidFragment")
        public SignatureFragment(ImageSaved onImageClicked) {
            saveListner = onImageClicked;
        }

        @Override
        public void onAttach(Activity activity) {
            super.onAttach(activity);
        }

        @Override
        public void onDetach() {
            saveListner = null;
            super.onDetach();
        }

        public SignatureFragment() {
        }

        @Override
        public void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setStyle(android.support.v4.app.DialogFragment.STYLE_NO_FRAME, R.style.Base_Theme_AppCompat_Light_Dialog);
        }

        @Override
        public Dialog onCreateDialog(Bundle savedInstanceState) {
            Dialog dialog = super.onCreateDialog(savedInstanceState);
            dialog.requestWindowFeature(Window.FEATURE_NO_TITLE);
            return dialog;
        }

        @Nullable
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.signature_view, container, false);
            try
            {
                ll_signature_view = (LinearLayout) rootView.findViewById(R.id.ll_signature_view);
                btn_done = (Button) rootView.findViewById(R.id.btn_done);
                btn_clear = (Button) rootView.findViewById(R.id.btn_clear);
                btnCancel = (Button) rootView.findViewById(R.id.btn_cancel);
                tempDir = Environment.getExternalStorageDirectory() + "/" + EXTERNAL_DIR + "/";
                File directory = getActivity().getCacheDir();
                prepareDirectory();
                String uniqueId = getTodaysDate() + "_" + getCurrentTime() + "_" + Math.random();
                String current = uniqueId + ".png";
                mypath = new File(directory, current);

                signature = new Signature(getActivity(), null, ll_signature_view, mypath);
                ll_signature_view.setBackgroundColor(Color.WHITE);
                ll_signature_view.addView(signature, LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.MATCH_PARENT);
                btn_done.setOnClickListener(this);
                btn_clear.setOnClickListener(this);
                btnCancel.setOnClickListener(this);
            }
            catch(Exception ex)
            {
                ex.printStackTrace();
            }

            return rootView;
        }

        @Override
        public void onClick(View v) {
            if (v == btn_done) {
                ll_signature_view.setDrawingCacheEnabled(true);
                String imagePath = signature.save(ll_signature_view);
                if (saveListner != null) {
                    saveListner.onImageSaved(imagePath);
                }
                dismiss();

            }
            if (v == btnCancel) {
                dismiss();
            }
            if(v==btn_clear)
            {
                signature.clear();
            }
        }

        private boolean prepareDirectory() {
            try {
                if (makedirs()) {
                    return true;
                } else {
                    return false;
                }
            } catch (Exception e) {
                e.printStackTrace();
                Toast.makeText(getActivity(), "Could not initiate File System.. Is Sdcard mounted properly?", Toast.LENGTH_SHORT).show();
                return false;
            }
        }

        private boolean makedirs() {
            File tempdir = new File(tempDir);
            if (!tempdir.exists()) {
                tempdir.mkdirs();
            }

            if (tempdir.isDirectory()) {
                File[] files = tempdir.listFiles();
                for (File file : files) {
                    if (!file.delete()) {
                        System.out.println("Failed to delete " + file);
                    }
                }
            }
            return (tempdir.isDirectory());
        }

        private String getTodaysDate() {

            final Calendar c = Calendar.getInstance();
            int todaysDate = (c.get(Calendar.YEAR) * 10000) +
                    ((c.get(Calendar.MONTH) + 1) * 100) +
                    (c.get(Calendar.DAY_OF_MONTH));
            Log.w("DATE:", String.valueOf(todaysDate));
            return (String.valueOf(todaysDate));

        }

        private String getCurrentTime() {

            final Calendar c = Calendar.getInstance();
            int currentTime = (c.get(Calendar.HOUR_OF_DAY) * 10000) +
                    (c.get(Calendar.MINUTE) * 100) +
                    (c.get(Calendar.SECOND));
            Log.w("TIME:", String.valueOf(currentTime));
            return (String.valueOf(currentTime));
        }
    }

}

Layout File
signature_view.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:layout_margin="8dp">

    <LinearLayout
        android:id="@+id/ll_signature_view"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:background="@drawable/edittext_border_white"
        android:orientation="horizontal">

    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginTop="8dp">

        <Button
            android:id="@+id/btn_done"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="DONE" />

        <Button
            android:id="@+id/btn_clear"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="CLEAR" />

        <Button
            android:id="@+id/btn_cancel"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="CANCEL" />

    </LinearLayout>
</LinearLayout>  

activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="@dimen/activity_vertical_margin"
    tools:context=".MainActivity"
    android:background="@android:color/white">

    <TextView
        android:id="@+id/tvWelcome"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/Welcome" />

    <Button
        android:layout_marginTop="10dp"
        android:id="@+id/btnOpen"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="@string/Open"
        android:gravity="center"
        android:layout_below="@+id/tvWelcome"/>

    <ImageView
        android:id="@+id/ivSign"
        android:layout_marginTop="10dp"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@android:color/white"
        android:layout_below="@+id/btnOpen"
        />

</RelativeLayout> 

mCanvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR)

try this instead

canvas.drawColor(0xFFFFFFFF);

and also set Layer Type

mCanvas.setLayerType(View.LAYER_TYPE_SOFTWARE, null);