Android canvas: draw transparent circle on image

I am creating a pixel-hunting game. So my activity shows an ImageView. And I want to create a hint "show me where is the object". For this I need to blur whole image except a circle around a point where the object is located. Instead of blur I can show a just semitransparent black background. There is there is no problem to draw a semitransparent rectangle on the Canvas. But I don't know how to crop a transparent circle from it. The result should look like like this:

Please help me to achieve same result on Android SDK.

So finally I managed to do this.

Firstly I draw a semitransparent black rectangle on whole view. After that using PorterDuff.Mode.CLEAR I cut a transparent circle to show cat's position.

I had problem with PorterDuff.Mode.CLEAR: firstly I was getting a black circle instead of a transparent one.

Thanks to Romain Guy's comments here: comment here I understood that my window is opaque and I should draw on another bitmap. And only after draw on View's canvas.

Here is my onDraw method:

private Canvas temp;
private Paint paint;
private Paint p = new Paint();
private Paint transparentPaint;

private void init(){
    Bitmap bitmap = Bitmap.createBitmap(canvas.getWidth(), canvas.getHeight(), Bitmap.Config.ARGB_8888);
    temp = new Canvas(bitmap);
    paint = new Paint();
    paint.setColor(0xcc000000);
    transparentPaint = new Paint();
    transparentPaint.setColor(getResources().getColor(android.R.color.transparent));
    transparentPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
}

protected void onDraw(Canvas canvas) {
    temp.drawRect(0, 0, temp.getWidth(), temp.getHeight(), paint);
    temp.drawCircle(catPosition.x + radius / 2, catPosition.y + radius / 2, radius, transparentPaint);
    canvas.drawBitmap(bitmap, 0, 0, p);
}

I found a solution without bitmap drawing and creation. Here is the result of my implementation:

You need to create a custom FrameLayout and drawCircle with Clear paint:

    mBackgroundPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));

Also do not forget to disable Hardware acceleration and call setWillNotDraw(false) because we will override onDraw method

    setWillNotDraw(false);
    setLayerType(LAYER_TYPE_HARDWARE, null);

The full example is here:

public class TutorialView extends FrameLayout {
    private static final float RADIUS = 200;

    private Paint mBackgroundPaint;
    private float mCx = -1;
    private float mCy = -1;

    private int mTutorialColor = Color.parseColor("#D20E0F02");

    public TutorialView(Context context) {
        super(context);
        init();
    }

    public TutorialView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public TutorialView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
    public TutorialView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
        init();
    }

    private void init() {
        setWillNotDraw(false);
        setLayerType(LAYER_TYPE_HARDWARE, null);

        mBackgroundPaint = new Paint();
        mBackgroundPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        mCx = event.getX();
        mCy = event.getY();
        invalidate();
        return true;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawColor(mTutorialColor);
        if (mCx >= 0 && mCy >= 0) {
            canvas.drawCircle(mCx, mCy, RADIUS, mBackgroundPaint);
        }
    }
}

PS: This implementation just draws hole inside itself, you need to put background in your layout and add this TutorialView on top.

I have done this way by creating custom LinearLayout:

Check the Screenshot:

CircleOverlayView.java

import android.annotation.TargetApi;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.PorterDuff;
import android.graphics.PorterDuffXfermode;
import android.graphics.RectF;
import android.os.Build;
import android.util.AttributeSet;
import android.widget.LinearLayout;

/**
 * Created by hiren on 10/01/16.
 */
public class CircleOverlayView extends LinearLayout {
    private Bitmap bitmap;

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

    public CircleOverlayView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public CircleOverlayView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
    public CircleOverlayView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }

    @Override
    protected void dispatchDraw(Canvas canvas) {
        super.dispatchDraw(canvas);

        if (bitmap == null) {
            createWindowFrame(); 
        }
        canvas.drawBitmap(bitmap, 0, 0, null);
    }

    protected void createWindowFrame() {
        bitmap = Bitmap.createBitmap(getWidth(), getHeight(), Bitmap.Config.ARGB_8888); 
        Canvas osCanvas = new Canvas(bitmap);

        RectF outerRectangle = new RectF(0, 0, getWidth(), getHeight());

        Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
        paint.setColor(getResources().getColor(R.color.colorPrimary));
        paint.setAlpha(99);
        osCanvas.drawRect(outerRectangle, paint);

        paint.setColor(Color.TRANSPARENT); 
        paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_OUT)); 
        float centerX = getWidth() / 2;
        float centerY = getHeight() / 2;
        float radius = getResources().getDimensionPixelSize(R.dimen.radius);
        osCanvas.drawCircle(centerX, centerY, radius, paint);
    }

    @Override
    public boolean isInEditMode() {
        return true;
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        super.onLayout(changed, l, t, r, b);
        bitmap = null; 
    }
}

CircleDrawActivity.java:

public class CircleDrawActivity  extends AppCompatActivity{

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

activity_circle_draw.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/rlParent"
    android:layout_width="match_parent"
    android:layout_height="match_parent">


    <ImageView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@drawable/lighthouse"
        android:scaleType="fitXY" />

    <common.customview.CircleOverlayView
        android:id="@+id/cicleOverlay"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

    </common.customview.CircleOverlayView>


</RelativeLayout>

colors.xml:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <color name="colorPrimary">#3F51B5</color>
    <color name="colorPrimaryDark">#303F9F</color>
    <color name="colorAccent">#FF4081</color>
</resources>

dimens.xml:

<resources>
    <!-- Default screen margins, per the Android Design guidelines. -->
    <dimen name="nav_header_vertical_spacing">16dp</dimen>
    <dimen name="nav_header_height">160dp</dimen>
    <!-- Default screen margins, per the Android Design guidelines. -->
    <dimen name="activity_horizontal_margin">16dp</dimen>
    <dimen name="activity_vertical_margin">16dp</dimen>
    <dimen name="fab_margin">16dp</dimen>

    <dimen name="radius">50dp</dimen>
</resources>

Hope this will help you.

@Robert's answer actually showed me how to solve this problem but his code doesn't work. So I have updated his solution and made it work:

public class CaptureLayerView extends View {

  private Bitmap bitmap;
  private Canvas cnvs;
  private Paint p = new Paint();
  private Paint transparentPaint = new Paint();;
  private Paint semiTransparentPaint = new Paint();;
  private int parentWidth;
  private int parentHeight;
  private int radius = 100;

  public CaptureLayerView(Context context) {
      super(context);
      init();
  }

  public CaptureLayerView(Context context, AttributeSet attrs) {
      super(context, attrs);
      init();
  }

  private void init() {
      transparentPaint.setColor(getResources().getColor(android.R.color.transparent));
      transparentPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));

      semiTransparentPaint.setColor(getResources().getColor(R.color.colorAccent));
      semiTransparentPaint.setAlpha(70);
  }

  @Override
  protected void onDraw(Canvas canvas) {
      super.onDraw(canvas);

      bitmap = Bitmap.createBitmap(parentWidth, parentHeight, Bitmap.Config.ARGB_8888);
      cnvs = new Canvas(bitmap);
      cnvs.drawRect(0, 0, cnvs.getWidth(), cnvs.getHeight(), semiTransparentPaint);
      cnvs.drawCircle(parentWidth / 2, parentHeight / 2, radius, transparentPaint);
      canvas.drawBitmap(bitmap, 0, 0, p);
  }

  @Override
  protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {

      parentWidth = MeasureSpec.getSize(widthMeasureSpec);
      parentHeight = MeasureSpec.getSize(heightMeasureSpec);

      this.setMeasuredDimension(parentWidth, parentHeight);
      super.onMeasure(widthMeasureSpec, heightMeasureSpec);
  }
}

And now use this view in any layout like this:

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
    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">

<SurfaceView
    android:id="@+id/surfaceView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center">
</SurfaceView>

<com.example.myapp.CaptureLayerView
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

<Button
    android:id="@+id/btnTakePicture"
    android:layout_width="match_parent"
    android:layout_height="80dp"
    android:onClick="onClickPicture"
    android:text="@string/take_picture">
</Button>

Here I wanted a semi transparent layer on the SurfaceView with transparent circle in the center. P.S. This code is not optimized one because it creates Bitmap in onDraw method, it is because I couldn't get parent view's width and height in init method, so I only could know them in onDraw.

A simple 2020 kotlin solution without any warnings.

---

1. Code for custom view with hole

class HolePosition(var x: Float, var y: Float, var r: Float)

---

class HoleView @JvmOverloads constructor(
    context: Context?,
    attrs: AttributeSet? = null,
    defStyleAttr: Int = 0
): View(context, attrs, defStyleAttr) {

    private val paint: Paint = Paint()
    private var holePaint: Paint = Paint()
    private var bitmap: Bitmap? = null
    private var layer: Canvas? = null

    //position of hole
    var holePosition: HolePosition = HolePosition(0.0f, 0.0f, 0.0f)
        set(value) {
            field = value
            //redraw
            this.invalidate()
        }

    override fun onDraw(canvas: Canvas) {
        super.onDraw(canvas)
        if (bitmap == null) { configureBitmap() }

        //draw background
        layer?.drawRect(0.0f, 0.0f, width.toFloat(), height.toFloat(), paint)
        //draw hole
        layer?.drawCircle(holePosition.x, holePosition.y, holePosition.r, holePaint);
        //draw bitmap
        canvas.drawBitmap(bitmap!!, 0.0f, 0.0f, paint);
    }

    private fun configureBitmap() {
        //create bitmap and layer
        bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888)
        layer = Canvas(bitmap!!)
    }

    init {
        //configure background color
        val backgroundAlpha = 0.8
        paint.color = ColorUtils.setAlphaComponent(resources.getColor(R.color.mainDark, null), (255 * backgroundAlpha).toInt() )

        //configure hole color & mode
        holePaint.color = resources.getColor(android.R.color.transparent, null)
        holePaint.xfermode = PorterDuffXfermode(PorterDuff.Mode.CLEAR)
    }
}

2. Layout example

<com.your_company.package.HoleView
    android:id="@+id/hole_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
</com.your_company.package.HoleView>

3. Hole set usage

val holeView = findViewById<HoleView>(R.id.hole_view)
holeView.holePosition = HolePosition(x = 350.0f, y = 350.0f, r = 180.0f)

4. Results

I don't have much to add to your answer, But if anyone's interested I moved the bitmap allocation and all to onSizeChanged so it's better performance wise.

Here you can find a FrameLayout with a "Hole" in the middle of it ;)

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.PorterDuff;
import android.graphics.PorterDuffXfermode;
import android.os.Handler;
import android.util.AttributeSet;
import android.util.Log;
import android.widget.FrameLayout;

/**
 * Created by blackvvine on 1/1/16.
 */
public class SteroidFrameLayout extends FrameLayout {

    private Paint transPaint;
    private Paint defaultPaint;

    private Bitmap bitmap;
    private Canvas temp;

    public SteroidFrameLayout(Context context) {
        super(context);
        __init__();
    }

    public SteroidFrameLayout(Context context, AttributeSet attrs) {
        super(context, attrs);
        __init__();
    }

    public SteroidFrameLayout(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        __init__();
    }

    private void __init__() {
        transPaint = new Paint();
        defaultPaint = new Paint();
        transPaint.setColor(Color.TRANSPARENT);
        transPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR));
        setWillNotDraw(false);
    }


    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        bitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        temp = new Canvas(bitmap);
    }

    @Override
    protected void dispatchDraw(Canvas canvas) {

        temp.drawColor(Color.TRANSPARENT);
        super.dispatchDraw(temp);
        temp.drawCircle(cx, cy, getWidth()/4, transPaint);

        canvas.drawBitmap(bitmap, 0, 0, defaultPaint);

        if (p < 1)
            invalidate();
        else
            animRunning = false;

    }

}

p.s: although it's much more efficient than the original answer it's still a relatively heavy task to do in a draw() method, so if you're using this technique in an animation like me, don't expect a 60.0fps smooth one

public class CircleBlur extends Activity implements View.OnTouchListener {
SeekBar seekBar;
ImageView image,image1;

private Paint paint;
Bitmap circle,blurimg;

private Matrix matrix = new Matrix();
private Matrix savedMatrix = new Matrix();
private static final int NONE = 0;
private static final int DRAG = 1;
private static final int ZOOM = 2;
private int mode = NONE;
private PointF start = new PointF();
private PointF mid = new PointF();
private float oldDist = 1f;
float newRot = 0f;
private float d = 0f;
private float[] lastEvent = null;
private float radius=12;
Bitmap blurbitmap;
@RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN_MR1)
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,WindowManager.LayoutParams.FLAG_FULLSCREEN);
    setContentView(R.layout.activity_blurimage);
    image=findViewById(R.id.image);
    seekBar=findViewById(R.id.seekbar);
    image1=findViewById(R.id.image1);

    paint = new Paint(Paint.ANTI_ALIAS_FLAG);

    //here your image bind to Imageview
    image.setImageResource(R.drawable.nas1);

    image1.setOnTouchListener(this);
    seekBar.setProgress(12);

    seekBar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
        @Override
        public void onProgressChanged(SeekBar seekBar, int i, boolean b) {
            radius = (float) CircleBlur.this.seekBar.getProgress();
            blurbitmap=createBlurBitmap(blurimg, radius);
            CircleBlur();
        }
        @Override
        public void onStartTrackingTouch(SeekBar seekBar) {
        }
        @Override
        public void onStopTrackingTouch(SeekBar seekBar) {
        }
    });

}
@RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN_MR1)
private Bitmap createBlurBitmap(Bitmap src, float r) {
    if (r <= 0) {
        r = 0.1f;
    } else if (r > 25) {
        r = 25.0f;
    }
    Bitmap bitmap = Bitmap.createBitmap(src.getWidth(), src.getHeight(), Bitmap.Config.ARGB_8888);
    RenderScript renderScript = RenderScript.create(this);

    Allocation blurInput = Allocation.createFromBitmap(renderScript, src);
    Allocation blurOutput = Allocation.createFromBitmap(renderScript, bitmap);

    ScriptIntrinsicBlur blur = ScriptIntrinsicBlur.create(renderScript, Element.U8_4(renderScript));
    blur.setInput(blurInput);
    blur.setRadius(r);
    blur.forEach(blurOutput);

    blurOutput.copyTo(bitmap);
    renderScript.destroy();

    return bitmap;
}

@RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN_MR1)
public void CircleBlur()
{
    Bitmap result;
    // your circle image
    circle = BitmapFactory.decodeResource(getResources(),R.drawable.cicleouter);
        result = Bitmap.createBitmap(blurimg.getWidth(), blurimg.getHeight(), Bitmap.Config.ARGB_8888);
        Canvas mCanvas = new Canvas(result);
        paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
        paint.setDither(true);
        mCanvas.drawBitmap(blurimg,0,0, null);
        mCanvas.drawBitmap(circle, matrix, paint);
        paint.setXfermode(null);
    image1.setImageBitmap(result);
}

@RequiresApi(api = Build.VERSION_CODES.JELLY_BEAN_MR1)
@Override
public boolean onTouch(View v, MotionEvent event) {
        image1 = (ImageView) v;
        float x = event.getX(), y = event.getY();
        switch (event.getAction() & MotionEvent.ACTION_MASK) {
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_DOWN:
                savedMatrix.set(matrix);
                start.set(x, y);
                mode = DRAG;
                lastEvent = null;
                break;
            case MotionEvent.ACTION_POINTER_DOWN:
                oldDist = spacing(event);
                if (oldDist > 10f) {
                    savedMatrix.set(matrix);
                    midPoint(mid, event);
                    mode = ZOOM;
                }
                lastEvent = new float[4];
                lastEvent[0] = event.getX(0);
                lastEvent[1] = event.getX(1);
                lastEvent[2] = event.getY(0);
                lastEvent[3] = event.getY(1);
                d = rotation(event);
                break;
            // case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_POINTER_UP:
                mode = NONE;
                lastEvent = null;
                break;
            case MotionEvent.ACTION_MOVE:
                if (mode == DRAG) {
                    matrix.set(savedMatrix);
                    float dx = x - start.x;
                    float dy = y - start.y;
                    matrix.postTranslate(dx, dy);
                } else if (mode == ZOOM) {
                    float newDist = spacing(event);
                    if (newDist > 10f) {
                        matrix.set(savedMatrix);
                        float scale = (newDist / oldDist);
                        matrix.postScale(scale, scale, mid.x, mid.y);
                    }
                    if (lastEvent != null && event.getPointerCount() == 2 || event.getPointerCount() == 3) {
                        newRot = rotation(event);
                        float r = newRot - d;
                        float[] values = new float[9];
                        matrix.getValues(values);
                        float tx = values[2];
                        float ty = values[5];
                        float sx = values[0];
                        float xc = (image.getWidth() / 2) * sx;
                        float yc = (image.getHeight() / 2) * sx;
                        matrix.postRotate(r, tx + xc, ty + yc);
                    }
                }
                break;
        }
    CircleBlur();
        return true;
}
private float spacing(MotionEvent event) {
    float x = event.getX(0) - event.getX(1);
    float y = event.getY(0) - event.getY(1);
    float s=x * x + y * y;
    return (float)Math.sqrt(s);
}
private void midPoint(PointF point, MotionEvent event) {
    float x = event.getX(0) + event.getX(1);
    float y = event.getY(0) + event.getY(1);
    point.set(x / 2, y / 2);
}
private float rotation(MotionEvent event) {
    double delta_x = (event.getX(0) - event.getX(1));
    double delta_y = (event.getY(0) - event.getY(1));
    double radians = Math.atan2(delta_y, delta_x);
    return (float) Math.toDegrees(radians);
}

}

This worked for me:

canvas.drawCircle(x,y,radius,new Paint(Color.TRANSPARENT))

If you're having trouble getting a transparent circle cutout over a view with an opaque background, see this answer. To get it to work, I set my custom layout view to have a transparent background in XML, then drew on the background color I wanted for the layout with the line

cv.drawColor(Color.BLUE); //replace with your desired background color

---

The full OnDraw method from the answer I linked above:

@Override
protected void onDraw(Canvas canvas) {

    int w = getWidth();
    int h = getHeight();
    int radius = w > h ? h / 2 : w / 2;

    bm.eraseColor(Color.TRANSPARENT);
    cv.drawColor(Color.BLUE);
    cv.drawCircle(w / 2, h / 2, radius, eraser);
    canvas.drawBitmap(bm, 0, 0, null);
    super.onDraw(canvas);
}

Kotlin version without Bitmap allocation. But instead of a circle, a rounded rectangle is drawn here. I'm sure you can adapt it to circle by yourself.

    private val scanAreaBorder = Paint().apply {
    color = ContextCompat.getColor(context, R.color.postyellow)
    style = Paint.Style.STROKE
    strokeWidth = dpToPx(context, resources.getDimension(R.dimen.scan_area_border_stroke_width).toInt())
}

private val scanArea = Paint().apply {
    color = ContextCompat.getColor(context, android.R.color.transparent)
    xfermode = PorterDuffXfermode(PorterDuff.Mode.CLEAR)
}

private val background = Paint().apply {
    color = ContextCompat.getColor(context, R.color.scanner_area)
}

 init {
    setLayerType(LAYER_TYPE_HARDWARE, null)
}

    override fun onDraw(canvas: Canvas) {
    super.onDraw(canvas)
    canvas.run {
        drawRect(0f, 0f, width.toFloat(), height.toFloat(), background)
        val top = (height / 2f) - dpToPx(context, resources.getDimension(R.dimen.scan_area_height).toInt())
        val bottom = (height / 2f) + dpToPx(context, resources.getDimension(R.dimen.scan_area_height).toInt())
        val margin = dpToPx(context, resources.getDimension(R.dimen.scan_area_margin).toInt())
        val cornerRadius = dpToPx(context, resources.getDimension(R.dimen.scan_area_corner_radius).toInt())
        drawRoundRect(margin, top, width.toFloat() - margin, bottom, cornerRadius, cornerRadius, scanArea)
        drawRoundRect(margin, top, width.toFloat() - margin, bottom, cornerRadius, cornerRadius, scanAreaBorder)
    }
}