Using clipRect - explanation

Question

public class POCII extends Activity { 

    myView mv = new myView(this); 

    @Override 
    public void onCreate(Bundle savedInstanceState) { 
        super.onCreate(savedInstanceState); 
        setContentView(mv); 
    }
}


class myView extends View { 

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

    @Override 
    public void onDraw(Canvas canvas) { 

        Paint paint = new Paint(); 

        canvas.drawRect(0,0,100,100, paint); 
        canvas.clipRect(0,0,50,50);
    } 
}

My question is, shouldn't the above code draw a rectangle and then crop the top left portion? The rectangle is not getting cropped.

Please explain what clipRect does. What is it actually clipping? Does it clip in the form of a rectangle, given the co-ordinates? If so, Why is the above code not working?

Answer 1

Canvas.clipRect(left, top, right, bottom) reduces the region of the screen that future draw operations can write to. It sets the clipBounds to be the spacial intersection of the current clipping rectangle and the rectangle specified. There are lot of variants of the clipRect method that accept different forms for regions and allow different operations on the clipping rectangle. If you want to explicitly set the clipping region, try:

canvas.clipRect(left, top, right, bottom, Region.Op.REPLACE);

The 5th argument means replace the clipping rectangle rather than creating the intersection with the previous version.

Try moving the clipRect statement before the drawRect statement. Or, try adding:

paint.setColor(Color.YELLOW);
drawRect(0,0,75,75);

after your existing clipRect statement. It should draw a 50x50 yellow square over what you had before.

Another note: (after long frustration with the apparently, largely undocumented View/ViewGroup/drawing code) I found that canvas.translate(x,y) also adjusts the clipRect. The interaction of clipRect and the drawing matrix is very confusing. It is helpful to point out:

canvas.getMatrix()

and

canvas.getClipBounds()

before and after modifications to the canvas and before drawing things.

Answer 2

To crop the top left portion, do:

canvas.clipRect(0,0,50,50, Region.Op.DIFFERENCE);
// secondly...
canvas.drawRect(0,0,100,100, paint); 

Answer 3

ICS and above ...

XOR, Difference and ReverseDifference clip modes are ignored by ICS if hardware acceleration is enabled.

Just disable 2D hardware acceleration in your view:

myView.setLayerType(View.LAYER_TYPE_SOFTWARE, null);

Reference Android: Howto use clipRect in API15