Canvas.clipPath(Path) not clipping as expected

Question

I'm trying to clip a canvas drawing operation to an arc-shaped wedge. However, I'm not getting the intended result after setting the clipping path to the canvas.

For illustration, here is what I'm doing:

path.reset();

//Move to point #1
path.moveTo(rect.centerX(), rect.centerY());

//Per the documentation, this will draw a connecting line from the current
//position to the starting position of the arc (at 0 degrees), add the arc
//and my current position now lies at #2.
path.arcTo(rect, 0, -30);

//This should then close the path, finishing back at the center point (#3)
path.close();

This works, and when I simply draw this path (canvas.drawPath(path, paint)) it draws the wedge as shown above. However, when I set this path as the canvas's clipping path and draw into it:

//I've tried it with and without the Region.Op parameter
canvas.clipPath(path, Region.Op.REPLACE);
canvas.drawColor(Color.BLUE);

I get the following result instead (the wedge is left just to show reference):

So it instead seems to clip to the bounding rect of the Path, and not the Path itself. Any ideas what's happening here?

EDIT Just as an update, I've found a much more efficient way of doing this that allows for hardware acceleration. First, draw the entire image (that you'd be clipping) into an offscreen bitmap. Make a BitmapShader using this Bitmap, set that shader to a Paint, then draw the wedge path using that Paint object:

drawMyBitmap(bitmap);
Shader shader = new BitmapShader(bitmap, Shader.TileMode.CLAMP, Shader.TileMode.CLAMP);
Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
paint.setShader(shader);

@Override
public void onDraw(Canvas canvas) {
    canvas.drawArc(rect,         //The rectangle bounding the circle
                   startAngle,   //The angle (CW from 3 o'clock) to start
                   sweepAngle,   //The angle (CW from 3 o'clock) of the arc
                   true,         //Boolean of whether to draw a filled arc (wedge)
                   paint         //The paint with the shader attached
    );
}

Mandarin

Are you using HC or above or otherwise using hardware acceleration? developer.android.com/guide/topics/graphics/hardware-accel.html. If so, clipPath is unsupported and problematic. – Mckoy

@Simon: Oh my god. Heh. Sadly I've been referring to that document a lot this week, but I totally overlooked that. Disabling HW Accel worked perfectly! If you'll post this as an answer I'll accept it. You're a lifesaver! – Mandarin

Glad to have helped. Good luck. – Mckoy

Beautiful question! – Adagio

How exactly do you accomplish this?: "First, draw the entire image (that you'd be clipping) into an offscreen bitmap" – Stopcock

Answer 1

Are you using HC or above or otherwise using hardware acceleration?

If so, clipPath is unsupported and problematic.

developer.android.com/guide/topics/graphics/hardware-accel.html.

Mckoy

That was the problem. :) Moved the clipping part of the drawing operation into an offscreen bitmap, then drew it back to the HW accelerated canvas and it works like a charm! Thanks again! – Mandarin

@kcoppock That sounds a bit backwards. Why don't you just convert the view's layer type into software, and still do your drawing in onDraw()? – Herzegovina

@RichardJ.RossIII That would have made more sense. :) I was a bit unfamiliar with that at the time. I've edited in the new way that I'm handling this problem now, thanks for the input though! – Mandarin

Answer 2

OP's question is specifically about using a clipping region and has been answered by @Simon. Bear in mind, however, that there's a more straightforward way of drawing a filled arc:

Create a Paint:

mPaint = new Paint();
mPaint.setColor(Color.BLUE);
mPaint.setStyle(Style.FILL);
mPaint.setAntiAlias(true);

When drawing, simply draw the path:

canvas.drawPath(path, mPaint);

Grasmere