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POPorterDuffXfermode DST_IN not working as expected
 

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  1. POPorterDuffXfermode DST_IN not working as expected
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    <p>So I'm trying to speed up some drawing we're doing (drawing a portion of an arc with alpha transparency) and was attempting to cache the entire arc into a separate bitmap, and show it selectively with an alpha mask. </p> <p>From the research I've done (the Xfermodes API demo for Android, <a href="http://www.ibm.com/developerworks/java/library/j-mer0918/" rel="nofollow noreferrer">this example</a>, and <a href="https://sites.google.com/site/drjohnbmatthews/composite" rel="nofollow noreferrer">this tool</a>), if I have for example the following two graphics:</p> <p><img src="https://i.stack.imgur.com/t7aw7.png" alt="enter image description here"></p> <p><img src="https://i.stack.imgur.com/nNZNM.png" alt="enter image description here"></p> <p>and draw using the following:</p> <pre><code>Xfermode DST_IN = new PorterDuffXfermode(PorterDuff.Mode.DST_IN); Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG); canvas.drawBitmap(circle, 0, 0, paint); paint.setXfermode(DST_IN); canvas.drawBitmap(arc, 0, 0, paint); paint.setXfermode(null); </code></pre> <p>I should get this result:</p> <p><img src="https://i.stack.imgur.com/7D7Ba.png" alt="enter image description here"></p> <p>Where the destination image (the circle) is clipped to the area where the source image (the arc) is drawn. Instead, I get the full circle. If I just draw the arc, it appears in the correct location, and if I use DST_OUT instead, I get the inverse of the expected result (the other three quadrants of the circle). </p> <p>I've also made sure to disable hardware rendering for this view, in case there was an issue with this Xfermode, but it doesn't make a difference.</p> <p>I broke it out into a separate project at the simplest level trying to get it to work, and using the following code, I still have the same problem:</p> <pre><code>public class ClippedView extends View { private Xfermode DST_IN, DST_OUT; private Paint paint; public ClippedView(Context context) { super(context); init(); } private void init() { setLayoutParams(new RelativeLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT)); this.setLayerType(View.LAYER_TYPE_SOFTWARE, null); paint = new Paint(Paint.ANTI_ALIAS_FLAG); DST_IN = new PorterDuffXfermode(PorterDuff.Mode.DST_IN); DST_OUT = new PorterDuffXfermode(PorterDuff.Mode.DST_OUT); } @Override protected void onDraw(Canvas canvas) { super.onDraw(canvas); paint.setColor(Color.GREEN); canvas.drawRect(0, 0, getWidth() / 2, getHeight() / 2, paint); paint.setColor(Color.BLACK); paint.setXfermode(DST_IN); canvas.drawCircle(getWidth() / 2, getHeight() / 2, getWidth() / 2, paint); paint.setXfermode(null); } } </code></pre> <p>Am I using it wrong? Am I just missing something? Have I found a bug? :)</p>
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