what is the relation between Canvas and Bitmap?
<p>What is the relation between Canvas and Bitmap?</p>

<pre><code>Bitmap drawingBitmap = Bitmap.createBitmap(bmp1.getWidth(),
bmp1.getHeight(), bmp1.getConfig());
canvas = new Canvas(drawingBitmap);
paint = new Paint();
canvas.drawBitmap(bmp1, 0, 0, paint);
paint.setXfermode(new PorterDuffXfermode(android.graphics.PorterDuff.Mode.SCREEN));
canvas.drawBitmap(bmp2, 0, 0, paint);
compositeImageView.setImageBitmap(drawingBitmap);
</code></pre>

<p>I don't understand this code.Why the drawingBitmap is the composition of bmp1 and bmp2? </p>
