Paint paint = new Paint();
paint.setAlpha(0x40); //设置透明程度
canvas.drawBitmap(this.bitmap, this.offsetX, this.offsetY,paint);
Drawable ibSelected;
...定义Drawable
ibSelected.setBounds(0, 0, itemImageWidth+2, itemImageHeight+2);
((BitmapDrawable)ibSelected).getPaint().setAlpha(0x40);
ibSelected.draw(canvas);