If you want to have transparent background and android:outlineProvider="bounds" doesn't work for you, you can create custom ViewOutlineProvider:
class TransparentOutlineProvider extends ViewOutlineProvider {
@Override
public void getOutline(View view, Outline outline) {
ViewOutlineProvider.BACKGROUND.getOutline(view, outline);
Drawable background = view.getBackground();
float outlineAlpha = background == null ? 0f : background.getAlpha()/255f;
outline.setAlpha(outlineAlpha);
}
}
And set this provider to your transparent view:
transparentView.setOutlineProvider(new TransparentOutlineProvider());
Sources: https://code.google.com/p/android/issues/detail?id=78248#c13
[EDIT] Documentation of Drawable.getAlpha() says that it is specific to how the Drawable threats the alpha. It is possible that it will always return 255. In this case you can provide the alpha manually:
class TransparentOutlineProvider extends ViewOutlineProvider {
private float mAlpha;
public TransparentOutlineProvider(float alpha) {
mAlpha = alpha;
}
@Override
public void getOutline(View view, Outline outline) {
ViewOutlineProvider.BACKGROUND.getOutline(view, outline);
outline.setAlpha(mAlpha);
}
}
If your view background is a StateListDrawable with default color #DDFF0000 that is with alpha DDx0/FFx0 == 0.86
transparentView.setOutlineProvider(new TransparentOutlineProvider(0.86f));