Android : Android: convert scrolling layout to Bitmap

I would like to convert the whole scrolling layout inside a ScrollView to a Bitmap

<ScrollView...>

<RelativeLayout android:id="@+id/content"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    ...>

</RelativeLayout>

</ScrollView>

If the content doesn't scroll, this code works:

RelativeLayout content = (RelativeLayout) findViewById(R.id.content);
content.setDrawingCacheEnabled(true);
content.buildDrawingCache();
Bitmap bm = content.getDrawingCache();

but if the content is scrolling, getDrawingCache() returns null.

Any suggestion on how I can export a scrolling view to a bitmap?