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Posted on November 10, 2006 by Grant Skinner
I recently had a very interesting discussion with Andreas Heim, Peter Hall and Werner Sharp (Flash Player Engineer) about DisplayObject.scrollRect, and learnt a lot about the specific mechanics of how it works in the current player.
For instance, setting the scrollRect as above would mask a 50×50 region of the sprite starting 20 pixels down, but the masked view would still be placed at the sprite’s origin, so it would appear to be scrolled 20 pixels up.
Note that this has the same considerations that are usually associated with cacheAsBitmap. If the display object is dirtied (ie. it changes visually), the Flash player will re-render the bitmap cache, so this technique generally does not make sense for content that is likely to change each frame (though it might if the content is much larger than the masked area). It also has memory overhead from having to maintain the bitmap cache – though I assume it would be lower because it is limited to the masked area.
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