ios5 - UIView self.layer.shouldRasterize = YES and performance issues

Question

I would like to share my experience from using self.layer.shouldRasterize = YES; flag on UIViews.

I have a UIView class hierarchy that has self.layer.shouldRasterize turned ON in order to improve scrolling performance (all of them have STATIC subviews that are larger than the screen of the device).

Today in one of the subclasses I used CAEmitterLayer to produce nice particle effects.

The performance is really poor although the number of particles was really low (50 particles).

What is the cause of this problem?

Answer 1

I'll just quote Apple Doc and explain:

@property BOOL shouldRasterize

When the value of this property is YES, the layer is rendered as a bitmap in its local coordinate space and then composited to the destination with any other content. Shadow effects and any filters in the filters property are rasterized and included in the bitmap. However, the current opacity of the layer is not rasterized. If the rasterized bitmap requires scaling during compositing, the filters in the minificationFilter and magnificationFilter properties are applied as needed.

So basically when shouldRasterize is set to YES, every pixel that will compose the layer is calculated and the whole layer is cached as a bitmap.

• When will you benefit from it ?

When you only need to draw it once. That means when you need just pure "simple" animation (eg moving, transform, scaling...) because CoreAnimation will actually use that layer without redrawing it every frame. It's a very powerful feature to cache complex layers (with shadows and corner radius) combined with CoreAnimation.

• When will it kill you framerate ?

When your layer is redisplayed many times, because on top of the drawing that is already taking effect, the shouldRasterize will process all pixels to cache the bitmap data.

So the real question you should ask yourself is this : "On which layer am I applying the shouldRasterize to YES ? And how often is this layer redrawn ?"

Hope this was clear enough.