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

Answer 1

Turning OFF self.layer.shouldRasterize increases performance to normal levels.

Why is that?

According to a video on apple's developers site (I cannot remember the video, help please?) the rule for self.layer.shouldRasterize is that simple: If all of your subviews are static (their position, contents etc, are not changing or animating) then it is beneficiary to turn self.layer.shouldRasterize ON. On the other side if any of the subviews are changing then the framework needs to re-cache the view hierarchy and this is a huge bottleneck. Under the hood the bottleneck is the memory copying between CPU and GPU.

Answer 2

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.