When does a view (or layer) require offscreen rendering?

I don't think there is a rule written down anywhere, but hopefully this will help:

First, let's clear up some definitions. I think offscreen vs onscreen rendering is not the overriding concern most of the time, because offscreen rendering can be as fast as onscreen. The main issue is whether the rendering is done in hardware or software.

There is also very little practical difference between using layers and views. Views are just a thin wrapper around CALayer and they don't introduce a significant performance penalty most of the time. You can override the type of layer used by a view using the +layerClass method if you want to have a view backed by a CAShapeLayer or CATileLayer, etc.

Generally, on iOS, pixel effects and Quartz / Core Graphics drawing are not hardware accelerated, and most other things are.

The following things are not hardware accelerated, which means that they need to be done in software (offscreen):

1. Anything done in a drawRect. If your view has a drawRect, even an empty one, the drawing is not done in hardware, and there is a performance penalty.

2. Any layer with the shouldRasterize property set to YES.

3. Any layer with a mask or drop shadow.

4. Text (any kind, including UILabels, CATextLayers, Core Text, etc).

5. Any drawing you do yourself (either onscreen or offscreen) using a CGContext.

Most other things are hardware accelerated, so they are much faster. However, this may not mean what you think it does.

Any of the above types of drawing are slow compared to hardware accelerated drawing, however they don't necessarily slow down your app because they don't need to happen every frame. For example, drawing a drop shadow on a view is slow the first time, but after it is drawn it is cached, and is only redrawn if the view changes size or shape.

The same goes for rasterised views or views with a custom drawRect: the view typically isn't redrawn every frame, it is drawn once and then cached, so the performance after the view is first set up is no worse, unless the bounds change or you call setNeedsDisplay on it.

For good performance, the trick is to avoid using software drawing for views that change every frame. For example, if you need an animated vector shape you'll get better performance using CAShapeLayer or OpenGL than drawRect and Core Graphics. But if you draw a shape once and then don't need to change it, it won't make much difference.

Similarly, don't put a drop shadow on an animated view because it will slow down your frame rate. But a shadow on a view that doesn't change from frame to frame won't have much negative impact.

Another thing to watch out for is slowing down the view setup time. For example, suppose you have a page of text with drop shadows on all the text; this will take a very long time to draw initially since both the text and shadows all need to be rendered in software, but once drawn it will be fast. You will therefore want to set up this view in advance when your application loads, and keep a copy of it in memory so that the user doesn't have to wait ages for the view to display when it first appears on screen.

This is probably the reason for the apparent contradiction in the WWDC videos. For large, complex views that don't change every frame, drawing them once in software (after which they are cached and don't need to be redrawn) will yield better performance than having the hardware re-composite them every frame, even though it will be slower to draw the first time.

But for views that must be redrawn constantly, like table cells (the cells are recycled so they must be redrawn each time one cell scrolls offscreen and is re-used as it scrolls back onto the other side as a different row), software drawing may slow things down a lot.

Offscreen-rendering is one of the worst defined topics in iOS rendering, today. When Apple's UIKit engineers refer to offscreen-rendering, it has a very specific meaning, and a ton of third-party iOS dev blogs are getting it wrong.

When you override "drawRect:", you're drawing via the CPU, and spitting out a bitmap. The bitmap is packaged up and sent to separate process that lives in iOS, the render server. Ideally, the render server just displays the data on screen.

If you fiddle with properties on CALayer, like turning on drop shadows, the GPU will perform additional drawing. This additional work is what UIKit engineers mean when they say "off-screen rendering." This is always performed with hardware.

The issue with off-screen drawing isn't necessarily the drawing. The off-screen pass requires a context switch, as the GPU switches its drawing destination. During this switch, the GPU is idle.

While I don't know a full list of properties that trigger an off-screen pass, you can diagnose this with the Core Animation Instrument's "Color Offscreen-rendered layer" toggle. I assume any property other than alpha is performed via an offscreen pass.

With early iOS hardware, it was reasonable to say "do everything in drawRect." Nowadays GPUs are better, and UIKit has features like shouldRasterize. Today, it's a balancing act between the time spent in drawRect, the number of off-screen passes, and the amount of blending. For the full details, watch the 2014 WWDC session 419, "Advanced Graphics and Animation for iOS Apps."

That all said, it's good to understand what's going on behind-the-scenes, and keep it in the back of your head so you don't do anything insane, but you should start from the simplest solution. Then test it on the slowest hardware you support. If you aren't hitting 60FPS, use Instruments to measure things and figure it out. There are a few possible bottlenecks, and if you aren't using data to diagnose things, you're just guessing.

Offscreen rendering / Rendering on the CPU

The biggest bottlenecks to graphics performance are offscreen rendering and blending – they can happen for every frame of the animation and can cause choppy scrolling.

Offscreen rendering (software rendering) happens when it is necessary to do the drawing in software (offscreen) before it can be handed over to the GPU. Hardware does not handle text rendering and advanced compositions with masks and shadows.

The following will trigger offscreen rendering:

• Any layer with a mask (layer.mask)

• Any layer with layer.masksToBounds / view.clipsToBounds being true

• Any layer with layer.allowsGroupOpacity set to YES and layer.opacity is less than 1.0
  When does a view (or layer) require offscreen rendering?

• Any layer with a drop shadow (layer.shadow*).
  Tips on how to fix: https://markpospesel.wordpress.com/tag/performance/

• Any layer with layer.shouldRasterize being true

• Any layer with layer.cornerRadius, layer.edgeAntialiasingMask, layer.allowsEdgeAntialiasing

• Any layer with layer.borderWith and layer.borderColor?
  Missing reference / proof

• Text (any kind, including UILabel, CATextLayer, Core Text, etc).

• Most of the drawings you do with CGContext in drawRect:. Even an empty implementation will be rendered offscreen.

This post covers blending and other things affecting performance: What triggers offscreen rendering, blending and layoutSubviews in iOS?