On iOS, how do CALayer bitmaps (CGImage objects) get displayed onto Graphics Card?

Question

On iOS, I was able to create 3 CGImage objects, and use a CADisplayLink at 60fps to do

self.view.layer.contents = (__bridge id) imageArray[counter++ % 3];

inside the ViewController, and each time, an image is set to the view's CALayer contents, which is a bitmap.

And this all by itself, can alter what the screen shows. The screen will just loop through these 3 images, at 60fps. There is no UIView's drawRect, no CALayer's display, drawInContext, or CALayer's delegate's drawLayerInContext. All it does is to change the CALayer's contents.

I also tried adding a smaller size sublayer to self.view.layer, and set that sublayer's contents instead. And that sublayer will cycle through those 3 images.

So this is very similar to back in the old days even on Apple ][ or even in King's Quest III era, which are DOS video games, where there is 1 bitmap, and the screen just constantly shows what the bitmap is.

Except this time, it is not 1 bitmap, but a tree or a linked list of bitmaps, and the graphics card constantly use the Painter's Model to paint those bitmaps (with position and opacity), onto the main screen. So it seems that drawRect, CALayer, everything, were all designed to achieve this final purpose.

Is that how it works? Does the graphics card take an ordered list of bitmaps or a tree of bitmaps? (and then constantly show them. To simplify, we don't consider the Implicit animation in the CA framework) What is actually happening down in the graphics card handling layer? (and actually, is this method almost the same on iOS, Mac OS X, and on the PCs?)

(this question aims to understand how our graphics programming actually get rendered in modern graphics cards, since for example, if we need to understand UIView and how CALayer works, or even use CALayer's bitmap directly, we do need to understand the graphics architecture.)

Answer 1

Modern display libraries (such as Quartz used in iOS and Mac OS) use hardware accelerated compositing. The workings is very similar to how computer graphics libraries such as OpenGL work. In essence, each CALayer is kept in as a separate surface that is buffered and rendered by the video hardware much like a texture in a 3D game. This is exceptionally well implemented in iOS and this is why the iPhone is so well-known for having a smooth UI.

In the "old days" (i.e. Windows 9x, Mac OS Classic, etc), the screen was essentially one big framebuffer, and everything that was exposed by e.g. moving a window had to be redrawn manually by each application. The redrawing was mostly done by the CPU, which put an upper limit on animation performance. Animation were usually very "flickery" due to the redrawing involved. This technique was mostly suited for desktop applications without too much animation. Notably, Android uses (or at least used to use) this technique, which is a big problem when porting iOS applications over to Android.

Games of the old days days (e.g. DOS, arcade machines, etc, also used a lot on Mac OS classic), something called sprite animation was used to improve performance and reduce flickering by keeping the moving images in offscreen buffers that were rendered by the hardware and synchronized with the monitor's vblank, which meant that animations were smooth even on very low-end systems. However, the size of these images were very limited and the screen resolutions were low, only about 10-15% of the pixels of even an iPhone screen of today.

Answer 2

You've got a reasonable intuition here, but there are still several steps between contents and the display. First off, contents doesn't have to be a CGImage. It is often a private class called CABackingStorage which is not quite the same thing. In many cases there are hardware optimizations going on to bypass rendering the image into main memory and then copying it to video memory. And since the contents of various layers are all composited together, you're still a ways from the "real" display memory. Not to mention that modifications to contents just directly impacts the model layer, not the presentation or render layers. Plus there are CGLayer objects that can store their image directly in video memory. There's a lot of different stuff going on.

So the answer is, no, the video "card" (chip; it's the PowerVR BTW) does not take an ordered bunch of layers. It takes lower-level data in ways that are not well documented. Some things (particularly parts of Core Animation, and perhaps CGLayer) appear to be wrappers around OpenGL textures, but others are probably Core Graphics directly accessing the hardware itself. Once you get to this level of the stack, it's all private and can change from version to version and from device to device.

You also may find Brad Larson's response useful here: iOS: is Core Graphics implemented on top of OpenGL?

You may also be interested in Chapter 6 of iOS:PTL. While it doesn't go into the implementation specifics, it does include a lot of practical discussion of how to improve drawing performance and best utilize the hardware with Core Graphics. Chapter 7 details all the developer-accessible steps involved in CALayer drawing.