Surface and SurfaceHolder

The Surface class has been part of the public API since 1.0. Its description simply says, "Handle onto a raw buffer that is being managed by the screen compositor." The statement was accurate when initially written but falls well short of the mark on a modern system.

The Surface represents the producer side of a buffer queue that is often (but not always!) consumed by SurfaceFlinger. When you render onto a Surface, the result ends up in a buffer that gets shipped to the consumer. A Surface is not simply a raw chunk of memory you can scribble on.

The BufferQueue for a display Surface is typically configured for triple-buffering; but buffers are allocated on demand. So if the producer generates buffers slowly enough -- maybe it's animating at 30fps on a 60fps display -- there might only be two allocated buffers in the queue. This helps minimize memory consumption. You can see a summary of the buffers associated with every layer in the dumpsys SurfaceFlinger output.

Canvas Rendering

Once upon a time, all rendering was done in software, and you can still do this today. The low-level implementation is provided by the Skia graphics library. If you want to draw a rectangle, you make a library call, and it sets bytes in a buffer appropriately. To ensure that a buffer isn't updated by two clients at once, or written to while being displayed, you have to lock the buffer to access it. lockCanvas() locks the buffer and returns a Canvas to use for drawing, and unlockCanvasAndPost() unlocks the buffer and sends it to the compositor.

As time went on, and devices with general-purpose 3D engines appeared, Android reoriented itself around OpenGL ES. However, it was important to keep the old API working, for apps as well as app framework code, so an effort was made to hardware-accelerate the Canvas API. As you can see from the charts on the Hardware Acceleration page, this was a bit of a bumpy ride. Note in particular that while the Canvas provided to a View's onDraw() method may be hardware-accelerated, the Canvas obtained when an app locks a Surface directly with lockCanvas() never is.

When you lock a Surface for Canvas access, the "CPU renderer" connects to the producer side of the BufferQueue and does not disconnect until the Surface is destroyed. Most other producers (like GLES) can be disconnected and reconnected to a Surface, but the Canvas-based "CPU renderer" cannot. This means you can't draw on a surface with GLES or send it frames from a video decoder if you've ever locked it for a Canvas.

The first time the producer requests a buffer from a BufferQueue, it is allocated and initialized to zeroes. Initialization is necessary to avoid inadvertently sharing data between processes. When you re-use a buffer, however, the previous contents will still be present. If you repeatedly call lockCanvas() and unlockCanvasAndPost() without drawing anything, you'll cycle between previously-rendered frames.

The Surface lock/unlock code keeps a reference to the previously-rendered buffer. If you specify a dirty region when locking the Surface, it will copy the non-dirty pixels from the previous buffer. There's a fair chance the buffer will be handled by SurfaceFlinger or HWC; but since we need to only read from it, there's no need to wait for exclusive access.

The main non-Canvas way for an application to draw directly on a Surface is through OpenGL ES. That's described in the EGLSurface and OpenGL ES section.

SurfaceHolder

Some things that work with Surfaces want a SurfaceHolder, notably SurfaceView. The original idea was that Surface represented the raw compositor-managed buffer, while SurfaceHolder was managed by the app and kept track of higher-level information like the dimensions and format. The Java-language definition mirrors the underlying native implementation. It's arguably no longer useful to split it this way, but it has long been part of the public API.

Generally speaking, anything having to do with a View will involve a SurfaceHolder. Some other APIs, such as MediaCodec, will operate on the Surface itself. You can easily get the Surface from the SurfaceHolder, so hang on to the latter when you have it.

APIs to get and set Surface parameters, such as the size and format, are implemented through SurfaceHolder.