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Quartz Performance: A Look Under the Hood

When your application draws to a window using Quartz 2D function calls, the drawing travels from your application to the window’s backing store. From there, Quartz composites all window backing stores, including those generated by OpenGL and video playback, and sends the composite to a frame buffer.

The Quartz Compositor acts like a video mixer in which every pixel onscreen can be shared among windows in real time. Sharing is a big break from traditional windowing systems, which use a switch model. In a switch model, every pixel onscreen belongs entirely to one window or the desktop. The switch model results in a less elegant interface, with abrupt switching between windows. The video mixer model, combined with the ability to make windows translucent, allows for smooth transitions between the states of a graphical user interface and gives Mac OS X its unique and elegant look.

Quartz Extreme, introduced in Mac OS X v10.2, harnesses the power of the graphics processing unit (GPU) to composite the window backing store to the frame buffer. For computers that do not have the minimum graphics hardware needed for Quartz Extreme, Quartz uses the CPU, just as it did prior to Mac OS X v10.2.

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