CALayer
Core Animation
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Quartz 2D Coordinate Systems
坐标系是什么？是定义了
defines the range of locations used to express the location and sizes of objects to be drawn on the page
Quartz使用，独立的坐标系－－映射到相应的坐标系
从user space－－－到device space，使用CTM－－当前转换矩阵－－－current transformation matrix
仿射矩阵－－－平移、旋转、缩放－－translation, rotation, and scaling operations
so,告诉我放射变换的目的？
1—what kind of context it is？
2—behavior to match the expectations of the context.
to match the behavior for a context provided by UIKit
行为匹配由UIKit提供的上下文
Some technologies set up their graphics contexts using a different default coordinate system than the one used by Quartz.
PDF graphics context－－－－不提供匹配
只知道有些是不提供匹配的
It is up to your application to adjust any Quartz calls it makes to a context that has a transform applied to it. For example, if you want an image or PDF to draw correctly into a graphics context, your application may need to temporarily adjust the CTM of the graphics context. In iOS, if you use a UIImage object to wrap a CGImage object you create, you do not need to modify the CTM. The UIImage object automatically compensates for the modified coordinate system applied by UIKit.
