NSImage的坐标系统

Image Coordinate Systems

Like views, NSImage objects use their own coordinate system to manage their content, which in this case is the image data itself. This internal coordinate system is independent of any containing views into which the image is drawn. Although you might think understanding this coordinate system is important for drawing images in your views, it actually is not. The purpose of the internal coordinate system is to orient the image data itself. As a result, the only time you should ever need to know about this internal coordinate system is when you create a new image by locking focus on an NSImage object and drawing into it.

Image objects have two possible orientations: standard and flipped. When you create a new, emptyNSImage object, you can set the orientation based on how you want to draw the image data. By default, images use the standard Cartesian (unflipped) coordinate system, but you can force them to use a flipped coordinate system by calling thesetFlipped: method of NSImage prior to drawing. You must always set the image orientation before you lock focus on the image and start drawing though. Changing the orientation of the coordinate system after alockFocus call has no effect. In addition, calling the setFlipped: method after you unlock focus again may not have the desired results and should be avoided.

When drawing images in your view, you can think of the image as just a rectangle with some data in it. Regardless of the orientation of its internal coordinate system, you always place an image relative to the current view’s coordinate system.Figure 6-1 shows two images drawn in an unflipped view. The code used to draw each image uses the coordinate points shown in the figure, which are in the view’s (unflipped) coordinate system. Because the first image uses a flipped coordinate system internally, however, it draws its content upside down.

Figure 6-1  Image orientation in an unflipped view