Coordinate Space
QuickDraw and Quartz coordinate space differ most in the location you draw to and the location of the origin. In Quartz, you draw to user space, which is device-independent and pixel-free. By drawing to user space, you can send the same drawing to any number of destinations—screen, printer, bitmap, PDF—and Quartz converts the user space coordinates to the appropriate device space coordinates. No math required on your part!
When you draw with Quartz, you need only to work in user space. If for some reason your application needs to obtain the affine transform that Quartz uses to convert between user and device space, you can call the function CGContextGetUserSpaceToDeviceSpaceTransform, introduced in Mac OS X v10.4. Quartz also adds functions in Mac OS X v10.4 that convert geometries (points, sizes, and rectangles) between device and user space.
The Quartz user space is modeled on the Cartesian plane—coordinates are single-precision, floating-point numbers, and by default, the positive y-axis extends upward. In a new graphics context, the origin corresponds to the lower-left corner of the page, as shown in Figure 2-1. QuickDraw, by comparison, uses integer coordinates whose origin corresponds to the upper-left corner of a page. HIView, uses floating-point coordinates whose origin corresponds to the upper-left corner of a page.
You can use Quartz translation and scaling functions to convert between coordinate systems. Listing 2-1 shows how to switch from Quartz coordinates to one whose origin is in the upper-left corner by modifying the context matrix prior to drawing.
Listing 2-1 Code that transforms the Quartz origin to be at the upper-left
CGContextSaveGState (myContext); |
CGContextTranslateCTM (myContext, 0, myOrigin.y + myPortHeight); |
CGContextScaleCTM (myContext, 1.0f, -1.0f); |
// Your drawing code here. |
CGContextRestoreGState (myContext); |
The function CGContextTranslateCTM translates the coordinate system so that the y values are moved toward the top of the HIView by the height of the HIView bounding rectangle. If you were to draw now, your drawing would be outside the HIView, not in a visible area.
The function CGContextScaleCTM code flips the y-coordinates by a factor of –1.0, effectively flipping the coordinates into the HIView. After this operation, the origin is at the lower left of the HIView, with the y values increasing from bottom to top. The x values are unchanged; they still increase from left to right.
Tip: Spend time to figure out the coordinates and to translate from one coordinate system to the other. Quartz transforms make it easy to switch back and forth, but it’s also easy to make mistakes until you have practiced. Be careful not to perform transformations in the wrong order. If drawing doesn’t show up as you expect, use the function CGContextGetClipBoundingBox to see if the bounding box of the drawing area is where you expect it to be. See Transforms in Quartz 2D Programming Guide for more information.
If you use HIView in conjunction with Quartz, and draw to a graphics context that you obtain from HIView, that graphics context uses HIView coordinates. HIView places the origin in the upper-left corner of the view, but it uses floating-point values just like Quartz does. HIView uses the upper left to ensure that the coordinates of objects, such as controls, do not change as the user resizes the window. When you want to draw a Quartz image (CGImage) to an HIView, make sure that you use the HIView function HIViewDrawCGImage, which orients the image appropriately for the HIView coordinate system.
Keep in mind that text drawing is affected by the coordinate system. Quartz, the HIToolBox, and ATSUI are among the APIs that provide support for drawing text. You need to be aware of the coordinate-system assumptions made by each text drawing function as well as the transformations you’ve performed on the Quartz coordinate system. For example, if you use the default Quartz coordinate system and the Quartz text drawing functions (CGContextShowText, CGContextShowTextAtPoint, CGContextShowGlyphs, and so forth), then text is drawn in the correct orientation. If you use the default Quartz coordinate system and the HIToolbox text drawing functions (such as HIThemeDrawTextBox), text appears inverted. In the case of HIThemeDrawTextBox you can remedy this by specifying the option kHIThemeOrientationInverted.
The details of text drawing and coordinate systems aren’t discussed here. It’s an issue you’ll want to investigate further. However, if you draw text and it either doesn’t appear or it appears inverted, take a close look at the coordinate system, the transformations you’ve performed, and the assumptions of the text drawing function you use. Also keep in mind that Quartz has a text matrix that can be transformed separately from default user space. For more information, see Text in Quartz 2D Programming Guide.
Last updated: 2006-09-05
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