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Anti-aliasing

Anti-aliasing is used in 2D graphics to smooth and soften the jagged (or aliased) edges you sometimes see when graphical objects such as text, line art, and images are drawn in a bitmap context. Anti-aliased objects are more accurately represented, more appealing to the eye, and more realistic.

Quartz provides a clear advantage over QuickDraw when it comes to anti-aliasing because Quartz uses anti-aliasing to draw shapes as well as text. Quartz also provides several levels of additional text anti-aliasing or text smoothing for LCD displays. QuickDraw supports anti-aliasing for text only, and its algorithm is limited to 16 shades of gray, with glyphs always positioned on pixel boundaries.

Quartz anti-aliasing maintains consistent high-quality rendering at any resolution by finding the best representation for a particular device. In graphics contexts that support anti-aliasing, by default everything drawn is anti-aliased. Images are drawn with anti-aliasing along their borders, causing them to appear to blend smoothly into the adjacent background.

Compared with QuickDraw, Quartz text anti-aliasing is more sophisticated. Quartz anti-aliasing uses a coverage model to compute the degree to which nearby pixels in device space are covered or contained by the drawing primitive. The coverage data determines the opacity of partially covered pixels. Anti-aliasing uses 8-bit opacity, which provides 256 different opacity levels. The opacity depth could increase in the future, as device capabilities and algorithms improve.

You can turn anti-aliasing off for a particular bitmap graphics context by calling the Quartz function CGContextSetShouldAntialias. The anti-aliasing setting is part of the graphics state.

Beginning in Mac OS X v10.4, you can also control whether or not to allow anti-aliasing for a particular bit-oriented graphics context by using the function CGContextSetAllowsAntialiasing. Pass true to this function to allow anti-aliasing, and false not to allow it. This setting is not part of the graphics state. Quartz performs anti-aliasing for a bit-oriented graphics context if you allow anti-aliasing (by passing true to CGContextSetAllowsAntialiasing) and you set the anti-aliasing setting graphics state parameter to true (by calling CGContextSetShouldAntialias).

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