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How Quartz 2D Draws Text

Quartz 2D uses fonts, which are sets of shapes that are associated with characters, to draw text. A character abstractly represents the concept of, for example, a lowercase “b”, the number “2”, or the arithmetic operation “+”. You do not ever see a character on a display device. What you see on a display device is a glyph, the path that serves as the visual representation of the character. One glyph can represent one character, such as a lowercase “b”; more than one character, such as the “fi” ligature, which is a single glyph representing two characters; or a non printing character, such as the space character.

The mapping from a byte code (sometimes called a character code) to a specific character is specified by a text encoding. For example, you use the numeric byte code value 65 to specify an uppercase “A” from a font encoded using the ASCII text encoding.

When you want to print or display text, you generally refer to characters rather than glyphs. Quartz renders glyphs using font data provided by the Apple Type Services (ATS) framework.

You specify the location of text in user space coordinates. The text matrix specifies the transform from text space to user space. The text position is stored in the tx and ty variables of the text matrix. When you first create a graphics context, it initializes the text matrix to the identity matrix, thus text space coordinates are initially the same as user space coordinates.

Quartz conceptually concatenates the text matrix with the current transformation matrix and other parameters from the graphics state to produce the final text rendering matrix, the matrix actually used to draw the text on the page. The text matrix does not include the font size, which is always expressed in text space.

When Quartz draws the text, it retrieves the relevant glyphs from the ATS font server and paints them using the current parameters in the graphics state, such as fill color (if the text is filled) and stroke color. After a drawing operation, the text position remains at the last point used to image the text.

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