Inside Macintosh: Imaging with QuickDraw
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Following the picSize and picFrame fields, a Picture record contains data in the form of opcodes , which are values that the DrawPicture procedure uses to determine what object to draw or what mode to change for subsequent drawing. Your application generally should not read or write these opcodes directly but should instead use the QuickDraw routines described in this chapter for generating and processing the opcodes. (For your application's debugging purposes, these opcodes are listed in Appendix A at the back of this book.)
In addition to compact QuickDraw drawing commands, opcodes can also specify picture comments. Created with the PicComment procedure, a picture comment contains data or commands for special processing by output devices, such as PostScript printers. If your application requires capability beyond that provided by QuickDraw drawing routines, the PicComment procedure allows your application to pass data or commands directly to the output device. For example, picture comments enable highly sophisticated drawing applications that process opcodes directly to reconstruct drawing instructions--such as rotating text--not found in QuickDraw. Picture comments are usually stored in the definition of a picture or are included in the code an application sends to a printer driver.
Unless your application creates highly sophisticated graphics, you typically use QuickDraw commands when drawing to the screen and use picture comments to include special drawing commands for printers only. For example, your application can use picture comments to specify commands--for rotating text and graphics and for drawing dashed lines, fractional line widths, and smoothed polygons--that are supported by some printers but are not accessible through standard QuickDraw calls. These picture comments are described in detail in Appendix B, "Using Picture Comments for Printing," in this book.