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     SECTION 2.1                                                           Imaging Model



2.1.2 Adobe Imaging Model

     The Adobe imaging model is a simple and unified view of two-dimensional
     graphics borrowed from the graphic arts. In this model, “paint” is placed on a
     page in selected areas:

     • The painted figures can be in the form of character shapes (glyphs), geometric
       shapes, lines, or sampled images such as digital representations of photographs.
     • The paint may be in color or in black, white, or any shade of gray. It may also
       take the form of a repeating pattern (PDF 1.2) or a smooth transition between
       colors (PDF 1.3).
     • Any of these elements may be clipped to appear within other shapes as they are
       placed onto the page.

     A page’s content stream contains operands and operators describing a sequence of
     graphics objects. A PDF consumer application maintains an implicit current page
     that accumulates the marks made by the painting operators. Initially, the current
     page is completely blank. For each graphics object encountered in the content
     stream, the application places marks on the current page, which replace or com-
     bine with any previous marks they may overlay. Once the page has been com-
     pletely composed, the accumulated marks are rendered on the output medium
     and the current page is cleared to blank again.

     PDF 1.3 and earlier versions use an opaque imaging model in which each new
     graphics object painted onto a page completely obscures the previous contents of
     the page at those locations (subject to the effects of certain optional parameters
     that may modify this behavior; see Section 4.5.6, “Overprint Control”). No mat-
     ter what color an object has—white, black, gray, or color—it is placed on the page
     as if it were applied with opaque paint. PDF 1.4 introduces a transparent imaging
     model in which objects painted on the page are not required to be fully opaque.
     Instead, newly painted objects are composited with the previously existing con-
     tents of the page, producing results that combine the colors of the object and its
     backdrop according to their respective opacity characteristics. The transparent
     imaging model is described in Chapter 7.

     The principal graphics objects (among others) are as follows:

     • A path object consists of a sequence of connected and disconnected points,
       lines, and curves that together describe shapes and their positions. It is built up

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