Previous Next

                                               38
      CHAPTER 2                                                                  Overview



      For each graphical element that is to appear on the page, the scan converter sets
      the values of the corresponding pixels. When the interpretation of the page de-
      scription is complete, the pixel values in memory represent the appearance of the
      page. At this point, a raster output process can render this representation (make it
      visible) on a printed page or display screen.

      Scan-converting a graphical shape, such as a rectangle or circle, entails determin-
      ing which device pixels lie inside the shape and setting their values appropriately
      (for example, to black). Because the edges of a shape do not always fall precisely
      on the boundaries between pixels, some policy is required for deciding how to set
      the pixels along the edges. Scan-converting a glyph representing a text character
      is conceptually the same as scan-converting an arbitrary graphical shape. How-
      ever, character glyphs are much more sensitive to legibility requirements and
      must meet more rigid objective and subjective measures of quality.

      Rendering grayscale elements on a bilevel device is accomplished by a technique
      known as halftoning. The array of pixels is divided into small clusters according to
      some pattern (called the halftone screen). Within each cluster, some pixels are set
      to black and others to white in proportion to the level of gray desired at that loca-
      tion on the page. When viewed from a sufficient distance, the individual dots be-
      come imperceptible and the perceived result is a shade of gray. This enables a
      bilevel raster output device to reproduce shades of gray and to approximate natu-
      ral images such as photographs. Some color devices use a similar technique.


 2.2 Other General Properties

      This section describes other notable general properties of PDF, aside from its im-
      aging model.


2.2.1 Portability

      PDF files are represented as sequences of 8-bit binary bytes. A PDF file is de-
      signed to be portable across all platforms and operating systems. The binary rep-
      resentation is intended to be generated, transported, and consumed directly,
      without translation between native character sets, end-of-line representations, or
      other conventions used on various platforms.

Previous Next