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                                                494
      CHAPTER 6                                                                     Rendering




          150 per inch at 45   100 per inch at 45   50 per inch at 45   75 per inch at 45
          Round dot screen     Round dot screen     Round dot screen       Line screen



                               FIGURE 6.1 Various halftoning effects


6.4.3 Threshold Arrays

      Another way to define a halftone screen is with a threshold array that directly
      controls individual device pixels in a halftone cell. This technique provides a high
      degree of control over halftone rendering. It also permits halftone cells to be arbi-
      trary rectangles, whereas those controlled by a spot function are always square.

      A threshold array is much like a sampled image—a rectangular array of pixel
      values—but is defined entirely in device space. Depending on the halftone type,
      the threshold values occupy 8 or 16 bits each. Threshold values nominally repre-
      sent gray levels in the usual way, from 0 for black up to the maximum (255 or
      65,535) for white. The threshold array is replicated to tile the entire device space:
      each pixel in device space is mapped to a particular sample in the threshold array.
      On a bilevel device, where each pixel is either black or white, halftoning with a
      threshold array proceeds as follows:

      1. For each device pixel that is to be painted with some gray level, consult the
         corresponding threshold value from the threshold array.

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