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      CHAPTER 3                                                                     Syntax



      The resulting PDF image object, then, contains the page information segment
      and the immediate text region segment and refers to a JBIG2Globals stream that
      contains the symbol dictionary segment.


3.3.7 DCTDecode Filter

      The DCTDecode filter decodes grayscale or color image data that has been
      encoded in the JPEG baseline format. (JPEG stands for the Joint Photographic
      Experts Group, a group within the International Organization for
      Standardization that developed the format; DCT stands for discrete cosine
      transform, the primary technique used in the encoding.)

      JPEG encoding is a lossy compression method, designed specifically for
      compression of sampled continuous-tone images and not for general data
      compression. Data to be encoded using JPEG consists of a stream of image
      samples, each consisting of one, two, three, or four color components. The color
      component values for a particular sample must appear consecutively. Each
      component value occupies an 8-bit byte.

      During encoding, several parameters control the algorithm and the information
      loss. The values of these parameters, which include the dimensions of the image
      and the number of components per sample, are entirely under the control of the
      encoder and are stored in the encoded data. DCTDecode generally obtains the
      parameter values it requires directly from the encoded data. However, in one
      instance, the parameter might not be present in the encoded data but must be
      specified in the filter parameter dictionary; see Table 3.11.

      The details of the encoding algorithm are not presented here but are in the ISO
      specification and in JPEG: Still Image Data Compression Standard, by Pennebaker
      and Mitchell (see the Bibliography). Briefly, the JPEG algorithm breaks an image
      up into blocks that are 8 samples wide by 8 samples shigh. Each color component
      in an image is treated separately. A two-dimensional DCT is performed on each
      block. This operation produces 64 coefficients, which are then quantized. Each
      coefficient may be quantized with a different step size. It is this quantization that
      results in the loss of information in the JPEG algorithm. The quantized coef-
      ficients are then compressed.

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