18. Logical Screen Descriptor.

      a. Description.  The Logical Screen Descriptor contains the parameters
      necessary to define the area of the display device within which the
      images will be rendered.  The coordinates in this block are given with
      respect to the top-left corner of the virtual screen; they do not
      necessarily refer to absolute coordinates on the display device.  This
      implies that they could refer to window coordinates in a window-based
      environment or printer coordinates when a printer is used.

      This block is REQUIRED; exactly one Logical Screen Descriptor must be
      present per Data Stream.

      b. Required Version.  Not applicable. This block is not subject to a
      version number. This block must appear immediately after the Header.

      c. Syntax.

      7 6 5 4 3 2 1 0        Field Name                    Type
     +---------------+
  0  |               |       Logical Screen Width          Unsigned
     +-             -+
  1  |               |
     +---------------+
  2  |               |       Logical Screen Height         Unsigned
     +-             -+
  3  |               |
     +---------------+
  4  | |     | |     |       <Packed Fields>               See below
     +---------------+
  5  |               |       Background Color Index        Byte
     +---------------+
  6  |               |       Pixel Aspect Ratio            Byte
     +---------------+

            viii) Pixel Aspect Ratio - Factor used to compute an approximation
            of the aspect ratio of the pixel in the original image.  If the
            value of the field is not 0, this approximation of the aspect ratio
            is computed based on the formula:

            Aspect Ratio = (Pixel Aspect Ratio + 15) / 64

            The Pixel Aspect Ratio is defined to be the quotient of the pixel's
            width over its height.  The value range in this field allows
            specification of the widest pixel of 4:1 to the tallest pixel of
            1:4 in increments of 1/64th.

            Values :        0 -   No aspect ratio information is given.
                       1..255 -   Value used in the computation.