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     CHAPTER 4                                                                     Graphics



     Note: For convenience, the following sections refer consistently to the object defining
     an image as an image dictionary. Although this term properly refers only to the
     dictionary portion of the stream object representing an image XObject, it should be
     understood to apply equally to the stream’s data portion or to the parameters and
     data of an inline image.


4.8.2 Sample Representation

     The source format for an image can be described by four parameters:

     • The width of the image in samples
     • The height of the image in samples
     • The number of color components per sample
     • The number of bits per color component
     The image dictionary specifies the width, height, and number of bits per compo-
     nent explicitly. The number of color components can be inferred from the color
     space specified in the dictionary.

     Note: For images using the JPXDecode filter (see Section 3.3.8, “JPXDecode Filter”),
     the number of bits per component is determined from the image data and not speci-
     fied in the image dictionary. The color space may or may not be specified in the dic-
     tionary.

     Sample data is represented as a stream of bytes, interpreted as 8-bit unsigned
     integers in the range 0 to 255. The bytes constitute a continuous bit stream, with
     the high-order bit of each byte first. This bit stream, in turn, is divided into units
     of n bits each, where n is the number of bits per component. Each unit encodes a
     color component value, given with high-order bit first; units of 16 bits are given
     with the most significant byte first. Byte boundaries are ignored, except that each
     row of sample data must begin on a byte boundary. If the number of data bits per
     row is not a multiple of 8, the end of the row is padded with extra bits to fill out
     the last byte. A PDF consumer application ignores these padding bits.

     Each n-bit unit within the bit stream is interpreted as an unsigned integer in the
     range 0 to 2n − 1, with the high-order bit first. The image dictionary’s Decode
     entry maps this integer to a color component value, equivalent to what could be
     used with color operators such as sc or g. Color components are interleaved sam-

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