2.2.2.2.1.1.1.4 Delta-Encoded Points (DELTA_PTS_FIELD)
The DELTA_PTS_FIELD structure is used to encode a series of points. Each point is expressed as an X and Y delta from the previous point in the series (the first X and Y deltas are relative to a base point that MUST be included in the order that contains the DELTA_PTS_FIELD structure). The number of points is order-dependent and is not specified by any field within the DELTA_PTS_FIELD structure. Instead, a separate field within the order that contains the DELTA_PTS_FIELD structure MUST be used to specify the number of points (this field SHOULD<1> be placed immediately before the DELTA_PTS_FIELD structure in the order encoding).
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zeroBits (variable) |
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deltaEncodedPoints (variable) |
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zeroBits (variable): A variable-length byte field. The zeroBits field is used to indicate the absence of an X or Y delta value for a specific point in the series. The size in bytes of the zeroBits field is given by ceil(NumPoints / 4) where NumPoints is the number of points being encoded. Each point in the series requires two zero-bits (four points per byte) to indicate whether an X or Y delta value is zero (and not present), starting with the most significant bits, so that for the first point the X-zero flag is set at (zeroBits[0] & 0x80), and the Y-zero flag is set at (zeroBits[0] & 0x40).
deltaEncodedPoints (variable): A variable-length byte field. The deltaEncodedPoints field contains a series of (X, Y) pairs, each pair specifying the delta from the previous pair in the series (the first pair in the series contains a delta from a pre-established coordinate).
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The presence of the X and Y delta values for a given pair in the series is dictated by the individual bits of the zeroBits field. If the zero bit is set for a given X or Y component, its value is unchanged from the previous X or Y component in the series (a delta of zero), and no data is provided. If the zero bit is not set for a given X or Y component, the delta value it represents is encoded in a packed signed format:
If the high bit (0x80) is not set in the first encoding byte, the field is 1 byte long and is encoded as a signed delta in the lower 7 bits of the byte.
If the high bit of the first encoding byte is set, the lower 7 bits of the first byte and the 8 bits of the next byte are concatenated (the first byte containing the high-order bits) to create a 15-bit signed delta value.