2.2.2.2.1.1.1.5 Delta-Encoded Rectangles (DELTA_RECTS_FIELD)
The DELTA_RECTS_FIELD structure is used to encode a series of rectangles. Each rectangle is encoded as a (left, top, width, height) quadruple with the left and top components of each quadruple containing the delta from the left and top components of the previous rectangle; the first rectangle in the series is implicitly assumed to be (0, 0, 0, 0). The number of rectangles is order-dependent and not specified by any field within the DELTA_RECTS_FIELD structure. Instead, a separate field within the order that contains the DELTA_RECTS_FIELD structure MUST be used to specify the number of rectangles (this field SHOULD be placed immediately before the DELTA_RECTS_FIELD structure in the order encoding). The maximum number of rectangles that can be encoded by this structure is 45.
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zeroBits (variable) |
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deltaEncodedRectangles (variable) |
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zeroBits (variable): A variable-length byte field. The zeroBits field is used to indicate the absence of a left, top, width, or height component. The size, in bytes, of the zeroBits field is given by ceil(NumRects / 2) where NumRects is the number of rectangles being encoded. Each rectangle in the series requires four zero-bits (two rectangles per byte) to indicate whether a left, top, width, or height component is zero (and not present), starting with the most significant bits, so that for the first rectangle the left-zero flag is set at (zeroBits[0] & 0x80), the top-zero flag is set at (zeroBits[0] & 0x40), the width-zero flag is set at (zeroBits[0] & 0x20), and the height-zero flag is set at (zeroBits[0] & 0x10).
deltaEncodedRectangles (variable): A variable-length byte field. The deltaEncodedRectangles field contains a series of (left, top, width, height) quadruples with the left and top components in each quadruple specifying the delta from the left and top components of the previous rectangle in the series; the first rectangle in the series is implicitly assumed to be (0, 0, 0, 0).
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Assume there are two rectangles specified in (left, top, right, bottom) quadruples:
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1: (L1, T1, R1, B1) 2: (L2, T2, R2, B2)
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Assuming Rectangle 1 is the first in the series, and by using the (left, top, width, height) quadruple encoding scheme, these two rectangles would be specified as:
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1: (L1, T1, R1 - L1, B1 - T1) 2: (L2 - L1, T2 - T1, R2 - L2, B2 - T2)
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The presence of the left, top, width, or height component for a given quadruple is dictated by the individual bits of the zeroBits field. If the zero bit is set for a given left, top, width, or height component, its value is unchanged from the previous corresponding left, top, width, or height value in the series (a delta of zero), and no data is provided. If the zero bit is not set for a left, right, width, or height component, its value 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.