Copyright © 2011 Open Geospatial Consortium
90 As you can see, level 5 or 6 is sufficient in most cases; additional compression brought
by higher levels costs a lot of CPU for only few percents compaction gain.
7.1.1.4 Thourth family
7.1.1.4.1 Lot of doubles
Java candidates:
Copyright © 2011 Open Geospatial Consortium
91 Native candidates:
Interpretation: All candidates “without deflate” perform nearly the same when dealing with floating
point numbers except EXI with schema knowledge. When we look closely at data and count characters, we see that a typical coordinate is:
-105.013035 40.2083175 It uses double, the whole pattern spaning on 23 characters.
IEEE 754 CWXML uses 8 bytes for each double, it will take just 16 bytes to store the raw data. If you add one additional byte for separation you reach 17
bytes compared to 23. The compression is only around 25. IEEE 754 binary format is made in such a way it doesn’t please deflate which needs to find at least
3 consecutive identical bytes to begin to consider placing a back-reference. For float it’s hard because the format is not byte aligned.
Copyright © 2011 Open Geospatial Consortium
92 For doubles we will have 11 bits of exponent not byte boundaries either and
mantissa 52 bits possible to get 6 consecutives bytes this time. EXI use its own way to store floating points numbers exi:double or just Float
which covers both xsd:float and xsd:double. The “Float” datatype representation is two consecutive integers signed:
o The first Integer represents the mantissa of the floating point number o and the second Integer represents the base-10 exponent of the floating
point number This representation relies on integer compression, which is the following:
A Boolean for the sign a single bit when deflate is not used, a full byte otherwise
An unsigned integer for the absolute value of the integer, which is encoded as a sequence of bytes terminated by a byte with its
most significant bit set to 0. The value of the unsigned integer is stored in the least significant 7 bits of the bytes as a sequence of 7-
bit bytes, with the least significant byte first.
As you see this storage using 7 real bits in 8 bits envelope can be bad for performance when using full fractions, as 23 bits of mantissa will make 4 bytes
in EXI. So basically when using deflate on a double, the input material provided by EXI could cost 8 bytes of mantissa, 2 bytes of exponent and 2 bytes of sign
total 12 bytes. That means that EXI doubles could take much place as ascii data equivalent.
7.1.2 Interpretation of results
