Add segy endian check #19
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SEGY may have BIG or LITTLE endian byte order. Machine also may have BIG or LITTLE endian order. If machine and SEGY have different byte order then we have to swap bytes while reading data. To check whether machine and SEGY byte order are same we can use 3225-3226 bytes (Data Sample Format). According to SEGY revision document this value may have only value in range 1-16. Than concerns any type greater > 1 byte (even INT16, FLOAT32 or IBMFLOAT32 or any other). For `bswap_needed` correct work it is important that stream (`s::IO)` is opened from the begining of the file. Thus we can"t use the stream when reading traces or trace header (there we pass stream that is opened from some offset). That is why in some functions we don"t have default value for `swap_bytes::Bool` argument.
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I had to revert it because it breaks a lot of tests in JUDI reading segy files incorrectly. Please reopen it and we can try to see what is wrong |
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PR reopened here #21 |
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Before that commit all SEGY file were supposed to be BIG endian and the host machine is LITTLE endian.
It weren"t working with LITTLE endian SEGY on LITTLE endian machine.
Now the endianness of machine is cheked using Base.ENDIAN_BOM value and SEGY endian is checked using 3225-3226 file binare header value (Data Sample Format code) wich is supposed to be in range 1-16 (from the SEGY Revision).
The most importnat here is the function
bswap_needed(s::IO)that checks whether swap bytes is needed or not.It is important that
s::IOwas opened from the beginning of the file.That is thrue when reading
file_headerbut this is not true when reading traces or trace_headers.That is why
swap_bytesargument for these functions doesn"t have default value.Also
bigendian::Boolargument for some functions was renamed toswap_bytes::Bool.I locally checked the correctness of reading and scanning IEEE little/big endian SEGY and IBMFLOAT32 big endian SEGY on my LITTLE endian machine and it is working correctly.