converting byte waves to floating point numbers

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Wed, 02/09/2011 - 08:35 pm

While analyzing incompletely documented binary files, I'm trying to convert bytes I previously read from a large file to IEEE doubles within Igor. There seems to be no ready-made conversion routine for this in Igor. I know I should probably just read the bytes again from the file using FBinRead with all its nice flags, but I would like to do this in memory due to speed reasons (the byte alignment in the file is not clear so I have to try several possibilities).

Anyway, does anyone have an easier way than type-casting via structures like so:

structure dbyte_struct
    char bytes[8]
endstructure

structure dval_struct
    double value
endstructure

function BytesToDouble(byteWave, byteOrder)
wave byteWave
variable byteOrder
    struct dbyte_struct db
    struct dval_struct dv
    variable k
    for( k = 0; k < 8; k += 1 )
        db.bytes[k] = byteWave[k]
    endfor
    string temp
    StructPut/S db, temp
    StructGet/S/B=(byteOrder) dv, temp
    return dv.value
end
Hi Wolfgang,
I've had to solve similar problems.
Check out the Sockit XOP.
Use SOCKITwavetoString to convert the bytes into a string, then use SOCKITstringtowave to put the bytes into the wavetype of your choice. It will do big endian conversions if you ask it to. It's all done in memory with memcpy.

Alternatively, you should be able to use redimension with the /E flag. That's probably safer if you're distributing code.

Andy

February 9, 2011 at 09:53 pm - Permalink

Oh, aye, the Redimension operation actually does the entire job. So all I have to do is
Redimension /D /E=1 bw
where bw is a byte wave made by Make/B/U bw as in my previous code. If the byte order is in doubt, I can then use
Redimension /D /E=2 bw
to swap the byte order back and forth...

This is brilliant. The documentation nearly hides this cool feature, the only information I could find in the help files was
[Redimension /E]=1: Force reshape without converting or moving data.

Uncharacteristically terse...although spot-on in hindsight.

Thanks for pointing that out, Andy!

February 10, 2011 at 02:39 am - Permalink