Re: Binary floating point format
Re: Binary floating point format
- Subject: Re: Binary floating point format
- From: Carl Hoefs <email@hidden>
- Date: Thu, 30 Mar 2017 21:03:22 -0700
> On Mar 30, 2017, at 8:10 PM, Greg Parker <email@hidden> wrote:
>
>>
>> On Mar 30, 2017, at 7:09 PM, Carl Hoefs <email@hidden <mailto:email@hidden>> wrote:
>>
>>
>>> On Mar 30, 2017, at 6:40 PM, Steve Bird <email@hidden <mailto:email@hidden>> wrote:
>>>
>>>
>>>> On Mar 30, 2017, at 8:25 PM, Carl Hoefs <email@hidden <mailto:email@hidden>> wrote:
>>>>
>>>> I have megabytes of raw legacy science datasets that I'm trying to read into my app and ingest into an array of doubles. The data is supposed to be organized as a stream of 8-byte doubles. I do not know how these datasets were generated, so I don't know what format (big/little endian, byte swapped, etc) they are in.
>>>> Here is a hex dump of 4 binary doubles:
>>>>
>>>> 49BF7DE372533C05 A8C02FE3135B4F09 86C22FE37E630B05 27C2C4E3E258BA08
>>>
>>> A few minutes of LabVIEW work shows some semi-reasonable values if you swap bytes:
>>> BF49E37D5372053C = -0.000790058
>>> C0A8E32F5B13094F = -3185.59
>>>
>>> Do you know that those values are wrong?
>>
>> Hmm, the -3185.59 doesn't look right. The data is _supposed_ to be:
>>
>> 1st double: Time value, monotonically increasing, usually starting near 0.00000
>> 2nd double: Sensor value, oscillating waveform, centered about some arbitrary value
>>
>> So I'm expecting data like:
>>
>> 0.011 0.57525634765625
>> 0.012 0.45166015625000
>> 0.013 0.29907226562500
>> 0.014 0.13275146484375
>> 0.015 -0.03173828125000
>> 0.016 -0.18218994140625
>> 0.017 -0.29602050781250
>> 0.018 -0.38055419921875
>> ...etc.
>>
>> or even:
>>
>> 0.018 2089.66467285156250
>> 0.020 2087.57525634765625
>> 0.022 2085.45166015625000
>> 0.024 2083.29907226562500
>> 0.026 2082.13275146484375
>> 0.028 2081.03173828125000
>> 0.030 2080.18218994140625
>> 0.032 2080.29602050781250
>> 0.034 2079.38055419921875
>> 0.036 2079.43060302734375
>> 0.038 2078.44738769531250
>> 0.040 2076.43640136718750
>> ...etc
>>
>> When I manually swap the bytes in my program, I still end up with invalid values.
>> -Carl
>>
>> 26 C3 E9 E2 6C 6A 38 0C 46 C3 E9 E2 6C 6A 40 0B 46 C3 E9 E2 83 6A DC 0A 66 C3 E9 E2 83 6A 0E 0B
>> A5 C3 EF E2 AF 6A 40 0B A5 C3 EF E2 C6 6A 40 0B C5 C3 F5 E2 C6 6A 16 0A C5 C3 F5 E2 DC 6A 0E 0B
>> 26 C3 3C E3 9E 62 B2 09 26 C3 3C E3 9E 62 B2 09 26 C3 36 E3 B4 62 BA 08 06 C3 36 E3 CB 62 C2 07
>> 45 C4 76 E3 44 6C 38 0C 25 C4 76 E3 5A 6C 06 0C 25 C4 76 E3 5A 6C DC 0A 05 C4 7D E3 70 6C DC 0A
>
>
> Byte-swapped VAX type D might be right, but it's so easy to be off by a factor of 2 here that it's hard to trust any guess.
>
> Using CFSwapInt64() from CoreFoundation/CFByteOrder.h and from_vax_d8() from https://pubs.usgs.gov/of/2005/1424/ <https://pubs.usgs.gov/of/2005/1424/>, your bytes above are:
> -41.7215935353
> -49.7215935353
> -49.7215935353
> -49.7215935366
> -49.7215935366
> -57.7215935366
> -57.7215935366
> -82.9432328547
> -82.9432328547
> -82.9432328574
> -82.9432328574
> -98.9432786338
> -98.9432786338
> -98.9432786363
> -98.9432786363
> -41.7219100388
>
Enticing results! When I use CFSwapInt64(), it does this to the bytes:
- raw bytes: 69BF5DE3 9F530306
- swp bytes: 0603539F E35DBF69
* [04] time_base: 2400969845129752384725738662412037010364031309686872696685853017835577483033453697155629104443247189829603912142488904070729903714756009814926441089319983058422454699494905107871645497808138444180815872.000000
The 'time_base' value is what I get when I printf the swapped-bytes value with a %f formatter.
Not sure why you're getting good results...
-Carl
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