[ncl-talk] dim_gbits issue

[Dennis Shea]

http://www.ncl.ucar.edu/Applications/HDF.shtml

provides an application example. As noted, this is a sequence of bits not bytes.


;******************************************************************
; hdf4eos_5.ncl
;
; Concepts illustrated:
;   - Plotting EOS-DIS data
;   - Reading HDF4 data
;   - Using dim_gbits to extract bits of information from a variable
[SNIP]
;******************************************************************
;    Bit fields within each byte are numbered from the left:
;    7, 6, 5, 4, 3, 2, 1, 0.
;    The left-most bit (bit 7) is the most significant bit.
;    The right-most bit (bit 0) is the least significant bit.
;
;    bit field    Description                        Key
;    ---------    -----------                        ---
;    2, 1         Unobstructed FOV Quality Flag    00 (0) = Cloudy
;                                                  01 (1) = Uncertain
;                                                  10 (2) = Probably  Clear
;                                                  11 (3) = Clear
;******************************************************************

===

FYI: There is another function that "Unpacks one-bit chunks from the
input array."

http://www.ncl.ucar.edu/Document/Functions/Built-in/getbitsone.shtml

Good luck


On Mon, Mar 2, 2015 at 7:45 AM, Rick Brownrigg <brownrig at ucar.edu> wrote:

> Hi Kevin,
>
> I get the same result. However, your case is not exactly like the examples
> page, which depicts a sequence of bits, whereas you are feeding a sequence
> of bytes into the function.  If you look at the series of bits within those
> bytes:
>
> 0000 0000 | 0000 0001 | 0000 0000 ...
> ^                ^                ^
> x[0]            x[1]           x[2]          ...
>
> and then perform the dim_gbits decomposition on that, you do indeed get (/
> 0, 0, 2, 0 /)
>
> Hope that helps...
> Rick
>
>
> On Mon, Mar 2, 2015 at 6:00 AM, Kevin Vermeesch <
> kevin.c.vermeesch at nasa.gov> wrote:
>
>>  Hi,
>> I'm trying to use the built-in function dim_gbits (
>> https://www.ncl.ucar.edu/Document/Functions/Built-in/dim_gbits.shtml)
>> and am not getting the expected result while trying the example in the link
>> above. Looking at the worked example in the Description Section, I used the
>> following commands:
>>
>> x = tobyte((/0,1,0,1,0,1,1,1,1,1,1,0,0,1,0,0,1,1,0,1,0,0/))
>> print(dim_gbits(x,5,2,3,4))
>>
>> I get: (/0,0,2,0/)
>> when I was expecting to get: (/3,2,1,0/) as the example indicates.
>>
>> I am using version 6.1.0 (Cygwin) and 6.2.1 (32-bit CentOS). Is this a
>> bug or am I not using/understanding the function correctly?
>> thanks,
>> Kevin
>>
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