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unpack('>f', b'\x00\x01\x00\x00')

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  unpack('>f', b'\x00\x01\x00\x00') kuaile xu <kuaile.xu@gmail.com> - 2011-11-30 14:24 -0800
    Re: unpack('>f', b'\x00\x01\x00\x00') Chris Rebert <clp2@rebertia.com> - 2011-11-30 15:02 -0800
      Re: unpack('>f', b'\x00\x01\x00\x00') kuaile xu <kuaile.xu@gmail.com> - 2011-11-30 15:25 -0800
      Re: unpack('>f', b'\x00\x01\x00\x00') Hrvoje Niksic <hniksic@xemacs.org> - 2011-12-01 11:21 +0100
        Re: unpack('>f', b'\x00\x01\x00\x00') Mark Dickinson <mdickinson@enthought.com> - 2011-12-02 08:25 -0800
    Re: unpack('>f', b'\x00\x01\x00\x00') Ian Kelly <ian.g.kelly@gmail.com> - 2011-11-30 16:49 -0700

#16456 — unpack('>f', b'\x00\x01\x00\x00')

Hi:

I am working on a python script that parses mp4 video header. Once of
the field is a 32-bit fixed-point number.

I know that the four bytes are: 00, 01, 00, 00. I have a third party
mp4 parsing program which displays this field's value is:1.0.

However, the struct.unpack gets a value of 0.0.

Python 3.2 (r32:88445, Feb 20 2011, 21:30:00) [MSC v.1500 64 bit
(AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from struct import *
>>> unpack('>f', b'\x00\x01\x00\x00')
(9.183549615799121e-41,)
>>>

In addition, there is another field which is a 16-bit fixed point
number. How do I unpack two bytes into a float?

Thank you very much,

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#16461

On Wed, Nov 30, 2011 at 2:24 PM, kuaile xu <kuaile.xu@gmail.com> wrote:
> Hi:
>
> I am working on a python script that parses mp4 video header. Once of
> the field is a 32-bit fixed-point number.
>
> I know that the four bytes are: 00, 01, 00, 00. I have a third party
> mp4 parsing program which displays this field's value is:1.0.
>
> However, the struct.unpack gets a value of 0.0.
>
> Python 3.2 (r32:88445, Feb 20 2011, 21:30:00) [MSC v.1500 64 bit
> (AMD64)] on win32
> Type "help", "copyright", "credits" or "license" for more information.
>>>> from struct import *
>>>> unpack('>f', b'\x00\x01\x00\x00')
> (9.183549615799121e-41,)

Floating-point and fixed-point are *separate* number formats with
distinct representations. You cannot expect to correctly (un)pack one
as if it was the other. Similarly, converting between the two formats
can introduce range and/or imprecision error.

C does not have a built-in fixed-point datatype, so the `struct`
module doesn't handle fixed-point numbers directly. You're going to
have to unpack it (or parts of it) as something more raw, and then do
the necessary bit manipulation yourself.

Cheers,
Chris
--
http://rebertia.com

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#16463

On Nov 30, 6:02 pm, Chris Rebert <c...@rebertia.com> wrote:
> On Wed, Nov 30, 2011 at 2:24 PM, kuaile xu <kuaile...@gmail.com> wrote:
> > Hi:
>
> > I am working on a python script that parses mp4 video header. Once of
> > the field is a 32-bit fixed-point number.
>
> > I know that the four bytes are: 00, 01, 00, 00. I have a third party
> > mp4 parsing program which displays this field's value is:1.0.
>
> > However, the struct.unpack gets a value of 0.0.
>
> > Python 3.2 (r32:88445, Feb 20 2011, 21:30:00) [MSC v.1500 64 bit
> > (AMD64)] on win32
> > Type "help", "copyright", "credits" or "license" for more information.
> >>>> from struct import *
> >>>> unpack('>f', b'\x00\x01\x00\x00')
> > (9.183549615799121e-41,)
>
> Floating-point and fixed-point are *separate* number formats with
> distinct representations. You cannot expect to correctly (un)pack one
> as if it was the other. Similarly, converting between the two formats
> can introduce range and/or imprecision error.
>
> C does not have a built-in fixed-point datatype, so the `struct`
> module doesn't handle fixed-point numbers directly. You're going to
> have to unpack it (or parts of it) as something more raw, and then do
> the necessary bit manipulation yourself.
>
> Cheers,
> Chris
> --http://rebertia.com
>
>

Thanks a lot.

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#16487

Chris Rebert <clp2@rebertia.com> writes:

> C does not have a built-in fixed-point datatype, so the `struct`
> module doesn't handle fixed-point numbers directly.

The built-in decimal module supports fixed-point arithmetic, but the
struct module doesn't know about it.  A bug report (or patch) by someone
who works with binary representations of fixed-point would be a good
start to improve it.

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#16554

On Dec 1, 10:21 am, Hrvoje Niksic <hnik...@xemacs.org> wrote:
> Chris Rebert <c...@rebertia.com> writes:
> > C does not have a built-in fixed-point datatype, so the `struct`
> > module doesn't handle fixed-point numbers directly.
>
> The built-in decimal module supports fixed-point arithmetic, but the
> struct module doesn't know about it.  A bug report (or patch) by someone
> who works with binary representations of fixed-point would be a good
> start to improve it.

Not really:  the decimal module is for floating-point, not fixed-
point, though its semantics for significant trailing zeros can help
give the appearance of fixed-point arithmetic for simple operations
(addition, subtraction) that stay within suitable bounds.  And decimal
fixed-point isn't so much use for a binary fixed-point format, anyway.

--
Mark

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#16465

On Wed, Nov 30, 2011 at 3:24 PM, kuaile xu <kuaile.xu@gmail.com> wrote:
> Hi:
>
> I am working on a python script that parses mp4 video header. Once of
> the field is a 32-bit fixed-point number.
>
> I know that the four bytes are: 00, 01, 00, 00. I have a third party
> mp4 parsing program which displays this field's value is:1.0.
>
> However, the struct.unpack gets a value of 0.0.
>
> Python 3.2 (r32:88445, Feb 20 2011, 21:30:00) [MSC v.1500 64 bit
> (AMD64)] on win32
> Type "help", "copyright", "credits" or "license" for more information.
>>>> from struct import *
>>>> unpack('>f', b'\x00\x01\x00\x00')
> (9.183549615799121e-41,)
>>>>

Like Chris said, you can't unpack it as a float.  Assuming that's Q16
fixed-point, what you can do is unpack it as an int and then multiply
by 2.0 ** -16 to get the corresponding float value.  There should be
no loss of precision converting to a float since floats store 52 bits
of precision, but be aware that you could have overflow or loss of
precision when converting in the opposite direction.

> In addition, there is another field which is a 16-bit fixed point
> number. How do I unpack two bytes into a float?

Same as above, just change the multiplier to match the scale.

Cheers,
Ian

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