On Wed, Nov 30, 2011 at 3:24 PM, kuaile xu <[email protected]> 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 -- http://mail.python.org/mailman/listinfo/python-list
