OpenCV and Numpy interacting badly
numpy, opencv, python
Solution
This isn't an answer, but it's too big for a comment. I played with the values a bit to find the limits.
Without loading `numpy` and `cv`:
>>> unpack("f", pack("i", 8388608))
(1.1754943508222875e-38,)
>>> unpack("f", pack("i", 8388607))
(1.1754942106924411e-38,)
After loading `numpy` and `cv`, the first line is the same, but the second:
>>> unpack("f", pack("i", 8388607))
(0.0,)
You'll notice that the first result is the lower limit for 32 bit floats. I then tried the same with `d`.
Without loading the libraries:
>>> unpack("d", pack("xi", 1048576))
(2.2250738585072014e-308,)
>>> unpack("d", pack("xi", 1048575))
(2.2250717365114104e-308,)
And after loading the libraries:
>>> unpack("d",pack("xi", 1048575))
(0.0,)
Now the first result is the lower limit for 64 bit float precision.
It seems that for some reason, loading the `numpy` and `cv` libraries, in that order, constrains `unpack` to use 32 and 64 bit precision and return 0 for lower values.
Problem
Can anyone explain why importing cv and numpy would change the behaviour of python's struct.unpack? Here's what I observe: ``` Python 2.7.3 (default, Aug 1 2012, 05:14:39) [GCC 4.6.3] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> from struct import pack, unpack >>> unpack("f",pack("I",31))[0] 4.344025239406933e-44 ``` This is correct ``` >>> import cv libdc1394 error: Failed to initialize libdc1394 >>> unpack("f",pack("I",31))[0] 4.344025239406933e-44 ``` Still ok, after importing cv ``` >>> import numpy >>> unpack("f",pack("I",31))[0] 4.344025239406933e-44 ``` And OK after importing cv and then numpy Now I restart python: ``` Python 2.7.3 (default, Aug 1 2012, 05:14:39) [GCC 4.6.3] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> from struct import pack, unpack >>> unpack("f",pack("I",31))[0] 4.344025239406933e-44 >>> import numpy >>> unpack("f",pack("I",31))[0] 4.344025239406933e-44 ``` So far so good, but now I import cv AFTER importing numpy: ``` >>> import cv libdc1394 error: Failed to initialize libdc1394 >>> unpack("f",pack("I",31))[0] 0.0 ``` I've repeated this a number of times, including on multiple servers, and it always goes the same way. I've also tried it with struct.unpack and struct.pack, which also makes no difference. I can't understand how importing numpy and cv could have any impact at all on the output of struct.unpack (pack remains the same, btw). The "libdc1394" thing is, I believe, a red-herring: ctypes error: libdc1394 error: Failed to initialize libdc1394 Any ideas? tl;dr: importing numpy and then opencv changes the behaviour of struct.unpack. UPDATE: Paulo's answer below shows that this is reproducible. Seborg's comment suggests that it's something to do with the way python handles subnormals, which sounds plausible. I looked into Contexts but that didn't seem to be the problem, as the context was the same after the imports as it had been before them.