CV_8U opencv's matrixes on no 8 bit systems

Question

I've read that the signed char and unsigned char types are not guaranteed to be 8 bits on every platform, but sometimes they have more than 8 bits. If so, using OpenCv how can we be sure that CV_8U is always 8bit? I've written a short function which takes a 8 bit Mat and happens to convert, if needed, CV_8SC1 Mat elements into uchars and CV_8UC1 into schar. Now I'm afraid it is not platform independent an I should fix the code in some way (but don't know how).

P.S.: Similarly, how can CV_32S always be int, also on machine with no 32bit ints?

I don't think you need to worry about this - just let OpenCV take care of the underlying implementation. Platforms with CHAR_BIT > 8 are somewhat unusual (typically DSPs). sizeof(int) > 4 is somewhat more common, but I'm pretty sure OpenCV will handle this. — Paul R
Actually OpenCV is promised to work only on ia32, amd64 and arm (since 2.3.0) platforms. All these platforms have 8 bit char and 32bit int. — Andrey Kamaev
@AndreyKamaev. And as to schar to uchar conversion and viceversa, do you think that can I rely on the code I've written? Are all those platform complement 2? — Giuseppe Dini

moooeeeep moooeeeep · Accepted Answer · 2011-11-19T14:22:41

Can you give a reference of this (I've never heard of that)? Probably you mean the padding that may be added at the end of a row in a cv::Mat. That is of no problem, since the padding is usually not used, and especially no problem if you use the interface functions, e.g. the iterators (c.f.). If you would post some code, we could see, if your implementation actually had such problems.

// template methods for iteration over matrix elements.
// the iterators take care of skipping gaps in the end of rows (if any)
template<typename _Tp> MatIterator_<_Tp> begin();
template<typename _Tp> MatIterator_<_Tp> end();

the CV_32S will be always 32-bit integer because they use types like those defined in inttypes.h (e.g. int32_t, uint32_t) and not the platform specific int, long, whatever.

CV_8U opencv's matrixes on no 8 bit systems

1 Answers