How to generate a cv::Mat type code?

Question

I've been using the c-style api to generate opencv type codes. For example:

cv::Mat(h, w, CV_8UC2);

CV_8UC2 is a macro defined in types_c.h (deprecated?):

#define CV_MAKETYPE(depth,cn) (CV_MAT_DEPTH(depth) + (((cn)-1) << CV_CN_SHIFT))

Is there a similar type code generation function in the c++ api, something like

Mat m(w,h, cv::Type(Vec<unsigned char, 2>).typecode()) ?

When you say "the C-style API"... are you referring to the macros? Aren't macros part of C++? :) — Яois
– Яois, Commented Jun 19, 2015 at 13:44
Also: in OpenCV 3.0, CV_MAKETYPE is defined at core/cvdefs.h, which is clearly not deprecated. — Яois
– Яois, Commented Jun 19, 2015 at 13:52
What I meant is that CV_MAKETYPE is still part of OpenCV, it's not like IplImage or any of the deprecated cvWhatever functions... — Яois
– Яois, Commented Jun 19, 2015 at 14:09

Community · Accepted Answer · 2020-06-20 09:12:55Z

As I said in my comments, CV_MAKETYPE is not deprecated, and afaik it is the standard way of generating those "type codes".

However (and just for fun), an alternative, more C++-ish, way of generating arbitrary codes (still in compile time) can be achieved by using TMP...

template <int depth, int cn> struct make_type { enum { // (yes, it is exactly the same expression used by CV_MAKETYPE) value = ((depth) & CV_MAT_DEPTH_MASK) + (((cn)-1) << CV_CN_SHIFT) }; }; // You can check that it works exactly the same as good, old `CV_MAKETYPE` cout << make_type<CV_8U,2>::value << " "<< CV_MAKETYPE(CV_8U,2) << endl;

... but don't do this. While tmp is fun and amazing, CV_MAKETYPE is the right way of doing things in this case.

EDIT: OpenCV has its own type traits utilities. In core/traits.hpp we can find class DataType:

The DataType class is basically used to provide a description of ... primitive data types without adding any fields or methods to the corresponding classes (and it is actually impossible to add anything to primitive C/C++ data types). This technique is known in C++ as class traits. It is not DataType itself that is used but its specialized versions ... The main purpose of this class is to convert compilation-time type information to an OpenCV-compatible data type identifier ...

So, such traits are used to tell OpenCV which data type you are working with, even if such a type is not native to OpenCV.

It's good to know this is still the standard. What about cv::DataType in core.hpp though? I see a cursory CV_MAKETYPE(depth, channels) in it (opencv rc1). How can you generate a type with it? I think whether using one or the other is better, is a different (and normative) question.
You're absolutely right. Seems that a trait- based version of basic type stuff already exists in OpenCV. I'll update my answer. Nevertheless, I think the standard way of doing what you wanted to do is still CV_MAKETYPE

Collectives™ on Stack Overflow

How to generate a cv::Mat type code?

1 Answer 1

2 Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Related