Construct OpenCV element type in runtime

Question

In OpenCV, when I need to create a cv::Mat, I will need to do something along the line of

cv::Mat new_mat(width, height, CV_32FC3)

What happens if I only know that I need the elements to be either float or double + whether I need 1/2/3 channels in runtime?

In other words, given the element type (float) and number of channel (int), how can I construct the term: CV_32FC3?

Reinderien · Accepted Answer · 2010-06-28 02:18:09Z

6

Read the source for cxtypes.h. It contains lines like the following:

#define CV_32FC1 CV_MAKETYPE(CV_32F,1)
#define CV_32FC2 CV_MAKETYPE(CV_32F,2)
#define CV_32FC3 CV_MAKETYPE(CV_32F,3)
#define CV_32FC4 CV_MAKETYPE(CV_32F,4)
#define CV_32FC(n) CV_MAKETYPE(CV_32F,(n))

#define CV_64FC1 CV_MAKETYPE(CV_64F,1)
#define CV_64FC2 CV_MAKETYPE(CV_64F,2)
#define CV_64FC3 CV_MAKETYPE(CV_64F,3)
#define CV_64FC4 CV_MAKETYPE(CV_64F,4)
#define CV_64FC(n) CV_MAKETYPE(CV_64F,(n))

CV_MAKETYPE is defined as:

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

This suggests the following code:

bool isdouble;
int nchannels;
// ...
if (isdouble) typeflag = CV_64FC(nchannels);
else typeflag = CV_32FC(nchannels);

I haven't tested this; let me know if it works. Also: I hate opencv's terrible type safety.

answered Jun 28, 2010 at 2:18

Reinderien

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2 Comments

Dat Chu Over a year ago

I use CV_MAKETYPE directly instead of the short-hand. Your solution works.

Renan Over a year ago

@Dat Chu: How do you find out the depth having only the typename of each element? (I'm trying to compute the type in a template method).

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