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How to understand Mat_<float>

asked 2017-11-07 00:20:42 -0500

yode gravatar image

updated 2017-11-07 00:21:40 -0500

I have such code

Mat srcImage = imread("1.jpg", 0);
Mat planes[] = {Mat_<float>(srcImage), Mat::zeros(srcImage.size(), CV_32F)};
Mat complexI;
merge(planes, 2, complexI);

How to understand this Mat_<float>(srcImage)? I cannot search any useful information about it. Could help to understand it?

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answered 2017-11-07 01:45:23 -0500

berak gravatar image

updated 2017-11-07 02:11:17 -0500

it's just a clever trick (maybe too clever, if it's hard to understand ...)

i guess, the context here is some dft analysis, which requires complex, float input.

normally, you would have to do:

Mat srcImage = imread("1.jpg", 0); // read the grayscale image. it's still uchar.
Mat realPart; 
srcImage.convertTo(realPart, CV_32F); // manual conversion
Mat imPart = Mat::zeros(srcImage.size(), CV_32F); 
Mat planes [] = { realPart, imPart };
merge(planes, 2, complexI);

the Mat_<float>(img) constructor does an internal, automatic conversion to float (it's a template object, it's type is determined at compile time, so it obviously cannot hold any other data type). it's a shortcut, saving some lines of code, simply.

those template Mat_'s also have some other, interesting properties:

since the type is already handled in the constructor, you no more need at<type> to access the elements, but can use a straightforward m(2,2)

then, it has an overloaded comma operator, so you can fill it like:

Mat_<float> T(3,3);
T << .2, .2, .4,
     .1, .2, .1,
     .2, .1, .3;

(which is quite useful for quickly setting up transformation Matrices, or filter kernels)

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You guess is right totally. What is your meaning of cannot hold any other data type? Where can I find more information about this usage of Mat_?

yode gravatar imageyode ( 2017-11-07 01:49:08 -0500 )edit
berak gravatar imageberak ( 2017-11-07 01:56:57 -0500 )edit

Thanks very much. It is useful fo me..

yode gravatar imageyode ( 2017-11-07 02:06:16 -0500 )edit
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Asked: 2017-11-07 00:20:42 -0500

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Last updated: Nov 07 '17