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How to Multiply cv::Mat with mask

asked 2015-07-14 02:57:07 -0500

Jony01 gravatar image

updated 2020-02-24 06:15:27 -0500

LBerger gravatar image

I'd like multiply two arrays(cv::Mat) with mask for speed up application.

In add and subtraction exists

mask - optional operation mask - 8-bit single channel array, that specifies elements of the output array to be changed

Provide OpenCV similar functionality for multiplication and divide?

I can solve it for full image and then copy data to output. But this is slow down.

cv::Mat input1(size, CV_8U|CV_16S|CV_32F);
cv::Mat input2(size, CV_8U|CV_16S|CV_32F); // or cv::Scalar
cv::Mat mask(size, CV_8U);
cv::Mat output(size, CV_8U|CV_16S|CV_32F); 

// per pixel multiplication input1 and input2 
cv::multiply(input1, input2, output);
//cv::multiply(input1, input2, output, mask);

way around for same result:

cv::Mat multiplyFull;
cv::multiply(input1, input2, multiplyFull);

// clear data in output
output.setTo(cv::Scalar::all(0), mask);

// set in output only multiplication given by mask
cv::add(output, multiplyFull, output, mask);
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I don't think it's possible without copying images.

LBerger gravatar imageLBerger ( 2015-07-14 03:15:09 -0500 )edit

IMHO would be nice if you ask your question like : here is my code to do ...... is there anyway to speed up my code. then maybe i learned something from your code

sturkmen gravatar imagesturkmen ( 2015-07-14 12:14:58 -0500 )edit

Hi did you solve this problem, I have the same question about multiplying mat using mask. Thanks!

BAHRAMUDIN ADIL gravatar imageBAHRAMUDIN ADIL ( 2019-02-12 03:21:02 -0500 )edit

you must write your own code

LBerger gravatar imageLBerger ( 2019-02-12 04:25:31 -0500 )edit

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answered 2019-02-12 05:18:42 -0500

LBerger gravatar image

updated 2019-02-12 05:24:41 -0500

You can try :

    multiply(x, y, z);

or you can try this :

template<typename Type>
class ParallelMultiplyWithMask : public ParallelLoopBody
    Mat &imgSrc1;
    Mat &imgSrc2;
    Mat &mask;
    Mat &dst;

    ParallelMultiplyWithMask<Type>(Mat& img1,Mat &img2, Mat &d, Mat &m) :
    virtual void operator()(const Range& range) const CV_OVERRIDE

        for (int y = range.start; y < range.end; y++)
            uchar *m = mask.ptr<uchar>(y);
            Type *vDst = dst.ptr<Type>(y);
            Type *vSrc1 = imgSrc1.ptr<Type>(y);
            Type *vSrc2 = imgSrc2.ptr<Type>(y);
            for (int x = 0; x < imgSrc1.cols; x++, vDst++, vSrc1++, vSrc2++, m++)
                if (*m)
                    *vDst = *vSrc1 * *vSrc2;
                    *vDst = *vSrc1;


void multiplywithMask(InputArray _src1, InputArray _src2,  OutputArray _dst, InputArray _mask)
    CV_Assert(_src1.sameSize(_src2) && _src1.type() == _src2.type());
    CV_Assert(_src1.sameSize(_mask) );
    Mat mask = _mask.getMat();
    CV_Assert(mask.type() == CV_8U);
    Mat dst(_src1.size(), _src1.type());
    Mat src1 = _src1.getMat(), src2 = _src2.getMat();
    if (_src1.type() == CV_8UC1)
        ParallelMultiplyWithMask<uchar> product(src1,src2, dst, mask);
        parallel_for_(Range(0, src1.rows), product);
    else if (_src1.type() == CV_32FC1)

        ParallelMultiplyWithMask<float> product(src1, src2, dst, mask);
        parallel_for_(Range(0, src1.rows), product);


multiplywithMask(x, y, z, mask);

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Thanks for the reply, it works. Now I have I new question if you can help how to achieve? Thanks!

BAHRAMUDIN ADIL gravatar imageBAHRAMUDIN ADIL ( 2019-02-13 02:01:40 -0500 )edit

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Asked: 2015-07-14 02:57:07 -0500

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Last updated: Feb 12 '19