Concatenate Mat in OpenCV

Question

I have a couple of images in Mat objects all with same dimensions I'd like to create one bix cv::Mat object to hold them all

So the dimension of the new matrix is: widthNew = widthOld x number of matrices, height remains unchanged.

I found that such a copy could be done using:

void cvCopy(const CvArr* src, CvArr* dst, const CvArr* mask=NULL)

but then, how could the mask be defined three different times for the three matrices?.

Regards, Moataz

Be careful that the max size for an IplImage is somewhere around 4000*4000. You might also have to reduce resolution of your input image — jlengrand, Jun 13 '12 at 18:16

score 14 · Answer 1 · answered Dec 16 '15 at 15:58

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I think there is an easy way to do this. OpenCV has a not documented methods called hconcat() and vconcat(). The first one is for horizontal concatenation and the second one for vertical concatenation.

You can use them in this way:

Mat A, B;
... //In this part you initialize the Mat A and Mat B.

Mat H, V; //These are the destination matrices
hconcat(A, B, H);
vconcat(A, B, V);

I hope this can help.

answered Dec 16 '15 at 15:58

Angie Quijano

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The destination can be the same as one of the sources so `vconcat(out, out1, out); vconcat(out, out2, out);` worked well for me in a multithreaded context (for combining output from individual threads). – Izaan Aug 13 '17 at 14:53

score 6 · Accepted Answer · edited May 23 '17 at 12:00

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You use an roi to define an image which is actually a region of the destination image and then copy to that. see Copy an cv::Mat inside a ROI of another one

edited May 23 '17 at 12:00

Community

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answered Jun 13 '12 at 16:42

Martin Beckett

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score 1 · Answer 3 · answered Jun 13 '12 at 21:14

You probably want to take look at the source code of cvShowManyImages(), which is a function that takes several images as input and concatenates them into a single 3-channel image to be displayed:

The method used is to set the ROIs of a Single Big image and then resizing and copying the input images on to the Single Big Image.

If you create a destination image big enough to hold your other images, you won't need to resize them.

If you want to know how to convert between IplImage <-> cv::Mat, check this thread.

sounds interesting, thnx. I think I'll copy the idea to implement something similar using pure cv::Mat instead of IpImage — Moataz Elmasry, Jun 14 '12 at 10:41

Concatenate Mat in OpenCV

3 Answers3