Show multiple (2,3,4,…) images in the same window in OpenCV

Question

I want to show 2, 3 or more images in the same window.

My problem is how to put the second, third image just on the right side (above, left or upper) the main image.

Answer 1

Try this code (see my comments):

Mat img = imread("lena.JPG");

CV::Mat chann[3], all; //  creating 

split(img, chann); // split an image into their color channel and n keep them inside

a 3 element array called chann

imshow("ppl", img);

hconcat(chann, 3, all); //  joining the images together in a horizontal manner, the 

array, number of array, and the destination

imshow("B :: G :: R",all);     // this just the little help i could provide 

Answer 2

The GUI included with OpenCV is pretty limited, if you need to do anything complicated, you really should use a GUi framework such as QT or VC++ on windows

Answer 3

I implemented this very recently. So thought of sharing it. It uses C++ API. The code is self-explanatory (hopefully).

    /**
     * @brief makeCanvas Makes composite image from the given images
     * @param vecMat Vector of Images.
     * @param windowHeight The height of the new composite image to be formed.
     * @param nRows Number of rows of images. (Number of columns will be calculated
     *              depending on the value of total number of images).
     * @return new composite image.
     */
    cv::Mat makeCanvas(std::vector<cv::Mat>& vecMat, int windowHeight, int nRows) {
            int N = vecMat.size();
            nRows  = nRows > N ? N : nRows; 
            int edgeThickness = 10;
            int imagesPerRow = ceil(double(N) / nRows);
            int resizeHeight = floor(2.0 * ((floor(double(windowHeight - edgeThickness) / nRows)) / 2.0)) - edgeThickness;
            int maxRowLength = 0;

            std::vector<int> resizeWidth;
            for (int i = 0; i < N;) {
                    int thisRowLen = 0;
                    for (int k = 0; k < imagesPerRow; k++) {
                            double aspectRatio = double(vecMat[i].cols) / vecMat[i].rows;
                            int temp = int( ceil(resizeHeight * aspectRatio));
                            resizeWidth.push_back(temp);
                            thisRowLen += temp;
                            if (++i == N) break;
                    }
                    if ((thisRowLen + edgeThickness * (imagesPerRow + 1)) > maxRowLength) {
                            maxRowLength = thisRowLen + edgeThickness * (imagesPerRow + 1);
                    }
            }
            int windowWidth = maxRowLength;
            cv::Mat canvasImage(windowHeight, windowWidth, CV_8UC3, Scalar(0, 0, 0));

            for (int k = 0, i = 0; i < nRows; i++) {
                    int y = i * resizeHeight + (i + 1) * edgeThickness;
                    int x_end = edgeThickness;
                    for (int j = 0; j < imagesPerRow && k < N; k++, j++) {
                            int x = x_end;
                            cv::Rect roi(x, y, resizeWidth[k], resizeHeight);
                            cv::Size s = canvasImage(roi).size();
                            // change the number of channels to three
                            cv::Mat target_ROI(s, CV_8UC3);
                            if (vecMat[k].channels() != canvasImage.channels()) {
                                if (vecMat[k].channels() == 1) {
                                    cv::cvtColor(vecMat[k], target_ROI, CV_GRAY2BGR);
                                }
                            } else {             
                                vecMat[k].copyTo(target_ROI);
                            }
                            cv::resize(target_ROI, target_ROI, s);
                            if (target_ROI.type() != canvasImage.type()) {
                                target_ROI.convertTo(target_ROI, canvasImage.type());
                            }
                            target_ROI.copyTo(canvasImage(roi));
                            x_end += resizeWidth[k] + edgeThickness;
                    }
            }
            return canvasImage;
    }

Here is sample output.

Answer 4

The answer depends on which interface you are using (C or C++). General workflow is

• Create an image (cv::Mat for C++, IplImage* for C) big enough to accomodate your composed image
• Copy your images into the big image
  • C++: Use the Mat::Mat(const Mat& m, const Range& rowRange, const Range& colRange) constructor to get a cv::Mat pointing to a subimage of your original window, then use the copyTo method to copy your small image into the big one
  • C: set a ROI in the big image and copy your small image into it
• Display your big image

Answer 5

Or just use:

Mat a, Mat b, Mat dst // a,b loaded

cv::hconcat(a, b, dst) // horizontal
cv::vconcat(a, b, dst) // vertical

Mat dst -> | a | b |

or do it with vector:

std::vector<cv::Mat> matrices = {
            a, b
    };
hconcat(matrices, dst);

Answer 6

You can find the answer on the OpenCV Wiki:

https://github.com/opencv/opencv/wiki/DisplayManyImages

:-)