OpenCV  4.1.1-dev
Open Source Computer Vision
Eroding and Dilating

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Goal

In this tutorial you will learn how to:

Note
The explanation below belongs to the book Learning OpenCV by Bradski and Kaehler.

Morphological Operations

Dilation

Erosion

Code

Explanation

  1. Most of the material shown here is trivial (if you have any doubt, please refer to the tutorials in previous sections). Let's check the general structure of the C++ program:

    • Load an image (can be BGR or grayscale)
    • Create two windows (one for dilation output, the other for erosion)
    • Create a set of two Trackbars for each operation:
      • The first trackbar "Element" returns either erosion_elem or dilation_elem
      • The second trackbar "Kernel size" return erosion_size or dilation_size for the corresponding operation.
    • Every time we move any slider, the user's function Erosion or Dilation will be called and it will update the output image based on the current trackbar values.

    Let's analyze these two functions:

  2. erosion:
    void Erosion( int, void* )
    {
    int erosion_type = 0;
    if( erosion_elem == 0 ){ erosion_type = MORPH_RECT; }
    else if( erosion_elem == 1 ){ erosion_type = MORPH_CROSS; }
    else if( erosion_elem == 2) { erosion_type = MORPH_ELLIPSE; }
    Mat element = getStructuringElement( erosion_type,
    Size( 2*erosion_size + 1, 2*erosion_size+1 ),
    Point( erosion_size, erosion_size ) );
    erode( src, erosion_dst, element );
    imshow( "Erosion Demo", erosion_dst );
    }
    • The function that performs the erosion operation is cv::erode . As we can see, it receives three arguments:
      • src: The source image
      • erosion_dst: The output image
      • element: This is the kernel we will use to perform the operation. If we do not specify, the default is a simple 3x3 matrix. Otherwise, we can specify its shape. For this, we need to use the function cv::getStructuringElement :

        Mat element = getStructuringElement( erosion_type,
        Size( 2*erosion_size + 1, 2*erosion_size+1 ),
        Point( erosion_size, erosion_size ) );

        We can choose any of three shapes for our kernel:

        • Rectangular box: MORPH_RECT
        • Cross: MORPH_CROSS
        • Ellipse: MORPH_ELLIPSE

        Then, we just have to specify the size of our kernel and the anchor point. If not specified, it is assumed to be in the center.

    • That is all. We are ready to perform the erosion of our image.
      Note
      Additionally, there is another parameter that allows you to perform multiple erosions (iterations) at once. However, We haven't used it in this simple tutorial. You can check out the reference for more details.
  3. dilation:

    The code is below. As you can see, it is completely similar to the snippet of code for erosion. Here we also have the option of defining our kernel, its anchor point and the size of the operator to be used.

    void Dilation( int, void* )
    {
    int dilation_type = 0;
    if( dilation_elem == 0 ){ dilation_type = MORPH_RECT; }
    else if( dilation_elem == 1 ){ dilation_type = MORPH_CROSS; }
    else if( dilation_elem == 2) { dilation_type = MORPH_ELLIPSE; }
    Mat element = getStructuringElement( dilation_type,
    Size( 2*dilation_size + 1, 2*dilation_size+1 ),
    Point( dilation_size, dilation_size ) );
    dilate( src, dilation_dst, element );
    imshow( "Dilation Demo", dilation_dst );
    }

    Results

Compile the code above and execute it with an image as argument. For instance, using this image:

Morphology_1_Tutorial_Original_Image.jpg

We get the results below. Varying the indices in the Trackbars give different output images, naturally. Try them out! You can even try to add a third Trackbar to control the number of iterations.

Morphology_1_Result.jpg