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    opencv5

    Basic Geometric Drawing

    OpenCV 5

    Basic Geometric Drawing

    Basic Geometric Drawing

    Basic Drawing {#tutorial_basic_geometric_drawing}

    @tableofcontents

    @next_tutorial{tutorial_random_generator_and_text}
    Original author Ana Huamán
    Compatibility OpenCV >= 3.0

    Goals

    In this tutorial you will learn how to:

    • Draw a line by using the OpenCV function line()
    • Draw an ellipse by using the OpenCV function ellipse()
    • Draw a rectangle by using the OpenCV function rectangle()
    • Draw a circle by using the OpenCV function circle()
    • Draw a filled polygon by using the OpenCV function fillPoly()

    OpenCV Theory

    @add_toggle_cpp For this tutorial, we will heavily use two structures: @ref cv::Point and @ref cv::Scalar : @end_toggle @add_toggle_java For this tutorial, we will heavily use two structures: @ref cv::Point and @ref cv::Scalar : @end_toggle @add_toggle_python For this tutorial, we will heavily use tuples in Python instead of @ref cv::Point and @ref cv::Scalar : @end_toggle

    Point

    It represents a 2D point, specified by its image coordinates \f$x\f$ and \f$y\f$. We can define it as: @add_toggle_cpp @code{.cpp} Point pt; pt.x = 10; pt.y = 8; @endcode or @code{.cpp} Point pt = Point(10, 8); @endcode @end_toggle @add_toggle_java @code{.java} Point pt = new Point(); pt.x = 10; pt.y = 8; @endcode or @code{.java} Point pt = new Point(10, 8); @endcode @end_toggle @add_toggle_python @code{.python} pt = (10, 0) # x = 10, y = 0 @endcode @end_toggle

    Scalar

    • Represents a 4-element vector. The type Scalar is widely used in OpenCV for passing pixel values.
    • In this tutorial, we will use it extensively to represent BGR color values (3 parameters). It is not necessary to define the last argument if it is not going to be used.
    • Let's see an example, if we are asked for a color argument and we give: @add_toggle_cpp @code{.cpp} Scalar( a, b, c ) @endcode @end_toggle @add_toggle_java @code{.java} Scalar( a, b, c ) @endcode @end_toggle @add_toggle_python @code{.python} ( a, b, c ) @endcode @end_toggle We would be defining a BGR color such as: Blue = a, Green = b and Red = c

    Code

    @add_toggle_cpp

    • This code is in your OpenCV sample folder. Otherwise you can grab it from here @include samples/cpp/tutorial_code/ImgProc/basic_drawing/Drawing_1.cpp @end_toggle

    @add_toggle_java

    • This code is in your OpenCV sample folder. Otherwise you can grab it from here @include samples/java/tutorial_code/ImgProc/BasicGeometricDrawing/BasicGeometricDrawing.java @end_toggle

    @add_toggle_python

    • This code is in your OpenCV sample folder. Otherwise you can grab it from here @include samples/python/tutorial_code/imgProc/BasicGeometricDrawing/basic_geometric_drawing.py @end_toggle

    Explanation

    Since we plan to draw two examples (an atom and a rook), we have to create two images and two windows to display them. @add_toggle_cpp @snippet cpp/tutorial_code/ImgProc/basic_drawing/Drawing_1.cpp create_images @end_toggle

    @add_toggle_java @snippet java/tutorial_code/ImgProc/BasicGeometricDrawing/BasicGeometricDrawing.java create_images @end_toggle

    @add_toggle_python @snippet python/tutorial_code/imgProc/BasicGeometricDrawing/basic_geometric_drawing.py create_images @end_toggle

    We created functions to draw different geometric shapes. For instance, to draw the atom we used MyEllipse and MyFilledCircle: @add_toggle_cpp @snippet cpp/tutorial_code/ImgProc/basic_drawing/Drawing_1.cpp draw_atom @end_toggle

    @add_toggle_java @snippet java/tutorial_code/ImgProc/BasicGeometricDrawing/BasicGeometricDrawing.java draw_atom @end_toggle

    @add_toggle_python @snippet python/tutorial_code/imgProc/BasicGeometricDrawing/basic_geometric_drawing.py draw_atom @end_toggle

    And to draw the rook we employed MyLine, rectangle and a MyPolygon: @add_toggle_cpp @snippet cpp/tutorial_code/ImgProc/basic_drawing/Drawing_1.cpp draw_rook @end_toggle

    @add_toggle_java @snippet java/tutorial_code/ImgProc/BasicGeometricDrawing/BasicGeometricDrawing.java draw_rook @end_toggle

    @add_toggle_python @snippet python/tutorial_code/imgProc/BasicGeometricDrawing/basic_geometric_drawing.py draw_rook @end_toggle

    Let's check what is inside each of these functions: @add_toggle_cpp @end_toggle

    MyLine

    @add_toggle_cpp @snippet cpp/tutorial_code/ImgProc/basic_drawing/Drawing_1.cpp my_line @end_toggle

    @add_toggle_java @snippet java/tutorial_code/ImgProc/BasicGeometricDrawing/BasicGeometricDrawing.java my_line @end_toggle

    @add_toggle_python @snippet python/tutorial_code/imgProc/BasicGeometricDrawing/basic_geometric_drawing.py my_line @end_toggle

    • As we can see, MyLine just call the function line() , which does the following:
      • Draw a line from Point start to Point end
      • The line is displayed in the image img
      • The line color is defined by ( 0, 0, 0 ) which is the RGB value correspondent to Black
      • The line thickness is set to thickness (in this case 2)
      • The line is a 8-connected one (lineType = 8)

    MyEllipse

    @add_toggle_cpp @snippet cpp/tutorial_code/ImgProc/basic_drawing/Drawing_1.cpp my_ellipse @end_toggle

    @add_toggle_java @snippet java/tutorial_code/ImgProc/BasicGeometricDrawing/BasicGeometricDrawing.java my_ellipse @end_toggle

    @add_toggle_python @snippet python/tutorial_code/imgProc/BasicGeometricDrawing/basic_geometric_drawing.py my_ellipse @end_toggle

    • From the code above, we can observe that the function ellipse() draws an ellipse such that:

      • The ellipse is displayed in the image img
      • The ellipse center is located in the point (w/2, w/2) and is enclosed in a box of size (w/4, w/16)
      • The ellipse is rotated angle degrees
      • The ellipse extends an arc between 0 and 360 degrees
      • The color of the figure will be ( 255, 0, 0 ) which means blue in BGR value.
      • The ellipse's thickness is 2.

    MyFilledCircle

    @add_toggle_cpp @snippet cpp/tutorial_code/ImgProc/basic_drawing/Drawing_1.cpp my_filled_circle @end_toggle

    @add_toggle_java @snippet java/tutorial_code/ImgProc/BasicGeometricDrawing/BasicGeometricDrawing.java my_filled_circle @end_toggle

    @add_toggle_python @snippet python/tutorial_code/imgProc/BasicGeometricDrawing/basic_geometric_drawing.py my_filled_circle @end_toggle

    • Similar to the ellipse function, we can observe that circle receives as arguments:

      • The image where the circle will be displayed (img)
      • The center of the circle denoted as the point center
      • The radius of the circle: w/32
      • The color of the circle: ( 0, 0, 255 ) which means Red in BGR
      • Since thickness = -1, the circle will be drawn filled.

    MyPolygon

    @add_toggle_cpp @snippet cpp/tutorial_code/ImgProc/basic_drawing/Drawing_1.cpp my_polygon @end_toggle

    @add_toggle_java @snippet java/tutorial_code/ImgProc/BasicGeometricDrawing/BasicGeometricDrawing.java my_polygon @end_toggle

    @add_toggle_python @snippet python/tutorial_code/imgProc/BasicGeometricDrawing/basic_geometric_drawing.py my_polygon @end_toggle

    • To draw a filled polygon we use the function fillPoly() . We note that:

      • The polygon will be drawn on img
      • The vertices of the polygon are the set of points in ppt
      • The color of the polygon is defined by ( 255, 255, 255 ), which is the BGR value for white

    rectangle

    @add_toggle_cpp @snippet cpp/tutorial_code/ImgProc/basic_drawing/Drawing_1.cpp rectangle @end_toggle

    @add_toggle_java @snippet java/tutorial_code/ImgProc/BasicGeometricDrawing/BasicGeometricDrawing.java rectangle @end_toggle

    @add_toggle_python @snippet python/tutorial_code/imgProc/BasicGeometricDrawing/basic_geometric_drawing.py rectangle @end_toggle

    • Finally we have the @ref cv::rectangle function (we did not create a special function for this guy). We note that:

      • The rectangle will be drawn on rook_image
      • Two opposite vertices of the rectangle are defined by ( 0, 7*w/8 ) and ( w, w )
      • The color of the rectangle is given by ( 0, 255, 255 ) which is the BGR value for yellow
      • Since the thickness value is given by FILLED (-1), the rectangle will be filled.

    Result

    Compiling and running your program should give you a result like this: