Adding borders to your images {#tutorial_copyMakeBorder}

@tableofcontents

@prev_tutorial{tutorial_filter_2d} @next_tutorial{tutorial_sobel_derivatives}

Goal

In this tutorial you will learn how to:

Use the OpenCV function copyMakeBorder() to set the borders (extra padding to your image).

Theory

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

-# In our previous tutorial we learned to use convolution to operate on images. One problem that

naturally arises is how to handle the boundaries. How can we convolve them if the evaluated
points are at the edge of the image?

-# What most of OpenCV functions do is to copy a given image onto another slightly larger image and

then automatically pads the boundary (by any of the methods explained in the sample code just
below). This way, the convolution can be performed over the needed pixels without problems (the
extra padding is cut after the operation is done).

-# In this tutorial, we will briefly explore two ways of defining the extra padding (border) for an

image:

-#  **BORDER_CONSTANT**: Pad the image with a constant value (i.e. black or \f$0\f$
-#  **BORDER_REPLICATE**: The row or column at the very edge of the original is replicated to
    the extra border.

This will be seen more clearly in the Code section.

What does this program do?
- Load an image
- Let the user choose what kind of padding use in the input image. There are two options:
  
  -# Constant value border: Applies a padding of a constant value for the whole border.
```
This value will be updated randomly each 0.5 seconds.
```
  -# Replicated border: The border will be replicated from the pixel values at the edges of
```
the original image.
```
  The user chooses either option by pressing 'c' (constant) or 'r' (replicate)
- The program finishes when the user presses 'ESC'

Code

The tutorial code's is shown lines below.

@add_toggle_cpp You can also download it from here @include samples/cpp/tutorial_code/ImgTrans/copyMakeBorder_demo.cpp @end_toggle

@add_toggle_java You can also download it from here @include samples/java/tutorial_code/ImgTrans/MakeBorder/CopyMakeBorder.java @end_toggle

@add_toggle_python You can also download it from here @include samples/python/tutorial_code/ImgTrans/MakeBorder/copy_make_border.py @end_toggle

Explanation

Declare the variables

First we declare the variables we are going to use:

@add_toggle_cpp @snippet cpp/tutorial_code/ImgTrans/copyMakeBorder_demo.cpp variables @end_toggle

@add_toggle_java @snippet java/tutorial_code/ImgTrans/MakeBorder/CopyMakeBorder.java variables @end_toggle

@add_toggle_python @snippet python/tutorial_code/ImgTrans/MakeBorder/copy_make_border.py variables @end_toggle

Especial attention deserves the variable rng which is a random number generator. We use it to generate the random border color, as we will see soon.

Load an image

As usual we load our source image src:

@add_toggle_cpp @snippet cpp/tutorial_code/ImgTrans/copyMakeBorder_demo.cpp load @end_toggle

@add_toggle_java @snippet java/tutorial_code/ImgTrans/MakeBorder/CopyMakeBorder.java load @end_toggle

@add_toggle_python @snippet python/tutorial_code/ImgTrans/MakeBorder/copy_make_border.py load @end_toggle

Create a window

After giving a short intro of how to use the program, we create a window:

@add_toggle_cpp @snippet cpp/tutorial_code/ImgTrans/copyMakeBorder_demo.cpp create_window @end_toggle

@add_toggle_java @snippet java/tutorial_code/ImgTrans/MakeBorder/CopyMakeBorder.java create_window @end_toggle

@add_toggle_python @snippet python/tutorial_code/ImgTrans/MakeBorder/copy_make_border.py create_window @end_toggle

Initialize arguments

Now we initialize the argument that defines the size of the borders (top, bottom, left and right). We give them a value of 5% the size of src.

@add_toggle_cpp @snippet cpp/tutorial_code/ImgTrans/copyMakeBorder_demo.cpp init_arguments @end_toggle

@add_toggle_java @snippet java/tutorial_code/ImgTrans/MakeBorder/CopyMakeBorder.java init_arguments @end_toggle

@add_toggle_python @snippet python/tutorial_code/ImgTrans/MakeBorder/copy_make_border.py init_arguments @end_toggle

Loop

The program runs in an infinite loop while the key ESC isn't pressed. If the user presses 'c' or 'r', the borderType variable takes the value of BORDER_CONSTANT or BORDER_REPLICATE respectively:

@add_toggle_cpp @snippet cpp/tutorial_code/ImgTrans/copyMakeBorder_demo.cpp check_keypress @end_toggle

@add_toggle_java @snippet java/tutorial_code/ImgTrans/MakeBorder/CopyMakeBorder.java check_keypress @end_toggle

@add_toggle_python @snippet python/tutorial_code/ImgTrans/MakeBorder/copy_make_border.py check_keypress @end_toggle

Random color

In each iteration (after 0.5 seconds), the random border color (value) is updated...

@add_toggle_cpp @snippet cpp/tutorial_code/ImgTrans/copyMakeBorder_demo.cpp update_value @end_toggle

@add_toggle_java @snippet java/tutorial_code/ImgTrans/MakeBorder/CopyMakeBorder.java update_value @end_toggle

@add_toggle_python @snippet python/tutorial_code/ImgTrans/MakeBorder/copy_make_border.py update_value @end_toggle

This value is a set of three numbers picked randomly in the range \f$[0,255]\f$.

Form a border around the image

Finally, we call the function copyMakeBorder() to apply the respective padding:

@add_toggle_cpp @snippet cpp/tutorial_code/ImgTrans/copyMakeBorder_demo.cpp copymakeborder @end_toggle

@add_toggle_java @snippet java/tutorial_code/ImgTrans/MakeBorder/CopyMakeBorder.java copymakeborder @end_toggle

@add_toggle_python @snippet python/tutorial_code/ImgTrans/MakeBorder/copy_make_border.py copymakeborder @end_toggle

The arguments are:

-# src: Source image -# dst: Destination image -# top, bottom, left, right: Length in pixels of the borders at each side of the image.
```
We define them as being 5% of the original size of the image.
```
-# borderType: Define what type of border is applied. It can be constant or replicate for
```
this example.
```
-# value: If borderType is BORDER_CONSTANT, this is the value used to fill the border
```
pixels.
```

Display the results

We display our output image in the image created previously

@add_toggle_cpp @snippet cpp/tutorial_code/ImgTrans/copyMakeBorder_demo.cpp display @end_toggle

@add_toggle_java @snippet java/tutorial_code/ImgTrans/MakeBorder/CopyMakeBorder.java display @end_toggle

@add_toggle_python @snippet python/tutorial_code/ImgTrans/MakeBorder/copy_make_border.py display @end_toggle

Results

-# After compiling the code above, you can execute it giving as argument the path of an image. The

result should be:

-   By default, it begins with the border set to BORDER_CONSTANT. Hence, a succession of random
    colored borders will be shown.
-   If you press 'r', the border will become a replica of the edge pixels.
-   If you press 'c', the random colored borders will appear again
-   If you press 'ESC' the program will exit.

Below some screenshot showing how the border changes color and how the *BORDER_REPLICATE*
option looks:

![](images/CopyMakeBorder_Tutorial_Results.jpg)

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