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Picture Mirroring

Image mirroring is divided into two types: horizontal mirroring and vertical mirroring. Horizontal mirroring uses the vertical center line of the image as the axis to swap the pixels of the image, i.e., swapping the left and right halves. Vertical mirroring uses the horizontal center line of the image as the axis to swap the upper and lower halves.

Transformation principle: Let the width of the image be width and the height be height. (x,y) are the transformed coordinates, (x0,y0) are the original image coordinates.

Horizontal mirror transformation

Forward mapping

Its inverse transformation is:

Backward mapping

Vertical mirror transformation

Its inverse transformation is:

Summary:

In horizontal mirroring, the entire image is traversed and each pixel is processed according to the mapping relationship. In fact, horizontal mirroring swaps the columns of the image coordinates from left to right, and from right to left. It can be done at the column unit level. Similarly, vertical mirroring can be done at the row unit level. Here we take vertical transformation as an example to see how Python implements it:

Code path:

~/dofbot_opencv/scripts/2.Transform/04_mirror_image.ipynb

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import cv2
import numpy as np
img = cv2.imread('yahboom.jpg',1)
#cv2.imshow('src',img)
imgInfo = img.shape
height = imgInfo[0]
width = imgInfo[1]
deep = imgInfo[2]
newImgInfo = (height*2,width,deep)
dst = np.zeros(newImgInfo,np.uint8)#uint8
for i in range(0,height):
    for j in range(0,width):
        dst[i,j] = img[i,j]
        #x y = 2*h - y -1
        dst[height*2-i-1,j] = img[i,j]
for i in range(0,width):
    dst[height,i] = (0,0,255) #BGR

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#bgr8 to jpeg format
import enum
import cv2
def bgr8_to_jpeg(value, quality=75):
    return bytes(cv2.imencode('.jpg', value)[1])

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import ipywidgets.widgets as widgets
image_widget1 = widgets.Image(format='jpg', )
# image_widget2 = widgets.Image(format='jpg', )
# create a horizontal box container to place the image widget next to eachother
# image_container = widgets.HBox([image_widget1, image_widget2])
# display the container in this cell's output
display(image_widget1)
#display(image_widget2)
image_widget1.value = bgr8_to_jpeg(dst)

The mirror image can be seen from the figure.