⚠️這個系列是自己瞎翻的，文法很醜，主要靠意會，跳着跳着撿重要的部分翻，翻錯了不負責，就這樣哈。
⚠️基於3.4.3，Image Gradients，附原文。
目標
在這一章，我們將學習：
- 找到圖像梯度、邊緣等等
- 我們會遇到以下函數：cv.Sobel()，cv.Scharr()，cv.Laplacian() 等等
理論
OpenCV提供三種梯度濾波器或高通濾波器，Sobel，Scharr和Laplacian。我們將會一個個來看。
1、索貝爾和沙爾（Sobel Scharr）派生物
Sobel operators is a joint Gausssian smoothing plus differentiation operation, so it is more resistant to noise. You can specify the direction of derivatives to be taken, vertical or horizontal (by the arguments, yorder and xorder respectively). You can also specify the size of kernel by the argument ksize. If ksize = -1, a 3x3 Scharr filter is used which gives better results than 3x3 Sobel filter. Please see the docs for kernels used.
2、拉普拉斯（Laplacian）派生物
It calculates the Laplacian of the image given by the relation, Δsrc=∂2src∂x2+∂2src∂y2 where each derivative is found using Sobel derivatives. If ksize = 1, then following kernel is used for filtering:
kernel=⎡⎣⎢⎢0101−41010⎤⎦⎥⎥
Code
Below code shows all operators in a single diagram. All kernels are of 5x5 size. Depth of output image is passed -1 to get the result in np.uint8 type.
import numpy as np
import cv2 as cv
from matplotlib import pyplot as plt
img = cv.imread('dave.jpg',0)
laplacian = cv.Laplacian(img,cv.CV_64F)
sobelx = cv.Sobel(img,cv.CV_64F,1,0,ksize=5)
sobely = cv.Sobel(img,cv.CV_64F,0,1,ksize=5)
plt.subplot(2,2,1),plt.imshow(img,cmap = 'gray')
plt.title('Original'), plt.xticks([]), plt.yticks([])
plt.subplot(2,2,2),plt.imshow(laplacian,cmap = 'gray')
plt.title('Laplacian'), plt.xticks([]), plt.yticks([])
plt.subplot(2,2,3),plt.imshow(sobelx,cmap = 'gray')
plt.title('Sobel X'), plt.xticks([]), plt.yticks([])
plt.subplot(2,2,4),plt.imshow(sobely,cmap = 'gray')
plt.title('Sobel Y'), plt.xticks([]), plt.yticks([])
plt.show()
Result:
image
One Important Matter!
In our last example, output datatype is cv.CV_8U or np.uint8. But there is a slight problem with that. Black-to-White transition is taken as Positive slope (it has a positive value) while White-to-Black transition is taken as a Negative slope (It has negative value). So when you convert data to np.uint8, all negative slopes are made zero. In simple words, you miss that edge.
If you want to detect both edges, better option is to keep the output datatype to some higher forms, like cv.CV_16S, cv.CV_64F etc, take its absolute value and then convert back to cv.CV_8U. Below code demonstrates this procedure for a horizontal Sobel filter and difference in results.
import numpy as np
import cv2 as cv
from matplotlib import pyplot as plt
img = cv.imread('box.png',0)
# Output dtype = cv.CV_8U
sobelx8u = cv.Sobel(img,cv.CV_8U,1,0,ksize=5)
# Output dtype = cv.CV_64F. Then take its absolute and convert to cv.CV_8U
sobelx64f = cv.Sobel(img,cv.CV_64F,1,0,ksize=5)
abs_sobel64f = np.absolute(sobelx64f)
sobel_8u = np.uint8(abs_sobel64f)
plt.subplot(1,3,1),plt.imshow(img,cmap = 'gray')
plt.title('Original'), plt.xticks([]), plt.yticks([])
plt.subplot(1,3,2),plt.imshow(sobelx8u,cmap = 'gray')
plt.title('Sobel CV_8U'), plt.xticks([]), plt.yticks([])
plt.subplot(1,3,3),plt.imshow(sobel_8u,cmap = 'gray')
plt.title('Sobel abs(CV_64F)'), plt.xticks([]), plt.yticks([])
plt.show()
查看以下結果：
Additional Resources
Exercises
下篇：【翻譯：OpenCV-Python教程】