OpenCV 提供了许多绘图函数来绘制几何形状和在图像上写入文本。让我们看看一些绘图函数并使用 OpenCV 在图像上绘制几何形状。
一些绘图功能是：
cv2.line() : Used to draw line on an image.
cv2.rectangle() : Used to draw rectangle on an image.
cv2.circle() : Used to draw circle on an image.
cv2.putText() : Used to write text on image.
为了演示上述函数的使用，我们需要一个大小为 400 X 400 的图像，填充纯色（在本例中为黑色）。为了做到这一点，我们可以利用numpy.zeroes函数来创建所需的图像。
# Python3 program to draw solid-colored
# image using numpy.zeroes() function
import numpy as np
import cv2
# Creating a black image with 3 channels
# RGB and unsigned int datatype
img = np.zeros((400, 400, 3), dtype = "uint8")
cv2.imshow('dark', img)
# Allows us to see image
# untill closed forcefully
cv2.waitKey(0)
cv2.destroyAllWindows()
输出 ： 
现在，让我们在这张纯黑色图像上绘制一些几何形状。
画一条线 ：
cv2.line(imageObjectName, (‘start_coordinates’), (‘end_coordinates’), (‘color_in_bgr’), ‘line_thickness’)
# Python3 program to draw line
# shape on solid image
import numpy as np
import cv2
# Creating a black image with 3 channels
# RGB and unsigned int datatype
img = np.zeros((400, 400, 3), dtype = "uint8")
# Creating line
cv2.line(img, (20, 160), (100, 160), (0, 0, 255), 10)
cv2.imshow('dark', img)
# Allows us to see image
# untill closed forcefully
cv2.waitKey(0)
cv2.destroyAllWindows()
输出 ： 
画一个矩形：
cv2.rectangle(imageObjectName, (‘top_left_vertex_coordinates’), (‘lower_right_vertex_coordinates’), (‘stroke_color_in_bgr’), ‘stroke_thickness’)
# Python3 program to draw rectangle
# shape on solid image
import numpy as np
import cv2
# Creating a black image with 3
# channels RGB and unsigned int datatype
img = np.zeros((400, 400, 3), dtype = "uint8")
# Creating rectangle
cv2.rectangle(img, (30, 30), (300, 200), (0, 255, 0), 5)
cv2.imshow('dark', img)
# Allows us to see image
# untill closed forcefully
cv2.waitKey(0)
cv2.destroyAllWindows()
输出 ： 
画一个圆圈：
cv2.circle(imageObjectName, (‘center_coordinates’), (‘circle_radius’), (‘color_in_bgr’), ‘stroke_thickness’)
# Python3 program to draw circle
# shape on solid image
import numpy as np
import cv2
# Creating a black image with 3
# channels RGB and unsigned int datatype
img = np.zeros((400, 400, 3), dtype = "uint8")
# Creating circle
cv2.circle(img, (200, 200), 80, (255, 0, 0), 3)
cv2.imshow('dark', img)
# Allows us to see image
# untill closed forcefully
cv2.waitKey(0)
cv2.destroyAllWindows()
输出 ： 
书写文字：
cv2.putText(imageObjectName, ‘TextContent’, (‘text_starting_point_coordinates’), ‘fontToBeUsed’, ‘font_size’, (‘text_color’, ‘text_thickness’, ‘line_type’)
# Python3 program to write
# text on solid image
import numpy as np
import cv2
# Creating a black image with 3
# channels RGB and unsigned int datatype
img = np.zeros((400, 400, 3), dtype = "uint8")
# writing text
font = cv2.FONT_HERSHEY_SIMPLEX
cv2.putText(img, 'GeeksForGeeks', (50, 50),
font, 0.8, (0, 255, 0), 2, cv2.LINE_AA)
cv2.imshow('dark', img)
# Allows us to see image
# untill closed forcefully
cv2.waitKey(0)
cv2.destroyAllWindows()
输出 ： 
在图像上绘制形状的应用：
- 绘制几何形状可以帮助我们突出图像的特定部分。
- 线条等几何形状可以帮助我们指出或识别图像中的特定区域。
- 在图像的某些区域上书写文本可以为该区域添加描述。
参考 ：
https://docs.opencv.org/2.4/modules/core/doc/drawing_functions.html