opencv学习笔记3:画线，画圆，方框，椭圆，文字

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• opencv学习笔记3:画线，画圆，方框，椭圆，文字

目标

1使用opencv画不同的形状
2我们将学会使用这些函数 : cv2.line(), cv2.circle() , cv2.rectangle(), cv2.ellipse(), cv2.putText() etc.

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参数解释

In all the above functions, you will see some common arguments as given below:

img : The image where you want to draw the shapes
color : Color of the shape. for BGR, pass it as a tuple, eg: (255,0,0) for blue. For grayscale, just pass the scalar value.
thickness : Thickness of the line or circle etc. If -1 is passed for closed figures like circles, it will fill the shape. default thickness = 1
lineType : Type of line, whether 8-connected, anti-aliased line etc. By default, it is 8-connected. cv2.LINE_AA gives anti-aliased line which looks great for curves.

Drawing Line 画线

画线，需要起始点和终点，设定线条颜色，
To draw a line, you need to pass starting and ending coordinates of line. We will create a black image and draw a blue line on it from top-left to bottom-right corners.

import numpy as np
import cv2
# Create a black image
img = np.zeros((512,512,3), np.uint8)
# Draw a diagonal blue line with thickness of 5 px
img = cv2.line(img,(0,0),(511,511),(255,0,0),5)

Drawing Rectangle 画矩形

需要提供左上和右下角的坐标，这次我们画一个绿色的矩形。

img = cv2.rectangle(img,(384,0),(510,128),(0,255,0),3)

Drawing Circle 画圆

为了画圆我们需要知道圆心和半径

img = cv2.circle(img,(447,63), 63, (0,0,255), -1)

Drawing Ellipse 画椭圆

To draw the ellipse, we need to pass several arguments. One argument is the center location (x,y). Next argument is axes lengths (major axis length, minor axis length). angle is the angle of rotation of ellipse in anti-clockwise direction. startAngle and endAngle denotes the starting and ending of ellipse arc measured in clockwise direction from major axis. i.e. giving values 0 and 360 gives the full ellipse. For more details, check the documentation of cv2.ellipse(). Below example draws a half ellipse at the center of the image.

img = cv2.ellipse(img,(256,256),(100,50),0,0,180,255,-1)

Drawing Polygon 画多边形

为了画多边形，首先你需要多边形的顶点坐标。

pts = np.array([[10,5],[20,30],[70,20],[50,10]], np.int32)
pts = pts.reshape((-1,1,2))
img = cv2.polylines(img,[pts],True,(0,255,255))

If third argument is False, you will get a polylines joining all the points, not a closed shape.

Note

cv2.polylines() can be used to draw multiple lines. Just create a list of all the lines you want to draw and pass it to the function. All lines will be drawn individually. It is more better and faster way to draw a group of lines than calling cv2.line() for each line.

图片上添加文字

To put texts in images, you need specify following things.
Text data that you want to write
Position coordinates of where you want put it (i.e. bottom-left corner where data starts).
Font type (Check cv2.putText() docs for supported fonts)
Font Scale (specifies the size of font)
regular things like color, thickness, lineType etc. For better look, lineType = cv2.LINE_AA is recommended.
We will write OpenCV on our image in white color.

font = cv2.FONT_HERSHEY_SIMPLEX
cv2.putText(img,'OpenCV',(10,500), font, 4,(255,255,255),2,cv2.LINE_AA)

画图结果显示

So it is time to see the final result of our drawing. As you studied in previous articles, display the image to see it.

Drawing Functions in OpenCV

Additional Resources

The angles used in ellipse function is not our circular angles. For more details, visit this discussion.