Image Drawing Functions

Here, we will cover some functions that draw shapes or add text to an image, which are particularly useful for making it so humans can see what a computer is trying to analyze.

NOTE: Before starting this module, please import the numpy library.

cv2.rectangle()

cv2.rectangle has 4 different arguments aside from its image matrix argument. Argument 2 and 3 are the upper left corner and bottom right corner coordinates of the rectangle respectively, while argument 4 is the color of the rectangle (don't forget this is BGR format.). Argument 5 is the thickness of said rectangle, but if set to -1, it will actually fill the rectangle.

• 1st Argument-Image Matrix

• 2nd Argument-Upper Left Coordinate

• 3rd Argument-Bottom Right Coordinate

• 4th Argument-Color

• 5th Argument-Line Thickness

cv2.circle()

cv2.circle has 4 arguments as well in addition to the image matrix argument. Argument 2 is the center of the circle while argument 3 is its radius. Argument 4 is the color, and argument 5 is the thickness. For arguments 4 and 5, the same rules apply here from the rectangle.

• 1st Argument-Image Matrix

• 2nd Argument-Center Coordinate

• 3rd Argument-Radius

• 4th Argument-Color

• 5th Argument-Line Thickness

cv2.line()

cv2.line is the last of the drawing functions. Arguments 2 and 3 are the start and end coordinates of the line, whereas arguments 4 and 5 are the color and thickness respectively. However, you cannot use -1 as a thickness here, as it will error the program.

• 1st Argument-Image Matrix

• 2nd Argument-Start Coordinate

• 3rd Argument-End Coordinate

• 4th Argument- Color

• 5th Argument-Line Thickness

cv2.putText()

Lastly, cv2.putText will place text on an image, that can be used to clarify something. Argument 2 is the text you want to place in an image, whereas argument 3 is the coordinate of the text. The constant on the 4th argument is the font style. You can use the font cv.FONT_HERSHEY_TRIPLEX as a font. Argument 5 is the scale factor, and argument 6 and 7 are color and thickness respectively.

• 1st Argument-Image Matrix

• 2nd Argument-Text String

• 3rd Argument-Text Coordinate

• 4th Argument-Font Style

• 5th Argument-Scale Factor

• 6th Argument- Color

• 7th Argument Thickness

Cropping and .shape

Remember, OpenCV stores all of its images as matrices, so cropping is not a library exclusive feature. Simply crop the matrix from one x coordinate to another and one y coordinate to another.

• i.e. img=img[200:300,400:500]

• where 200:300 represents only pixels on this x axis range to be displayed.

• where 400:500 represents only pixels on this y axis range to be displayed.

.shape, when attached to an image matrix will yield the width and height of the image.

• i.e img.shape[0] will yield the image's width and img.shape[1] will yield the image's height

NOTE: .shape is actually a numpy function, which is why you need to import the libarary.

You can also use some of these draw functions in more unorthodox ways, such as setting the radius of a circle to a radius of one pixel so that it can indicate a single pixel.

Try it yourself!

Like the last module, these windows will open and close one at a time so make sure to press a key to close it. And yes, you can use these on regular images and not just blank numpy matrices.

blank = np.zeros((500,500,3),dtype='uint8')

cv.imshow('Blank',blank)

cv.waitKey(0)

blank[200:300,300:400]=[0,0,255]

cv.imshow('Green',blank)

cv.waitKey(0)

##for the following functions, you may remove the first argument with your image matrix (img) from the previous modules.

cv.rectangle(blank, (0,0), (250,250), (0,255,0), thickness=2)

cv.imshow('rectangle',blank)

cv.waitKey(0)

cv.rectangle(blank,(0,0),(250,250),(0,255,0),thickness=cv.FILLED)

cv.imshow('rect',blank)

cv.waitKey(0)

cv.circle(blank,(250,250),40,(0,0,255),thickness=3)

cv.imshow('circ', blank)

cv.waitKey(0)

cv.line(blank, (0,0), (250,250), (255,255,255), thickness=3)

cv.imshow('line',blank)

cv.waitKey(0)

cv.putText(blank,'Hello',(img.shape[1]//2,img.shape[0]//2),cv.FONT_HERSHEY_TRIPLEX,1.0,(255,255,255),thickness=2)

cv.imshow('text', blank)

cv.waitKey(0)

• Legend:

  • img=your specified image matrix

  • x= an integer value

  • y= an integer value

  • z=an integer value

  • 'text'=the text you want to type in

  • cv2.CONSTANT= a value that tells OpenCV what to do specifically with the said function. Think of it as a setting if you will.

• Functions:

  • cv2.rectangle(img,(x,y),(x,y),(x,y,z),thickness=x)

  • cv2.circle(img,(x,y),x,(x,y,z),thickness=x)

  • cv2.line(img,(x,y),(x,y),(x,y,z),thickness=x)

  • cv2.putText(img,'text',(x,y),cv.CONSTANT, z,(x,y,z),thickness=x)

  • width, height=img.shape