OpenCV Basics

Reading Images

imread(filename)

image=cv2.imread("images/rose.jpeg")

cv2 library works on images as numpy matrices, if you want to see it just print the image like

print(image)

To display image,

cv2.imshow(windowname,image_matrix(cv2 image))

cv2.imshow("image",image)

the image will be blinked to see the image we need to add delay

cv2.waitKey(milliseconds)
if we give 0 then it is infinite delay

cv2.waitKey(0) 
cv2.waiKey(1000) # delay of 1 sec

Blur

blur_image=cv2.blur(source_image,kernel)
we don’t need to worry about kernel just change values to get desired depth of blur

blur_image=cv2.blur(image,(3,3))  # average blur

To know more about different types of blur available in cv2 see this

color convertion

Usually we use RGB channels but in cv2 color channels convention is BGR
we will oftenly use grayscale images

gray_scale=cv2.cvtColor(image,cv2.COLOR_BGR2GRAY)

To convert to RGB

rgb_image=cv2.cvtColor(image,cv2.COLOR_BGR2RGB)

To HSV

hsvImage=cv2.cvtColor(image,cv2.COLOR_BGR2HSV)

and many more modes available

shape function

cv2 module has shape function that gives us
height
width
and no.of color channels ex: 3 for RGB

height=image.shape[0]
width=image.shape[1]

Resize

cv2.resize(source,(width,height))

resized_image=cv2.resize(image,(width/2,height/2))

crop

as we know cv2 image objects are numpy arrays, we can crop images using array properties
image[(height)from:to,(width)from:to]

cropped_image=image[0:400,0:400]

Drawing on Images

Line

cv2.line(image,(x1,y1),(x2,y2),color,thickness)

cv2.line(image,(0,0),(100,100),(0,0,0),2)

rectangle

cv2.rectangle(image,(x1,y1),(x2,y2),color,thickness)

cv2.rectangle(image,(10,10),(190,100),(255,255,255),2)

circle

cv2.circle(image,center=(100,100),radius=50,color=(255,255,255),thickness=1)

similarly we can draw ellipse,polygon

Text

cv2.putText(image,text="Text",org=(200,200),fontFace=cv2.FONT_ITALIC,fontScale=1,color=(255,0,0),thickness=2)

cv2 also has some other fonts

Canny Edge Detection

canny=cv2.Canny(image=image,threshold1=100,threshold2=102)
canny2=cv2.Canny(image=image,threshold1=150,threshold2=50)

#try different threshold values

Listen to Mouse

cv.setMouseCallback(windowName,functionToExecute)

def functionToExecute(event,x,y,flags,param):

• (x,y)-pixel positions of mouse click
• event - type of mouse action
  • EVENT_LBUTTONDOWN - left mouse buttton down
  • EVENT_LBUTTONCLICK - left button click

  and there are lot more other events