Merge pull request #3259 from yogeshojha/master

[opencv/en]

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+---
+category: tool
+tool: OpenCV
+filename: learnopencv.py
+contributors:
+    - ["Yogesh Ojha", "http://github.com/yogeshojha"]
+---
+### Opencv
+
+OpenCV (Open Source Computer Vision) is a library of programming functions mainly aimed at real-time computer vision. 
+Originally developed by Intel, it was later supported by Willow Garage then Itseez (which was later acquired by Intel). 
+Opencv currently supports wide variety of languages like, C++, Python, Java etc
+
+#### Installation
+Please refer to these articles for installation of OpenCV on your computer.
+
+* Windows Installation Instructions: [https://opencv-python-tutroals.readthedocs.io/en/latest/py_tutorials/py_setup/py_setup_in_windows/py_setup_in_windows.html#install-opencv-python-in-windows]()
+* Mac Installation Instructions (High Sierra): [https://medium.com/@nuwanprabhath/installing-opencv-in-macos-high-sierra-for-python-3-89c79f0a246a]()
+* Linux Installation Instructions (Ubuntu 18.04): [https://www.pyimagesearch.com/2018/05/28/ubuntu-18-04-how-to-install-opencv]()
+
+### Here we will be focusing on python implementation of OpenCV
+
+```python
+# Reading image in OpenCV
+import cv2
+img = cv2.imread('cat.jpg')
+
+# Displaying the image
+# imshow() function is used to display the image
+cv2.imshow('Image',img)
+# Your first arguement is the title of the window and second parameter is image
+# If you are getting error, Object Type None, your image path may be wrong. Please recheck the pack to the image
+cv2.waitKey(0)
+# waitKey() is a keyboard binding function and takes arguement in milliseconds. For GUI events you MUST use waitKey() function.
+
+# Writing an image
+cv2.imwrite('catgray.png',img)
+# first arguement is the file name and second is the image
+
+# Convert image to grayscale
+gray_image = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
+
+# Capturing Video from Webcam
+cap = cv2.VideoCapture(0)
+#0 is your camera, if you have multiple camera, you need to enter their id
+while(True):
+    # Capturing frame-by-frame
+    _, frame = cap.read()
+    cv2.imshow('Frame',frame)
+    # When user presses q -> quit
+    if cv2.waitKey(1) & 0xFF == ord('q'):
+        break
+# Camera must be released
+cap.release()
+
+# Playing Video from file
+cap = cv2.VideoCapture('movie.mp4')
+while(cap.isOpened()):
+    _, frame = cap.read()
+    # Play the video in grayscale
+    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
+    cv2.imshow('frame',gray)
+    if cv2.waitKey(1) & 0xFF == ord('q'):
+        break
+cap.release()
+
+# Drawing The Line in OpenCV
+# cv2.line(img,(x,y),(x1,y1),(color->r,g,b->0 to 255),thickness)
+cv2.line(img,(0,0),(511,511),(255,0,0),5)
+
+# Drawing Rectangle
+# cv2.rectangle(img,(x,y),(x1,y1),(color->r,g,b->0 to 255),thickness)
+# thickness = -1 used for filling the rectangle
+cv2.rectangle(img,(384,0),(510,128),(0,255,0),3)
+
+# Drawing Circle
+cv2.circle(img,(xCenter,yCenter), radius, (color->r,g,b->0 to 255), thickness)
+cv2.circle(img,(200,90), 100, (0,0,255), -1)
+
+# Drawing Ellipse
+cv2.ellipse(img,(256,256),(100,50),0,0,180,255,-1)
+
+# Adding Text On Images
+cv2.putText(img,"Hello World!!!", (x,y), cv2.FONT_HERSHEY_SIMPLEX, 2, 255)
+
+# Blending Images
+img1 = cv2.imread('cat.png')
+img2 = cv2.imread('openCV.jpg')
+dst = cv2.addWeighted(img1,0.5,img2,0.5,0)
+
+# Thresholding image
+# Binary Thresholding
+_,thresImg = cv2.threshold(img,127,255,cv2.THRESH_BINARY)
+# Adaptive Thresholding
+adapThres = cv2.adaptiveThreshold(img,255,cv2.ADAPTIVE_THRESH_GAUSSIAN_C, cv2.THRESH_BINARY,11,2)
+
+# Blur Image
+# Gaussian Blur
+blur = cv2.GaussianBlur(img,(5,5),0)
+# Median Blur
+medianBlur = cv2.medianBlur(img,5)
+
+# Canny Edge Detection
+img = cv2.imread('cat.jpg',0)
+edges = cv2.Canny(img,100,200)
+
+# Face Detection using Haar Cascades
+# Download Haar Cascades from https://github.com/opencv/opencv/blob/master/data/haarcascades/
+import cv2
+import numpy as np
+face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')
+eye_cascade = cv2.CascadeClassifier('haarcascade_eye.xml')
+
+img = cv2.imread('human.jpg')
+gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
+
+aces = face_cascade.detectMultiScale(gray, 1.3, 5)
+for (x,y,w,h) in faces:
+    cv2.rectangle(img,(x,y),(x+w,y+h),(255,0,0),2)
+    roi_gray = gray[y:y+h, x:x+w]
+    roi_color = img[y:y+h, x:x+w]
+    eyes = eye_cascade.detectMultiScale(roi_gray)
+    for (ex,ey,ew,eh) in eyes:
+        cv2.rectangle(roi_color,(ex,ey),(ex+ew,ey+eh),(0,255,0),2)
+
+cv2.imshow('img',img)
+cv2.waitKey(0)
+
+cv2.destroyAllWindows()
+# destroyAllWindows() destroys all windows. 
+# If you wish to destroy specific window pass the exact name of window you created.
+```
+
+### Further Reading:
+
+* Download Cascade from [https://github.com/opencv/opencv/blob/master/data/haarcascades]()
+* OpenCV drawing Functions [https://docs.opencv.org/2.4/modules/core/doc/drawing_functions.html]()
+* An up-to-date language reference can be found at [https://opencv.org]()
+* Additional resources may be found at [https://en.wikipedia.org/wiki/OpenCV]()
+* Good OpenCv Tutorials
+    * [https://opencv-python-tutroals.readthedocs.io/en/latest/py_tutorials/py_tutorials.html]()
+    * [https://realpython.com/python-opencv-color-spaces]()
+    * [https://pyimagesearch.com]()
+    * [https://www.learnopencv.com]()