Category: vision

image histogram

python png

OpenCV (cv2) can be used to extract data from images and do operations on them. We demonstrate some examples of that below:

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Image properties
We can extract the width, height and color depth using the code below:

import cv2
import numpy as np

# read image into matrix.
m =  cv2.imread("python.png")

# get image properties.
h,w,bpp = np.shape(m)

# print image properties.
print "width: " + str(w)
print "height: " + str(h)
print "bpp: " + str(bpp)

Access pixel data
We can access the pixel data of an image directly using the matrix, example:

import cv2
import numpy as np

# read image into matrix.
m =  cv2.imread("python.png")

# get image properties.
h,w,bpp = np.shape(m)

# print pixel value
y = 1
x = 1
print m[y][x]

To iterate over all pixels in the image you can use:

import cv2
import numpy as np

# read image into matrix.
m =  cv2.imread("python.png")

# get image properties.
h,w,bpp = np.shape(m)

# iterate over the entire image.
for py in range(0,h):
for px in range(0,w):
print m[py][px]

Image manipulation
You can modify the pixels and pixel channels (r,g,b) directly. In the example below we remove one color channel:

import cv2
import numpy as np

# read image into matrix.
m =  cv2.imread("python.png")

# get image properties.
h,w,bpp = np.shape(m)

# iterate over the entire image.
for py in range(0,h):
for px in range(0,w):
m[py][px][0] = 0

# display image
cv2.imshow('matrix', m)
cv2.waitKey(0)

To change the entire image, you’ll have to change all channels:   m[py][px][0], m[py][px][1], m[py][px][2].

Save image
You can save a modified image to the disk using:

cv2.imwrite('filename.png',m)

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face detection python

In this tutorial you will learn how to apply face detection with Python. As input video we will use a Google Hangouts video. There are tons of Google Hangouts videos around the web and in these videos the face is usually large enough for the software to detect the faces.

Detection of faces is achieved using the OpenCV (Open Computer Vision) library. The most common face detection method is to extract cascades. This technique is known to work well with face detection. You need to have the cascade files (included in OpenCV) in the same directory as your program.

Related course
Master Computer Vision with OpenCV and Python

Video with Python OpenCV

To analyse the input video we extract each frame.  Each frame is shown for a brief period of time. Start with this basic program:

#! /usr/bin/python

import cv2

vc = cv2.VideoCapture('video.mp4')
c=1
fps = 24

if vc.isOpened():
    rval , frame = vc.read()
else:
    rval = False

while rval:
    rval, frame = vc.read()
    cv2.imshow("Result",frame)
    cv2.waitKey(1000 / fps);
    vc.release()

Upon execution you will see the video played without sound. (OpenCV does not support sound). Inside the while loop we have every video frame inside the variable frame. 

Face detection with OpenCV

#! /usr/bin/python

import cv2

face_cascade = cv2.CascadeClassifier('lbpcascade_frontalface.xml')
vc = cv2.VideoCapture('video.mp4')

if vc.isOpened():
    rval , frame = vc.read()
else:
    rval = False

roi = [0,0,0,0]

while rval:
    rval, frame = vc.read()

    # resize frame for speed.
    frame = cv2.resize(frame, (300,200))
    
    # face detection.
    faces = face_cascade.detectMultiScale(frame, 1.8, 2)
    nfaces = 0
    for (x,y,w,h) in faces:
    cv2.rectangle(frame,(x,y),(x+w,y+h),(0,0,255),2)
    nfaces = nfaces + 1
    roi = [x,y,w,h]

    # undetected face, show old on position.
    if nfaces == 0:
        cv2.rectangle(frame,(roi[0],roi[1]),(roi[0]+roi[2],roi[1]+roi[3]),(0,0,255),2)

    # show result
    cv2.imshow("Result",frame)
    cv2.waitKey(1);
    vc.release()

In this program we simply assumed there is one face in the video screen. We reduced the size of the screen to speed up the processing time. This is fine in most cases because detection will work fine in lower resolutions.  If you want to execute the face detection in “real time”, keeping the computational cycle short is mandatory. An alternative to this implementation is to process first and display later.

A limitation of this technique is that it does not always detect faces and faces that are very small or occluded may not be detected. It may show false positives such as a bag detected as face.  This technique works quite well on certain type of input videos.

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