summary about loading image data in Python

There are several options to load image data in Python, provided by:

1. OpenCV

2. matplotlib

3. PIL 

1. in OpenCV: 

    import cv2

    import numpy as np

    img = cv2.imread('examples.png')    # 默认是读入为彩色图，即使原图是灰度图也会复制成三个相同的通道变成彩色图

    img_gray = cv2.imread('examples.png',0)    # 第二个参数为0的时候读入为灰度图，即使原图是彩色图也会转成灰度图

    print(type(img), img.dtype, np.min(img), np.max(img))

    print(img.shape)

    print(img_gray.shape)    # opencv读进来的是numpy array，类型是uint8

2. in matplotlib:

    import matplotlib.pylab as plt

    import matplotlib.image as mpimg

    img=plt.imread("xxx.png") #numpy array

    or 

    img=mpimg.imread("xxx.png") #all the data loaded is in float32 format numpy array

3. in PIL:

    from PIL import Image

    img=Image.open("xxx.png")    #an image object, not numpy array, need to implement the following to convert:

    img=np.array(img)    or 

    img=np.asarray(img)