matplotlib.image和cv2.imread之间的区别

Question

When i read a grayscale image with matplotlib.image it shows dimension (512,512) and type (float32) but when i use cv2.imread for the same image it shows (512,512,3) and type uint8. Why is it so? Is cv2.imread command automatically converted image to BGR format, because of default flag. ( I am using python 3 and opencv2.) Following is code:

import cv2
import matplotlib.image as mpimage
img = cv2.imread('image_1.png')
img1=mpimage.imread('image_1.png')

Answer 1

Yes, by default it reads BGR and if you want to read a grayscale image you need to use the second argument in imread. From the docs :

Read an image

Use the function cv2.imread() to read an image. The image should be in the working directory or a full path of image should be given.

Second argument is a flag which specifies the way image should be read.

1. cv2.IMREAD_COLOR : Loads a color image. Any transparency of image will be neglected. It is the default flag.
2. cv2.IMREAD_GRAYSCALE : Loads image in grayscale mode
3. cv2.IMREAD_UNCHANGED : Loads image as such including alpha channel Note Instead of these three flags, you can simply pass integers 1, 0 or -1 respectively.

See the code below:

 import numpy as np import cv2 #Load an color image in grayscale img = cv2.imread('messi5.jpg',0) 

So, if you want to read in a grayscale image by default you need to have the second argument in imread as a 1, not 0. This subtle change to your code below should read in the same image format in both instances

import cv2
import matplotlib.image as mpimage
img = cv2.imread('image_1.png', 1)
img1=mpimage.imread('image_1.png')