为什么 Opencv python 包装器中的数据类型必须是“uint8”？

Question

I'd like to create an image of gray color using the following code:

import numpy as np
import cv2
cv=cv2

s=np.zeros((240, 320, 3), dtype=int)
s[s==0]=128
cv.imshow('3', s)
cv.waitKey()
cv.destroyAllWindows()

but I get a totally black image. When I write the image to a file, it indeed is a gray image:

fn='example.jpg'
cv.imwrite(fn, s)

So I have to change int to uint8 , then everything works well. But I'm still curious that why I have to use uint8 instead of int , is there any docs describing this?

Answer 1

From the documentation , OpenCV changes its behavior according to the type of the array:

• If the image is 8-bit unsigned, it is displayed as is.
• If the image is 16-bit unsigned or 32-bit integer, the pixels are divided by 256. That is, the value range [0,255*256] is mapped to [0,255].
• If the image is 32-bit floating-point, the pixel values are multiplied by 255. That is, the value range [0,1] is mapped to [0,255].

Hence, the following will all produce the same image grayscale image:

s = np.zeros((240, 320), dtype=np.uint8)
s[s==0] = 128

int32 :

s = np.zeros((240, 320), dtype=np.int32)
s[s==0] = 128 * 256

float32 :

s = np.zeros((240, 320), dtype=np.float32)
s[s==0] = 128 / 256.0

Answer 2

因为OpenCV读取图像的默认dtype是'uint8'，所以为了兼容，在该图像上进行操作的所有内容都必须是'uint8'类型

Answer 3

uint8 is an unsigned 8-bit integer that can represent values 0..255. int on the other hand is usually a 32-bit signed integer. When you create array using dtype=int, each element in that aray takes 4 bytes. OpenCV apparently expect array to be made of 8-bit tuples representing red, green and blue. So when you pass array of integers, the memory will contain something like this:

0x00000080 0x00000080 0x00000080.......

Which openCV interprets as:

{R=0x00,G=0x00,B=0x00} {R=0x80,G=0x00,B=0x00}, {R=0x00, G=0x00, B=0x00}...

My guess is what you've got very dark image which, on closer inspection would show very dim red, green and blue pixels.

When you changed type to uint8, your array would look like this:

0x80 0x80 0x80 0x80 0x80 0x80 0x80....

Which is then interpreted by OpenCV as gray RGB values:

{R=0x80,G=0x80,B=0x80}

Answer 4

If you want a gray image as output then must try this below code to to get an expecting result. Well nice try.

 import cv2 as cv import numpy as np img = cv.imread('C:/Users/Admin/Downloads/Opencv/tiger.png') #cv.imshow('tiger', img) #this code will shows you a normal image #cv.waitKey(0) #cv.destroyAllWindows() gray = cv.cvtColor(img, cv.COLOR_BGR2GRAY) cv.imshow('Gray', gray) #this above code will shows you a gray colour image cv.waitKey(0) cv.destroyAllWindows()