How does np.zeros() create a black background?

opencv

asked 2015-10-28 08:28:10 -0600

arp1561
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updated 2015-10-28 08:29:31 -0600

I am creating a very simple line drawing program. The problem is that i am still in the learning stage and I jsut stumbled across a problem.

import numpy as np import cv2

img = np.zeros((512,512,3), np.uint8)

img = cv2.line(img,(0,0),(511,511),(255,0,0),5)

how does np.zeros((512,512,3), np.uint8) create a black back ground? What if i want to create a red or a green background? And also, what does these 2 parameters mean?

I hope this answers my problem! Thanks!! :)

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answered 2015-10-28 09:15:13 -0600

StevenPuttemans

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Since you are using np.zeros, you are explicitly telling it to fill the complete image with zeros. Adding (0,0,0) at every position in the image results in a black image. Setting it all to (255,255,255) would be white, equalling np.ones() function. If you want color then you should assign values to the image positions.

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I tried using np.ones() but that is also giving me a black image. What should I do to get a white image?

meghasoni ( 2017-03-03 06:38:17 -0600 )edit

Can you try np.ones() * 255? It might be that your range is not correctly detected. If not, please open a new issue with more details.

StevenPuttemans ( 2017-03-03 07:10:01 -0600 )edit

but what is the significane of 3 in (255,255,3 )

supr8sung ( 2017-04-21 17:56:42 -0600 )edit

its a 255x255 image with 3 channels

StevenPuttemans ( 2017-04-24 06:04:14 -0600 )edit

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answered 2018-09-17 07:06:31 -0600

dumbledore
1

I've also been facing the same issue, try doing this (it solved the issue for me):

img = np.ones((512,512,3)) #img is the background image which appears when you run the program

For getting a black background you can use the np.zeros() function

img = np.zeros((512,512,3))

And if you want any other color then try filling the matrix with your choice of numbers corresponding to RGB values of your choice as this is (512 X 512 X 3) matrix.

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answered 2020-05-03 08:27:18 -0600

Mujeeb Ishaque
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The following python code worked for me for white background. Thanks to Steven P.

DEFAULT_SIZE_WHITE_CHANNEL = (640, 480, 1)
white_channel_image = np.ones(DEFAULT_SIZE_WHITE_CHANNEL, dtype = "uint8") * 255

The image produced will have a single channel of size 640 x 480. You will get a white image. Merge it with different channels using cv2.merge() function to get a RGB image.

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How does np.zeros() create a black background?

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