Image from numpy array appears colorless

Question

I would like to create following 2x2 image from numpy array using OpenCV.

I tried to create it with this code snippet.

import cv2
import numpy as np

blue = (255, 0, 0)
green = (0, 255, 0)
red = (0, 0, 255)

image = np.array([
    [blue, green],
    [red, blue]
])

cv2.imwrite('2x2.jpg', image)

I used BGR color format, yet when I view the output image, I see a colorless image.

What am I missing?

Environment

OpenCV 4.5.1.48
Python 3.9.1
Windows 10

With such a small image, it might be that the JPEG encoding is over compressing your original data. Try saving the file as a PNG instead, with cv2.imwrite('2x2.png', image). — stateMachine
– stateMachine, Commented Apr 28, 2021 at 22:46
Good point about jpg compression for images smaller than the jpg 8x8 block size. — fmw42
– fmw42, Commented Apr 28, 2021 at 23:10
@fmw42 It's not about the size, same happens if you tile this pattern. It's the effect of chroma subsampling used by JPEG. I think 4:2;0 variant (the most common), which would mean that colour is sampled at half the resolution (i.e. one sample for a 2x2 tile). Seems to fit, the result appears to only have varying luminance and same hue. Really shows why JPEG is bad choice for synthetic images like this. — Dan Mašek
– Dan Mašek, Commented Apr 29, 2021 at 9:29
OpenCV doesn't provide a way to directly control the subsampling method, however there is a side effect -- when you specify both IMWRITE_JPEG_LUMA_QUALITY and IMWRITE_JPEG_CHROMA_QUALITY and they are not equal, subsampling is turned off: github.com/opencv/opencv/blob/3.4/modules/imgcodecs/src/… | Unfortunately this needs JPEG_LIB_VERSION >= 70, and sadly the library shipped with OpenCV and used to build the standard releases (at least on Windows) is only version 62. — Dan Mašek
– Dan Mašek, Commented Apr 29, 2021 at 9:59
If JPEG format is required, Pillow allows direct control over subsampling: Image.fromarray(image[...,::-1]).save('2x2_PIL.jpg', quality=100, subsampling=0) — Dan Mašek
– Dan Mašek, Commented Apr 29, 2021 at 10:07

stateMachine · Accepted Answer · 2021-04-28 22:47:42Z

2

I think the jpg compressing method cause this issue on a few pixel shaped images

Instead of:

cv2.imwrite('2x2.jpg', image)

try to save it as png:

cv2.imwrite('2x2.png', image)

and always use uint8 on image array.

edited Apr 28, 2021 at 22:47

stateMachine

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answered Apr 28, 2021 at 22:26

Norbert Tiborcz

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Comments

fmw42 · Accepted Answer · 2021-04-28 21:41:53Z

2

I believe that your issue may be not specifying the dtype for your array. The following works fine for me in Python/OpenCV.

import cv2
import numpy as np

blue = (255, 0, 0)
green = (0, 255, 0)
red = (0, 0, 255)

image = np.array([
    [blue, green],
    [red, blue]
], dtype=np.uint8)

image = cv2.resize(image, (200,200), interpolation = cv2.INTER_AREA)

cv2.imwrite('2x2.jpg', image)

answered Apr 28, 2021 at 21:41

fmw42

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3 Comments

sanitizedUser Over a year ago

This doesn't solve it. When you change the size from (200, 200) to (2, 2) the output is the same colorless image. Specifying the dtype alone does nothing.

fmw42 Over a year ago

It works fine for me. If I remove the dtype, the resize crashes for me with an OpenCV error message. Sorry, if it does not work for you. I am using Python 3.7.5 and OpenCV 3.4.8

sanitizedUser Over a year ago

I use OpenCV 4.5.1.48 and if I remove the dtype it produces the same output. No crashes.

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