Wrong HSV conversion from RGB

Question

I have these lists containing blue values in RGB format.

low = [
    [0, 0, 128],
    [65, 105, 225],
    [70, 130, 180],
    [72, 61, 139],
    [83, 104, 120]
]

What I want to make is this: convert all the values from, for example, first list from RGB to HSV.

I made this code:

import cv2
import numpy as np

for v in low:
    rgb = np.uint8([[v]])
    print("RGB: ", rgb)

    hsv = cv2.cvtColor(rgb, cv2.COLOR_RGB2HSV)
    print("HSV: ", hsv)
    print("\n")

The problem is when I go to check if the colors (RGB-HSV) are the same. Here I discovered that it's not.

Let's take the last value from low list.

RGB:  [[[ 83 104 120]]]
HSV:  [[[103  79 120]]]

RGB is RGB input value and HSV is output. But this last one it's not the same color as RGB. First is a shade of blue and last is green. Why?

I used this tool to check values. It also says that the right HSV for this RGB should be 205, 30, 47.

Where is my mistake?

the chances of getting answers relate to the quality of code provided. See [minimal verifyable complete example](https://stackoverflow.com/help/mcve). - we do not need 500 colors to see the behavior. Also : you are lacking your imports (f.e. cv2)? Please edit and shrink your code, thanks. — Patrick Artner, May 21 '18 at 13:11
[Found this](http://shervinemami.info/colorConversion.html)...Hope will help — lucians, May 21 '18 at 13:38
The tool you used for verification has range [0,359] for Hue, [0,100] for Saturation and Value. OpenCV's HSV range are [0,179] for Hue, [0,255] for Saturation and Value. — FBergo, May 21 '18 at 13:55
Possible duplicate: https://stackoverflow.com/questions/43307959/how-to-get-hsv-and-lab-color-space/43319977#43319977 — Rick M., May 22 '18 at 07:23

FBergo · Accepted Answer · 2018-05-21T14:27:50.253

9

The tool you used for verification has range [0,359] for Hue, and range [0,100] for Saturation and Value. OpenCV's HSV ranges are [0,179] for Hue, [0,255] for Saturation and Value.

Multiply by 2, 1/2.55, 1/2.55 and you will get the expected values, off by minor integer truncation errors: [103 79 120] * [2 1/2.55 1/2.55] = [206 31 47]

edited May 21 '18 at 14:27

answered May 21 '18 at 14:05

FBergo

1,010
8
11

score 2 · Answer 2 · answered May 22 '18 at 09:00

2

Although @FBergo's answer is correct, I'd like to add that these conversions (multiply by...) are type-dependent and care must be taken when using conversions for 8UC3, 16SC3, 32SC3, 32FC3 and so on.

answered May 22 '18 at 09:00

Rick M.

3,045
1
21
39

Wrong HSV conversion from RGB

2 Answers2

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