getting the HUE of a pixel

dinasset · **Posted:** Thu Dec 03, 2015 10:37 am **(#1)**

GIMP Version: 2.8.14
Operating System: Windows
GIMP Experience: New User

Hi all.
Is there a native of add-on function to get the HUE of a pixel?
If not, because I read by googling that it's possible to derive the HUE from R,G,B

Preucil[7] describes a color hexagon, similar to a trilinear plot described by Evans, Hanson, and Brewer,[8] which may be used to compute hue from RGB. To place red at 0°, green at 120°, and blue at 240°,

hue(rgb) = atan2(sqrt{3} * (G - B), 2 * R - G - B ).

can some guru "translate" this formula in python?
Thanks!

(I don't know whether I reported the formula correctly, but experts will know it)

dinasset · **Posted:** Thu Dec 03, 2015 4:21 pm **(#2)**

edit:

continuing googling, after the above calculation it's necessary to multiply the result by 60 then if negative add 360

the previously indicated formula in python is easy,:
"import math"
"hue1 = math.atan2(math.sqrt(3) * (G - B), 2 * R - G - B)"

(then we multiply by 60 and add 360 if necessary)

experimented on python the result seems to be approximate IMO:
- an RGB with a value only on Green gives Hue=125.6...(shouldn't be 120?)
- an RGB with a value only on Blue gives Hue=234.3....(shouldn't be 240?)
when the RGB has value only on R the Hue is exact: 0.0

there is also a second formula, but it gives the same results.

is there something better around?

saulgoode · **Posted:** Thu Dec 03, 2015 7:42 pm **(#3)**

For what it's worth, here is my Scheme function for converting from RGB to HSV. All inputs and outputs should be normalized to fall within the range 0.0 to 1.0 .

Code: Select all

; (rgb->hsv r g b) ==> (h s v)
; inputs and outputs all normalized from 0.0 to 1.0
; 
(define rgb->hsv 
  (lambda rgb
    (let* ((v (apply max rgb))
           (d (- v (apply min rgb)))
           (s (if (zero? v)
                0.0
                (/ d v)))
           (h (cond
                ((zero? v)
                  0.0)
                ((< d 0.00001)
                  v)
                ((= (car rgb) v)
                  (/ (- (cadr rgb) (caddr rgb)) 6 d))
                ((= (cadr rgb) v)
                  (+ (/ 3) (/ (- (caddr rgb) (car rgb)) 6 d)))
                ((= (caddr rgb) v)
                  (+ (/ 2 3) (/ (- (car rgb) (cadr rgb)) 6 d))))))
      (list (if (< h 0)
              (+ h 1)
              h)
            s
            v))))

RobA · **Joined:** Jul 14, 2010 **Posts:** 697

And here are the scheme functions I use to convert back and forth ('cause I can't think in Lisp like SG can...

):

Code: Select all

(define (huefromrgb R G B)
  (let* ()
    (cond 
      ((= R G B) 0)
     ((and (>= R G) (>= G B)) (round (* 60 (/ (- G B) (- R B)))))
     ((and (> G R) (>= R B)) (round (* 60 (- 2 (/ (- R B) (- G B))))))
     ((and (>= G B) (> B R)) (round (* 60 (+ 2 (/ (- B R) (- G R))))))
     ((and (> B G) (> G R)) (round (* 60 (- 4 (/ (- G R) (- B R))))))
     ((and (> B R) (>= R G)) (round (* 60 (+ 4 (/ (- R G) (- B G))))))
     ((and (>= R B) (> B G)) (round (* 60 (- 6 (/ (- B G) (- R G))))))
    )))
    
(define (rgbfromhue hue)
  (let* ()
    (list
    ; Red
    (round (cond 
      ((< hue 60) 255)
      ((< hue 120) (* 255 (- 1 (/ (- hue 60) 60))))
      ((< hue 240) 0)
      ((< hue 300) (* 255 (/ (- hue 240) 60)))
      ((< hue 360) 255)
    ))
    ; Green
    (round (cond 
      ((< hue 60) (* 255 (/ hue 60)))
      ((< hue 180) 255)
      ((< hue 240) (* 255 (- 1 (/ (- hue 180) 60))))
      ((< hue 360) 0)
    ))
    ; Blue
    (round( cond 
      ((< hue 120) 0)
      ((< hue 180) (* 255 (/ (- hue 120) 60)))
      ((< hue 300) 255)
      ((< hue 360) (* 255 (- 1 (/ (- hue 300) 60))))
    ))
    )
  )
)

-Rob A>

dinasset · **Posted:** Fri Dec 04, 2015 12:24 am **(#5)**

thanks a lot, Saul and Rob!
I'll "translate" into python (or call as they are...)

dinasset · **Posted:** Fri Dec 04, 2015 2:40 pm **(#6)**

@RobA
Rob, I'm experimenting your formula, first part (from RGB to hue), after a "translation" to python.
One comment: unless I'm misinterpreting your formula, at the end should be necessary IMO to add a statement: if resulting hue is negative, then multiply it by -1 (in other words: it's the absolute value which represents the hue).
I say that because in my first experiments while the red, blue, cyan and yellow colors return a positive (and corresponding) value, respectively 0, 240, 180, 60, the green and magenta colors return a value which is correct in absolute value but with a minus in front, that is -120 and -300.
Am I wrong?

trandoductin · **Posted:** Fri Dec 04, 2015 7:02 pm **(#7)**

I am curious to know what you're working on dinasset.

dinasset · **Posted:** Sat Dec 05, 2015 1:10 am **(#8)**

an easy way to identify a colour by one single value, to understand "colours proximity"