Auto detect threshold for skin detection w/ HSV channel

[davlovsky]

Hello forum,
I have some code that gets the HSV color channels for a given image.
I need to be able to automatically get a lower and upper threshold value based on these values for skin detection.
Basically, I cannot hard-code the threshold values -they have to be extracted from the histogram.

from mpl_toolkits.mplot3d import Axes3D
import matplotlib
import matplotlib.pyplot as plt
import numpy as np
import cv2
from scipy.signal import argrelextrema
import sys
from scipy.signal import savgol_filter

np.set_printoptions(threshold=sys.maxsize)

# Getting HSV values
img = cv2.imread("face_dark.bmp")
hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV)
h,s,v = cv2.split(hsv)

# Getting YCrCb values
YCrCb = cv2.cvtColor(img, cv2.COLOR_BGR2YCR_CB)  
Y,Cr,Cb = cv2.split(YCrCb)

fig = plt.figure()

ax = fig.add_subplot(111, projection='3d')
for x, c, z in zip([h,s,v], ['r', 'g', 'b'], [30, 20, 10]):
    xs = np.arange(256)
    ys = cv2.calcHist([x], [0], None, [256], [0,256])
    cs = [c] * len(xs)
    cs[0] = 'c'
    ax.bar(xs, ys.ravel(), zs=z, zdir='y', color=cs, alpha=0.8)

ax.set_xlabel('X')
ax.set_ylabel('Y')
ax.set_zlabel('Z')

array = np.asarray(img)
arr = (array.astype(float))/255.0
img_hsv = matplotlib.colors.rgb_to_hsv(arr[...,:3])

# hard-coded thresholds
lower = np.array([0, 10, 60], dtype = "uint8") 
upper = np.array([20, 150, 255], dtype = "uint8")

mask = cv2.inRange(hsv, lower, upper)
res  = cv2.bitwise_and(img, img, mask=mask)

cv2.imshow('frame',img)
cv2.imshow('mask',mask)
cv2.imshow('res',res)

plt.show()

Based on the following HSV histogram:


how would I extract upper and lower thresholds for the skin detection?

Thanks!

[Larz60+]

np.argmax(n)?