## 10/26/2018

### Dithering python opencv source code (Floyd–Steinberg dithering)

This is dithering example, it make image like a stippling effect.

I referenced to blew website.
wiki page:
https://en.wikipedia.org/wiki/Floyd–Steinberg_dithering
In the source code, there are two functions those are :
dithering_gray, dithering_color
One is for Gary image (1 channel), other is for color (3 channel).
It's very easy to use, just call function~ ^^

import cv2
import numpy as np

def minmax(v):
if v > 255:
v = 255
if v < 0:
v = 0
return v

def dithering_gray(inMat, samplingF):
#https://en.wikipedia.org/wiki/Floyd–Steinberg_dithering
#input is supposed as color
# grab the image dimensions
h = inMat.shape
w = inMat.shape
# loop over the image
for y in range(0, h-1):
for x in range(1, w-1):
# threshold the pixel
old_p = inMat[y, x]
new_p = np.round(samplingF * old_p/255.0) * (255/samplingF)
inMat[y, x] = new_p
quant_error_p = old_p - new_p

# inMat[y, x+1] = minmax(inMat[y, x+1] + quant_error_p * 7 / 16.0)
# inMat[y+1, x-1] = minmax(inMat[y+1, x-1] + quant_error_p * 3 / 16.0)
# inMat[y+1, x] = minmax(inMat[y+1, x] + quant_error_p * 5 / 16.0)
# inMat[y+1, x+1] = minmax(inMat[y+1, x+1] + quant_error_p * 1 / 16.0)
inMat[y, x+1] = minmax(inMat[y, x+1] + quant_error_p * 7 / 16.0)
inMat[y+1, x-1] = minmax(inMat[y+1, x-1] + quant_error_p * 3 / 16.0)
inMat[y+1, x] = minmax(inMat[y+1, x] + quant_error_p * 5 / 16.0)
inMat[y+1, x+1] = minmax(inMat[y+1, x+1] + quant_error_p * 1 / 16.0)

# quant_error := oldpixel - newpixel
# pixel[x + 1][y ] := pixel[x + 1][y ] + quant_error * 7 / 16
# pixel[x - 1][y + 1] := pixel[x - 1][y + 1] + quant_error * 3 / 16
# pixel[x ][y + 1] := pixel[x ][y + 1] + quant_error * 5 / 16
# pixel[x + 1][y + 1] := pixel[x + 1][y + 1] + quant_error * 1 / 16

# return the thresholded image
return inMat

def dithering_color(inMat, samplingF):
#https://en.wikipedia.org/wiki/Floyd–Steinberg_dithering
#input is supposed as color
# grab the image dimensions
h = inMat.shape
w = inMat.shape
# loop over the image
for y in range(0, h-1):
for x in range(1, w-1):
# threshold the pixel
old_b = inMat[y, x, 0]
old_g = inMat[y, x, 1]
old_r = inMat[y, x, 2]
new_b = np.round(samplingF * old_b/255.0) * (255/samplingF)
new_g = np.round(samplingF * old_g/255.0) * (255/samplingF)
new_r = np.round(samplingF * old_r/255.0) * (255/samplingF)

inMat[y, x, 0] = new_b
inMat[y, x, 1] = new_g
inMat[y, x, 2] = new_r

quant_error_b = old_b - new_b
quant_error_g = old_g - new_g
quant_error_r = old_r - new_r

inMat[y, x+1, 0] = minmax(inMat[y, x+1, 0] + quant_error_b * 7 / 16.0)
inMat[y, x+1, 1] = minmax(inMat[y, x+1, 1] + quant_error_g * 7 / 16.0)
inMat[y, x+1, 2] = minmax(inMat[y, x+1, 2] + quant_error_r * 7 / 16.0)
inMat[y+1, x-1, 0] = minmax(inMat[y+1, x-1, 0] + quant_error_b * 3 / 16.0)
inMat[y+1, x-1, 1] = minmax(inMat[y+1, x-1, 1] + quant_error_g * 3 / 16.0)
inMat[y+1, x-1, 2] = minmax(inMat[y+1, x-1, 2] + quant_error_r * 3 / 16.0)

inMat[y+1, x, 0] = minmax(inMat[y+1, x, 0] + quant_error_b * 5 / 16.0)
inMat[y+1, x, 1] = minmax(inMat[y+1, x, 1] + quant_error_g * 5 / 16.0)
inMat[y+1, x, 2] = minmax(inMat[y+1, x, 2] + quant_error_r * 5 / 16.0)

inMat[y+1, x+1, 0] = minmax(inMat[y+1, x+1, 0] + quant_error_b * 1 / 16.0)
inMat[y+1, x+1, 1] = minmax(inMat[y+1, x+1, 1] + quant_error_g * 1 / 16.0)
inMat[y+1, x+1, 2] = minmax(inMat[y+1, x+1, 2] + quant_error_r * 1 / 16.0)

# quant_error := oldpixel - newpixel
# pixel[x + 1][y ] := pixel[x + 1][y ] + quant_error * 7 / 16
# pixel[x - 1][y + 1] := pixel[x - 1][y + 1] + quant_error * 3 / 16
# pixel[x ][y + 1] := pixel[x ][y + 1] + quant_error * 5 / 16
# pixel[x + 1][y + 1] := pixel[x + 1][y + 1] + quant_error * 1 / 16

# return the thresholded image
return inMat

#color ditering
outMat_color = dithering_color(inMat.copy(), 1)
cv2.imwrite('out_color.jpg', outMat_color)

#gray ditering
grayMat = cv2.cvtColor(inMat, cv2.COLOR_BGR2GRAY)
outMat_gray = dithering_gray(grayMat.copy(), 1)
cv2.imwrite('out_gray.jpg', outMat_gray)

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