Thank you everyone! That was some good reading.
I made a sketch (based on the sketch by @solub) to compare different weightings, and in my humble opinion the Relative Luminance is best (on a screen). You can see for yourselves because I have uploaded the results at the end of the message.
I have also uploaded a conversion made in Adobe Illustrator > Edit Colors > Convert to Grayscale. Seems like Adobe also uses Relative Luminance…?
from __future__ import print_function
def setup():
size(1070, 892)
global img
img = loadImage("2018_431_IN3_RET.png")
def draw():
if mouseX <= img.width/4:
image(img, 0, 0)
elif mouseX > img.width/4 and mouseX <= img.width/2:
weight_R = 54
weight_G = 183
weight_B = 18
image(relative_luminance(img, weight_R, weight_G, weight_B), 0, 0)
elif mouseX > img.width/2 and mouseX <= (img.width/4)*3:
weight_R = 77
weight_G = 151
weight_B = 28
image(relative_luminance(img, weight_R, weight_G, weight_B), 0, 0)
else:
weight_R = 85
weight_G = 85
weight_B = 85
image(relative_luminance(img, weight_R, weight_G, weight_B), 0, 0)
line(img.width/4, img.height - 50, img.width/4, img.height)
line(img.width/2, img.height - 50, img.width/2, img.height)
line((img.width/4)*3, img.height - 50, (img.width/4)*3, img.height)
if ((keyPressed) and (key == 'p')):
save("something.jpg")
def relative_luminance(img, weight_R, weight_G, weight_B):
"""
Convert the image to grayscale using true relative luminance weights and bit shifting.
Reference -> https://en.wikipedia.org/wiki/Relative_luminance
"""
lum_img = createImage(img.width, img.height, RGB)
img.loadPixels()
lum_img.loadPixels()
for i in range(len(img.pixels)):
col = img.pixels[i]
# Extract RGB components using bit shifts
r = (col >> 16) & 0xff # Red component
g = (col >> 8) & 0xff # Green component
b = col & 0xff # Blue component
# Calculate the true relative luminance using scaled weights:
lum = (weight_R * r + weight_G * g + weight_B * b) >> 8 # Bit-shift by 8 (dividing by 256)
# Set the grayscale pixel by combining the luminance value into RGB format
lum_img.pixels[i] = (col & 0xff000000) | (lum << 16) | (lum << 8) | lum
lum_img.updatePixels()
return lum_img
