Episode 032: Fade to Gray - Monochrome conversion (1)
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Today I’ll start a series about mono- chrome or black and white conversion of digital images. It’s a new topic for me - so I start with a bit of background, some images from the Masters and a bit about the easy ways to make a quick conversion by desaturation and tweaking the curves a bit.
The image here shows the output of the three options of the desaturation dialogue without further manipulation. From top left to bottom right: lightness, luminosity and average. Which is best? Depends on the image.
In the next episode I’ll show you a better but more complicated way to do this with the channel mixer.
You can find the files used here on the Download Page.
Some links
If you really want to know what’s behind the three options in the desaturate dialogue, go to the GIMP documentation. It’s math. 
The images shown in the beginning of the podcast are from Wikimedia. They have a glorious collection of copyright free images from Ansel Adams and others.
A gold mine is the “American Memory” collection of the US Library of Congress, especially the images made for the Farm Security Administration. A must to check out - don’t let the site design put you off.
You find the Focus Ring feed and the new Focus Ring at the Photocast Network. Check out the other podcasts as well, but stay subscribed to this one.
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Tags: b&w, desaturate, monochrome
February 12th, 2008 at 10:17 pm
This is going to be an interesting series. Now that you mentioned Ansel Adams here, I begin to wonder if you figured out some kind of zone mapping for the picture of the leaf at 23HQ.com.
Very nice show, Serge
February 12th, 2008 at 11:16 pm
No, zone mapping has to start with the exposure - and I shot this with my point&shoot by simply pointing and shooting.
The Zone System is in part translatable to the digital world - Chris Marquardt has done something about that at Tips from the Top Floor..
February 13th, 2008 at 7:02 am
I like to mention something remarkable.
Have you ever tried to convert a chroom object into monochrome? I have.
And only the background turned into monchrome. The chroom object already was monochrome. Isn’t that remarkable? I never thought about it.
February 13th, 2008 at 5:52 pm
Rolf,
Thanks for the link to TftTF. That was elucidating.
It sounds simple, now I have to experiment with it in real life.
February 16th, 2008 at 12:10 am
these are the formulas how the grey value is calculated with the various desaturate methods (from the gimp doc)
Average Brightness = (R + G + B) / 3
Luminosity = 0.21 * R + 0.71 * G + 0.07 * B
Lightness = ½ * (max(R,G,B) + min(R,G,B))
the average method is the simplest method to create a greyscale image… it just treats each color channel equal. this is more a technical way
the luminosity is adapted to the human eye. it reflects the sensitivity of the eye to the different colors
well i don’t know where the lightness method comes from
near the end of the video you showed those small differences in the rgb values (i tried to reproduce this… but somehow i could not find pixels with different rgb values).
but i do not think that these are rounding errors.
this can be a method to virtually increase the number of grey values
it also uses the fact that the sensitivity of red green and blue of the eye is different
for example
0,0,0 <- base color
0,1,0 <- this is a little brighter for the human eye
1,1,1 <- this would be next real grey value
the color fault is so small that it is not visible
if you e.g. draw a gradient from one color to another then this method is also used
February 17th, 2008 at 12:15 am
now i also found out that the lightness method would be the lightness layer when converting/decomposing the image to HSL
February 17th, 2008 at 3:47 pm
Yes, there are countless ways to come to the same result.
An other way is to grab the green channel (least noise) and try to work from there. But channel mixing is even better. See next episode…
March 9th, 2008 at 12:26 am
[...] Episode 032: Fade to Gray - Monochrome conversion (1) [...]
March 27th, 2008 at 10:35 pm
Very good!
July 18th, 2008 at 2:45 pm
Great to find that collection of Ansel Adams photographs which you can use on your web site! I’m a fan of this guy and will be happy to feature some examples of his work on my site, /Link removed by Rolf - commercial site for selling posters./