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In image editors, especially powerful ones like GIMP and Photoshop, there happen to be a bevvy of ways to turn a color image into a black and white one, but not all techniques are the same.

How do you analyze an image for black and white conversion to determine which black and white conversion methods would work best?

What are some different methods for converting black and white images?

Some good questions related to black & white here on Photo.SE:

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I think this should be CW, but my understanding is, answers to CW questions are automatically made CW, and authors won't get rep. I'd like to see more opportunities for answers to get rep, so I am leaving this non-CW for now. –  Alan Sep 18 '10 at 15:55
    
— yeah, there's no reason for this to be CW. –  mattdm May 21 '11 at 12:11
    
    
except this is the older question, so should that one be the dupe? –  Paul Cezanne Mar 19 '13 at 14:09
    
Yes, how is an older question a duplicate? –  Alan Mar 20 '13 at 16:39

4 Answers 4

A good place to start is by looking at each individual colour channel. This is easily done in Photoshop (and I assume GIMP) by opening up the channels palette and simply selecting either the red, green or blue channel. Usually one channel will simply look better than the others, sometimes you'll want to add a bit of another channel in, this is where the channel mixer tool comes in really handy. You can even subtract channels, so for example your final mix might be 0.5*R + G - 0.25*G

It can take a bit of experimentation but I would recommend looking at each colour channel in isolation to give you a good idea where to start!

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The easiest method is to just drop your saturation to zero. That leeches all the color out of the image. It may not be the effect that you're looking for, however.

Another method is to add the results of different color channels in various combinations to one another to make a single intensity value at each pixel location.

So, say, for instance, you could do this:

R + G + B = I

Where R is the red intensity at a given location, G is green, B is blue, and I is your final intensity.

You can then weight them:

R*r1 + G*g1 + b*b1 = I

Where those r1, g1, and b1 are constants for each channel.

You can also transform them, by, say, applying a histogram equalization to each channel prior to combination:

T1(R)*r1 + T2(G)*g1 + T3(B)*b1 = I

Where your T1 is a transfer function (mapping of one set of pixel values into another). Simple transfer functions are things like histogram equalizations, contrast adjustments, and other single-pixel modifications.

This is all from a mathematics perspective, ie, you're doing modifications in a program or in something like Matlab. You can also see how the transfer functions could get increasingly complex, combining neighborhood information and the like.

If you don't want to go that route (and I don't, not often, anyway), there are off the shelf solutions, in Lightroom or Nik. Here's a good review article on the last two, and this article and this article are also good primers.

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Thank you so much for "R*r1 + G*g1 + b*b1 = I". I was having a really hard time to figure out what the Channel Mixer in darktable actually does exactly. –  gojira Jan 21 at 3:16

GIMP has a "Channel mixer" tool that allows you to convert to grayscale using arbitrary ratios. If you have areas where different colors dominate, you can use it to emphasize contrast.

There's an example below, but it's really best to play with it and see for yourself. Checking "Preserve luminosity" uses just ratios of the channels, so you don't have to care about clipping the result by adding everything on 200%.

channel mixer 1channel mixer 2

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I'd suggest a film simulator, which simulates how various black and white films would respond to the given color signal. I use the iNDA plugin for Bibble, but it's based on a GIMP plugin I believe. It produces results that I like, it's very quick to try B&W on an image, and I like not having to think too much about all the parameters other answerers mention (YMMV of course).

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