Getting Around in GIMP - Black and White Conversion (Part 5)

So here we are at the end (not really, but too much more and we'll be just beating a dead horse).

What I feel many of posts become...

My intention at this point is to document some sort of workflow that might prove useful to others (and myself - you'd be surprised at how much I learn writing these posts...). So I'll touch on the final topic I wanted to talk about, blending different bits from various layers and working through a couple of examples.

The rest of my GIMP tutorials can be found here: 
Getting Around in GIMP
The rest of the tutorials in this series are here:
B&W Conversion - Part 1 (Desaturate)
B&W Conversion - Part 2 (Channel Mixer)
B&W Conversion - Part 3 (Decompose)
B&W Conversion - Part 4 (Pseudogrey, c2g, Blending)

Bits and Pieces

If you'll recall in Part 3 of this series, I had created a small Script-Fu to automatically decompose an image into various color component decompositions. I think this is a handy script to use if you're still finding your own workflow for grayscale conversions.

What's nice about it is that it will automatically generate a handful of automatic conversions for you as layers on your image. This way, you can visually inspect the results and find areas from each conversion that you like. Then you can use Layer Masks (you can go review my old Layer Masks tutorial to brush up if you'd like) to isolate specific areas and to blend it with others.

Enough talk, let's dive into an example...

Getting Around in GIMP - Black and White Conversion (Part 4)

The first part of this tutorial looked at GIMP desaturate to convert to grayscale, the second part investigated using the Channel Mixer to decompose to grayscale with varying contributions from red, green, and blue, and the third part looked at decomposing an image into its constituent color channels in various modes.

This part of the tutorial will focus on a couple of semi-automated methods for converting to black and white, as well as utilizing GIMP layer blending modes.

The rest of my GIMP tutorials can be found here: 
Getting Around in GIMP
The rest of the tutorials in this series are here:
B&W Conversion - Part 1 (Desaturate)
B&W Conversion - Part 2 (Channel Mixer)
B&W Conversion - Part 3 (Decompose)
B&W Conversion - Part 5 (Putting it All Together)

Getting Around in GIMP - Black and White Conversion (Part 3)

In the first part of this tutorial we had a look at using the Desaturate command to convert images to grayscale. The second part of this tutorial examined the use of Channel Mixer to adjust the contributions of each Red, Green, and Blue channel to the final grayscale result. This part of the tutorial will focus on decomposing the entire image to its component parts for (possibly) further manipulations.

The rest of my GIMP tutorials can be found here:
Getting Around in GIMP
The rest of the tutorials in this series are here:
B&W Conversion - Part 1 (Desaturate)
B&W Conversion - Part 2 (Channel Mixer)
B&W Conversion - Part 4 (Pseudogrey/c2g/Layers)
B&W Conversion - Part 5 (Putting it All Together)

To get a good grasp of what we are about to do, it helps to remember the very first part of this tutorial when we looked at what goes into producing a color pixel on your screen (you remember the R, G, B sub-pixels, right?).

I've also written a small Script-Fu helper script to decompose an image layer to all the different color mode decompositions I'm listing here. Find it at the end of the page!

Decomposing Colors

Previously, in the Channel Mixer we saw that we can adjust the contribution of each of the RGB channels to our final grayscale conversion. From that post, you should have seen that you can isolate a specific channel by setting its value to 100, and leaving the others at 0.

Chad White by Ryan Abel on Flickr (CC-BY)

If you wanted each of the RGB channel contributions isolated into its own layer, it would be tedious to do it manually for each channel. Luckily for us, GIMP has a built in command to automatically Decompose your image into different channels:

Getting Around in GIMP - Black and White Conversion (Part 2)

In the first part of this tutorial we had a look at using the Desaturate command to convert images to grayscale, and how the different options in that command work to produce their results.

The rest of my GIMP tutorials can be found here:
Getting Around in GIMP
The rest of the tutorials in this series are here:
B&W Conversion - Part 1 (Desaturate)
B&W Conversion - Part 3 (Decompose)
B&W Conversion - Part 4 (Pseudogrey/c2g/Layers)
B&W Conversion - Part 5 (Putting it All Together)

We saw how the Desaturate command can use straight numerical evaluations for conversion (Lightness and Average) as well as using the relative luminosity model for how our eyes will perceive brightness based on color.

These are fantastic and easy to use conversion options that require no extra work on your part to get to a grayscale image. Almost every other option from here on out will require you to make choices and adjustments to get your results.

This time around we are going to have a look at another very widely used method for converting to grayscale, the Channel Mixer.

Channel Mixer

Using Desaturate let you convert to grayscale based on pre-defined functions for calculating the final value, but what if you wanted even further control? What if you wanted to decide just how much the red channel should influence the final gray value, or to have more control over the ratios and weightings of each of the different channels independently? That is precisely what the Channel Mixer will allow you to do.

For the examples below, I'm going to mix it up with a different color gradient test map, blue to blue HSV gradient, with a gradient to black vertically. This represents our entire colorspace (feel free to download this image to follow along).

Gradient representing all colors and shades in our colorspace.
To compare using Desaturate: Original - Lightness - Average - Luminosity

So let's have a look.

Getting Around in GIMP - Black and White Conversion (Part 1)

This is a long topic, so to keep you from wanting to put your eyes out with a spoon, I've tried to break things up a bit. In this first part, I'll look at using the GIMP Desaturate command to reduce your images to grayscale and to hopefully shed some light on just how the options calculate exactly what level of gray each pixel should be.

The rest of my GIMP tutorials can be found here: 
Getting Around in GIMP
The rest of the tutorials in this series are here:
B&W Conversion - Part 2 (Channel Mixer)
B&W Conversion - Part 3 (Decompose)
B&W Conversion - Part 4 (Pseudogrey/c2g/Layers)
B&W Conversion - Part 5 (Putting it All Together)

What We're Trying to Achieve

B&W photography deserves a much longer look than I can afford to bore you with here. However, there are a few things I would like to focus on in regards to preparing your images for B&W.

Aspens (no title), Ansel Adams (© The Ansel Adams Publishing Rights Trust)

What you want to keep in mind is that by removing the color information you have effectively left yourself with only tonal data (and composition) to convey your intentions.

This can be both liberating, and confining.

By liberating yourself of color data, you can focus much more clearly on the subjects and composition with whats left. (Indeed, this is often felt to be one of the primary reasons street photography is normally associated with B&W images - with no colors to distract you, the focus is on the subjects and composition even more).

Without color, the form and tones are all that's left. (Edward Weston, Pepper #30)

Tonality

What I tend to refer to when using this term is the presence and relationships between different values of gray in the image. This can be subtle, with smooth, even differences among values, or much more pronounced.

Contrast

Contrast is the relative different in brightness between parts of an image. High contrast will have a much sharper differentiation between lighter and darker portions of an image, while low contrast will show less differences. Often a straight conversion to B&W can result in gray values that are all very similar, yielding a visually "flat" image.

Globally, it may refer to the overall distribution of lights and darks. Locally the same definition applies, but applied across a smaller section of the image.

Dynamic Range

Dynamic range is just the overall range of values being captured in your image. It represents the maximum dark and light that is captured for a given exposure. (Extending this dynamic range in an image is the topic for a later tutorial).

The Approach

The approach I will take here follows similar approaches I had taken in film days. I'll attempt to use different methods of grayscale conversion (and blending them) to get to a working image that is as full of tonal detail overall as possible. (Petteri Sulonen refers to this as his "digital negative") - if you want a great look at a digital B&W workflow, head over and read his article.

Then with the image containing as much tonal detail as possible, I will approach it with adjustments of various types to produce a final result that pleases my eye.

Before we can head down that road, we have to step back and consider the tools we are using. So...

Let's have a look at how an image gets displayed on your monitor!