Lesson 6: Removing and decomposing color

At this point you have gained significant experience on manipulating color. GIMP also offers possibilities to handle color not focusing on specific color ranges but affecting the whole dynamic range of the image in more generic ways. Apart from the fact that these techniques can be a lot of fun, you can use them to be creative at altering an image drastically in imaginative ways.

The tricks exposed in this lesson are important at first because they offer one more sharp insight of what a digital image is from the technical aspect. There is an ongoing analysing approach in this School. Progressively you are exploring ways to extract different types of single-channel information out of multi-channel information. These tools will prove to be invaluable later, when applying effective tricks that will be saving you a lot of time and effort.

The concept of drawable

In GIMP's terminology a drawable is any "surface" that you can "draw" on - alter pixels by drawing or painting or using any color or transformation tools. It is important to understand, that these tools can be used on any drawable. Some tools are not usable on the image itself only, that is to say intended for an immediate impact on the image -- they can also be applied on secondary items like channels and masks, thus being important and critical in the context of secondary tasks inside your workflow.

Removing color from a color photograph

"Colors" > "Posterize..."

Quite simple and well explained in short in the GIMP's Help. It reduces the number of colors in a controlled way. Typically not used in photography, since we might not ever want to reduce the number of colors in a photograph, for hell's sake. It is sometimes used however by graphic artists for special iconic effects. It is simple and easy to understand and if you play with it on a couple of photos you will get the hold of it absolutely.

"Colors" > "Threshold..."

This is going to be important in the process, in workflows like the selection mask workflow. Now please just pay attention to understand the Threshold tool itself.

Sidenote: using the Threshold tool to enhance OCR

You may see some interesting printed text written somewhere - lets us say on a print, like a book or newspaper - which you may want to convert it to electronic text later. This procedure may be of help:

1. you shoot the text with your photocamera, tablet, mobile or other digital device (you can even probably use a snapshot from a video or a screenshot),
2. you open this image data in GIMP,
3. with the Threshold tool you "clear it up" by converting it to real black-and-white, taking care to make it as readable as possible when dragging the two sliders inside the Threshold Tool's dialog,
4. then you import the image data in your OCR (Optical Character Recognition) application.

Converting to the Grayscale color mode

"Colors" > "Desaturate..."

"Colors" > "Components" > "Channel Mixer..."

The Channel Mixer is an effective tool to get rid of color on a color image. You can use it for converting a color image to a monochrome, with a good control of the process. While using Desaturate you convert via predefined recalculation functions, with the Channel Mixer you can play with each channel independently on how much it contributes to the result. Still more, this tool offers the possibility to play with the channels, if you want to keep the color.

"Colors" > "Components" > "Decompose..."

With this tool you can extract channels from a multi-channel image in various ways.

A final note for now

During this Lesson you probably posed a question which seemed to be reasonable enough. Why should i ever be interested in this weird thing called decomposing?

To sum up for now, there are two reasons why extracting an arbitrary 8bit channel out of your color image might be important and useful:

• If you want to leave color, you should want to control the conversion to a Grayscale as much as you can, with the most options available.
• If you want to stay in color, various options for separations offer possibilities to achieve selections and masks that can save you a lot of time and effort, either with the intention to affect color selectively or with the intention to separate/replace optical elements with less effort.

The hidden power of the Channel Mixer can be used in the same fashion. The Channel Mixer used with the "monochrome" option, may be more important than in the case of removing color: you can use it to extract 8bit information. In Lesson 8 you will understand the power of this concept - you can say this a promise.

The key of success on this kind of targeted separation procedures will be your imagination together with the knowing of what exactly you want to accomplish. In the coming lessons you will see ways to exploit these extracts to quickly achieve effective selections or masks.