5. Colorspaces and Blending Modes
It is amazing that almost every color we see can be decomposed into a
mixture of just three primary
colors. Most children
learn this at school and know that the primary colors are blue,
yellow, and red. Using finger paints, kids can experiment making the
so-called secondary colors green, orange, and purple. If hearing
worked this way, every musical note could be decomposed into a sum of
just three tones. Three tones might make a nice guitar chord, but
it's not enough to rock-and-roll.
This interesting fact, that three colors suffice, is due to the way
human vision functions. There are three different types of cones in
the eye (red, green, and blue); this is what accounts for only three
colors being needed. The fact that all the colors can be reproduced
by mixing only three is extremely useful. It's like an alphabet.
With a small collection of letters, any word can be made and any
meaning can be conveyed. The lesson is the same for color. With only
three color phosphors a computer monitor can, in principle, express
any color; with only three color inks, in principle, a printer can
print any color.
There are some practical limitations to the concept of reproducing all
colors with only three primaries. However, without going into more
detail, it suffices to know that the set of colors that can be made by
adding different amounts of three primaries is called a colorspace.
The shape of the space defined in this way is a cube.
In principle, the three primaries used to produce a colorspace are not
unique. In theory, any triple of independent colors can be used to
decompose a colorspace. However, in practice, colorspaces created by
two different sets of primaries are not identical. Typically, there
is a part of the color range of one space that cannot be reproduced by
The best choice of primaries usually depends on the type of physical
device used to create the color. Monitors use red, green, and blue
phosphors to create a colorspace called RGB. Printers use cyan,
magenta, and yellow inks and work in a colorspace called CMY. It is
the physical way colors are combined that explains the two spaces.
Monitors emit light, whereas inks only reflect light.
When we shine a white light on ink, what we see is the color component
of the white light that is reflected and not absorbed by the ink.
Knowing about colorspaces is important in the GIMP for several
reasons. To effectively correct color in an image requires some
notions about how colors interact. Furthermore, looking at different
color components of an image can often be quite useful for making
selections and masks.
This chapter is composed of two main parts. The first part begins
with a tutorial on three colorspaces: RGB, HSV, and CMYK. The RGB
space is discussed in detail, and this is followed by a description of
the relationship between the RGB and HSV spaces. HSV is a more
perceptually useful space than RGB and finds many uses in the GIMP.
Although the GIMP is not CMYK-capable, this important pre-press
colorspace is also discussed. Its benefits and drawbacks are
described and its importance to pre-press is explained.
The second part of the chapter presents transparency and the GIMP's 16
blending modes from a colorspace perspective. Blending modes are
powerful tools for combining colors between layers. You can also use
them to control how the painting tools apply color. The colorspace
presentation in the first part of this chapter will be very useful in
understanding how these modes work. The final section of this chapter
presents some practical uses of the blending modes.
- 5.1 The RGB Colorspace
- 5.2 The HSV Colorspace
- 5.3 Relating HSV to RGB
- 5.4 The Subtractive Color Systems CMY
- 5.5 Conversion to Grayscale
- 5.6 The Blending Modes
- 5.6.1 The Normal, Dissolve, and Behind Blending Modes
- 5.6.2 The Addition, Subtract, and Difference Blending Modes
- 5.6.3 The Multiply (Burn), Divide (Dodge), Screen, and Overlay Blending Modes
- 5.6.4 The Darken Only and Lighten Only Blending Modes
- 5.6.5 The Hue, Saturation, Value, and Color Blending Modes
- 5.7 Opacity and Transparency
- 5.8 Practical Uses of Blending Modes
- 5.9 Common Problems and
Frequently Asked Questions