Color is an important concept in many areas, and because of that, any cross-discipline discussion of color risks to be quite confusing to its participants. When artists, for example, use the word ‘tone’, they can mean both ‘hue’ and ‘lightness’. In the world of digital images, however, the term “luminosity” gives way to “brightness” – a word, which for an artist would indicate the level of saturation rather than lightness of a hue. So what do I mean, when I use those terms (and some more)?
First, let’s deal with brightness. Since there are other, less ambiguous definitions for the parameters it describes, I will not use it at all. Instead, I will employ two straightforward terms for two vital attributes of color – lightness and saturation. Following age-old artistic tradition, I will regard tone and lightness as synonyms. I will mostly talk about lightness and use tone as an alternative to avoid sounding too repetitive. This leaves the third parameter of color – hue, i.e. the color itself (the word color is used to signify the combination of all three attributes: hue, lightness and saturation). And while we are on the subject, let me just briefly note on the word shade. In everyday usage, shade can mean the presence of a different hue in a given color, i.e. chromatic heterogeneity. It can also refer to differences in lightness and saturation – both tint and shade – of all hues, including achromatic ones (as in, “shades of gray”). Where the context allows, I will use the word shade to denote any variation of color.
To recap: when discussing color properties, I am going to stick to tradition and talk about lightness, hue and saturation. This is how these three terms are understood in the context of this book:
Lightness is a perceived brightness of an area in the field of view. It is evaluated against the subjective brightness of any object viewed as white in the given conditions of lightning (luminosity in case of computer screen). Lightness depends on several factors, most notably, on the level of lighting (luminosity) around an object, and on the ability of this object to reflect and/or absorb light. Other factors influencing lightness include the angle of light and distance to the observed object (optical haze). Lightness of the observed object can thus change if:
- lighting conditions (screen luminosity) are altered;
- the hue is tinted (by adding white) or shaded (by adding black);
- there is a combination of the first two changes.
Lightness of a color does not depend on the way this color is reproduced. Whether an image is printed on paper or viewed digitally, each individual viewer will determine its lightness. Hue is a characteristic of color (chromatic) part of perception. The basic hues that humans perceive are red, blue, green, orange, violet, etc. Saturation is a property of color that tells us about its intensity. That is, the level of its perceived difference from an achromatic hue of similar lightness.
Before we begin talking about color in digital images, we should agree on the principles of numeric representation of color values in different color spaces. Throughout this book, I intend to analyze the colors of many images and describe the changes that occur after color correction manipulations. To do that, I prefer to use the value descriptors of two color spaces: RGB and Lab. There are no unified international standard of numeric/textual description of digital colors, so let’s at least agree on something that will hold stable across this text. To simplify the notation, I decided to go numerical and adopt the following scheme:
For RGB color space: RGB (x, y, z), where x, y and z are color values in R (Red), G (Green) and B (Blue) channels.For example: RGB (117, 92, 246).
For Lab color space: Lx ya za, where x, y and z are values in L (Lightness), while a and b are channels with color information. Note, that both a and b can take on negative values, too. To indicate lightness, L can be used separately from a and b. For example: 47L -15a 23b or 64L.
I do hope this short discussion of terms and definitions helped clear the initial confusion, and we can finally start talking about color.
LIFELIKE: A book on color in digital photography