This week, we’ll be looking at the way color is represented in digital images. Proper digital asset management requires that color is handled properly so that files reproduce predictably.
At the heart of digital imaging is the translation of visual information into numbers. Digital images are basically a rectangle of dots, each representing a color or tone, commonly referred to as a bitmap or a pixmap. There are certain characteristics of a bitmap which are common to most traditional digital images. These characteristics include the color model, color profiles and bit depth. Let’s look at these.
There are a number of different ways of turning color into numbers. Some use the physics of light waves, some rely on the way the eye perceives color, and some are built around the way ink combines to create colors. Each of these color models is useful in different ways. The vast majority of computer-based digital images use the RGB model.
Let’s look at some of the options.
The earliest digital images were often made with indexed color. In this arrangement, the color for each pixel is chosen from a list of possible colors. This can range from 1-bit (two colors, usually pure black and pure white) up to a 8-bit color (256 distinct colors). Indexed color is very economical in file size, but it does not do a very good job rendering photographic images. It was soon replaced with RGB color as a standard method.
RGB is a color model that employs the three primary colors of light: red, green and blue. These can be mixed to make all other colors. Conventional RGB digital images will assign a numerical value for red, green and blue for each pixel. The table below shows examples of the relationship between RGB values and the colors that are produced.
RGB is the native underlying color model for most digital cameras, computer monitors, and lots of image editing software. There are a lot of variations of exactly how to implement an RGB color space, as we will see in the next post.
Here’s a diagram that simulates RGB color. As you add one color to another, the resulting color is a brighter combination of the two. When all three colors are added together, you get white light. This is often referred to as Additive Color.
In addition to RGB, it’s also very common to run across grayscale images: ones that have brightness information, but no color information. In a grayscale image, each pixel can have a value from black to white. Grayscale is often used for black and white imagery (but RGB color is also frequently used). Grayscale is also be used for alpha channels and masks.
Commercial printing, like that which is used for books and magazines, is typically done with a combination of cyan, magenta, yellow and black ink. The first three are the primary colors in pigments, and black is added for extra punch, since it’s difficult to get a deep black with CMY inks.
CMYK color is, therefore, a color model that is useful for preparing images for print reproduction. In CMYK color, the numbers appear to run in reverse compared to RGB. Low numbers indicate less ink, so they describe lighter colors. And high numbers indicate a lot of ink and therefore darker colors.
Because it has four color channels instead of three, a CMYK version of an image is 1/3 bigger than an equivalent RGB image.
Here is a simulation of CMYK color. Since ink absorbs light, as you combine the color together the resulting color is darker. This is the opposite of RGB color and is often referred to as Subtractive Color.
Other color models
In the next post, we will look at color profiles, which are specific flavors of the color models described above.