CS 486 Data Visualizaztion
Color Lecture 2

Instructor: David Thompson
Last Revised: October 28, 1999

Introduction to Hue, Saturation, and Intensity

Color Description
Light Spectra (continuous) Always exact: impractical
RGB red, green, blue Matches eye hardware: used for color displays
Opponent Colors red-green, yellow-blue, intensity Output from opponent-color cells
HSI or HSV hue, saturation, intensity Close to perception: used for color description

RGB and Opponent Colors were described in previous lecture

Describe these colors to each other.

How did you describe them? I'm sure you used the terms lighter/darker--maybe blue, red, rust. I'm pretty sure that when describing the second three you didn't say that the chip on the right has more red and green light than the one to its left.

Dueling Color Perception Theories

Young-Helmholtz Theory

1704, Isaac Newton dicovers that ordinary light is made up of a mixture of different colored light; however artists said they could make any color with three basic colors.

1802- Thomas Young

Herring Theory

Do not think in terms of difference in RG and B as the Y-H theory would suggest.

Trouble recognizing boundaries between colors that only differ in Hue -- for example in the following image, it is quite easy to see the 20% difference in Intensity (the right side is darker), but the 20% of hue change is across the first two thirds is more difficult to detect. Hint, Hint!!

Beginning of this century Ewald Herring

Physicists like Young-Helmholtz where as Psychologists like the Herring theory. The two camps didn't talk.

Modern Synthesis

As we know both are right

Defining Hue, Saturation, and Intensity


In general, people convert the sensation of light intensity to the perception of

For example a wall with a shadow on it is still a wall.

Experiments show that our perception of brightness is not directly proportional to light intensity but is directly proportional to the logarithm of light intensity. Hint Hint!!

For example think about light bulbs. The difference between a 50 watt bulb and a 100 watt bulb is much bigger than that of a 100 watt bulb and a 150 watt bulb.

Some hues appear brighter than others. For example, yellow always appear brighter than blue. Why? Because the brain only seems to use R+G for intensity.



Non-Rainbow Colors

A Missing Pure Hue for the majority of humans

Using the three cone system, what are the possible combinations that produce color?

Red Green Blue Code
off off off ---
off off on --B
off on off -G-
off on on -GB
on off off R--
on off on R-B
on on on RGB

Can a tuneable laser which produces all the frequencies of a rainbow produce all eight entries in the table?

Actually, the laser can not provide a pure green signal that doesn't stimulate other cones.

What is R-B? It's purple. In a rainbow, a purple can not be created because it doesn't exist as a single wavelength. Instead it is two wavelengths that we perceive as one color.

The Ultimate Green
Normally we cannot just stimulate green cones; red cones are also stimulated, because of overlapping sensitivities. Consider however, protanopes who are missing red cones. They would be able to stimulate only green cones, and so perceive a type of color that the rest of us cannot imagine. Of course, there is no way they can explain to us how what they see is different than what we see...


There are two additional classes of color (non-pure hues) that are not rainbow colors.

Color Temperature

What does it mean when a monitor says it's color temperature is 5,600 K.