## CS 486 Data Visualizaztion Color Lecture 2

Instructor: David Thompson
Last Revised: October 28, 1999

## Introduction to Hue, Saturation, and Intensity

 Color Description Variables Comments 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

• Eye contains three sensors, correlating to the three colors that artists had used.
• Correctly summizes why both Newton and artists are correct
• Light is made up of thousands of colors
• Since our eyes have only three sensors, the three primary colors can make up all light that we see.
• Later Hermann von Helmholtz championed the theory.

### 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

• Eye - brain has three opponent systems.
• red-green
• yellow-blue
• black-white
• partially explains why we don't talk about RGB
• explained why we never think about red-green or yellow-blue because they are at opposite ends of the spectrum.

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

• 1959 - eye cone response curve was measured
• 1950 - experiments showed existence of cells that convert RGB to opponent signals.
• Opponent cells - found in the retina, LGN, and visual cortex
• Convert signals to (R - G, Y - B, I) where Y=G + B; and I = R + G
• We still don't think in terms of R-G and Y-B so is there some higher functioning cells?

## Defining Hue, Saturation, and Intensity

### Intensity

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

• brightness - ambient brightness of an object
• reflectivity - perceived reflecting brightness

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.

### Hue

• Can be referred to as color.
• Colors of the Rainbow
• Colors generated by Laser of 400nm to 700nm
• Colors separated by a prism.
• A pure hue refers to just one rainbow color (one frequency)
• One pure color that we perceive but does not exist.

### Saturation

• The purity of color
• 100% saturation means no other colors mixed in
• 0% saturation corresponds to all white (all colors mixed in)
• Need saturation to describe sensations that stimulate all three cones.

• Hue is the location of the peak of the light specturm
• Intensity is the absolute height of that peak
• Saturation measures the releative height of the peak over the background light

## 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?

• At 400nm blue cones are stimulated (Code = --B)
• At 500nm both blue and green cones (Code = -GB)
• At 600nm the blue cone drops out, but pick up the red cone (Code = RG-)
• At 700nm one red cones (Code = R--)
• Laser generated 4
• Code = --- is black; lack of light
• Code = RGB is white and must have all frequencies luminant
• So what's left? (Code = R-B)

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...

### Brown

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

• Desaturated colors such as pink, light blue, tan -- so on.
• The other is brown
• Cannot be produced by a combination of any of the three primaries
• Is only produced by yellow surrounded by brighter areas.
• Try covering your entire vision except for that part looking at a brown object
• This reflectivity is a higher-level activity in our brain.
• Considered to be a psychological color.

### Color Temperature

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

• Color temperature is a shorthand to describe the color balance of a device.
• How much RGB are added to make white.
• All objects give off radiation (heat).
• Most give off in the infared spectrum
• objects in the thousands degrees give off visible light
• The spectrum of thermal radiation is usually known as the black body curve

• Notice as temp increases, luminosity increases
• the peak of luminosity moves to shorter wavelengths
• shape of the spectra are independent of temperature (on log scale)
• Look at only visible section between dashes (next fig)

• Take sections out of visible wavelengths and then center them all about one point to just look at the curves.
• Clearly relative cool light (2000° K) objects generate primarily red photons.
• How about 6000° K, 20,000° K?
• red hot, blue hot, sun.
• So is our monitor really that hot?
• Other ways to generate light other than thermally.
• Incandescents are that hot but Vacuum shields us.
• Fluorescents use quantum mechanics o energy levels in molecules