ColorColor

• To understand how to make realistic images, we need a basic understanding of the physics and physiology of vision.

Basics Of ColorBasics Of Color

• Elements of color:

Basics of ColorBasics of Color

• Physics: – Illumination

• Electromagnetic spectra– Reflection

• Material properties• Surface geometry and microgeometry (i.e., polished

versus matte versus brushed)

• Perception– Physiology and neurophysiology– Perceptual psychology

Physiology of VisionPhysiology of Vision

• The eye:• The retina

– Rods– Cones

• Color!

Physiology of VisionPhysiology of Vision

• The center of the retina is a densely packed region called the fovea. – Cones much denser here than the

periphery

Physiology of Vision: ConesPhysiology of Vision: Cones

• Three types of cones:– L or R, most sensitive to red light (610 nm) – M or G, most sensitive to green light (560 nm)– S or B, most sensitive to blue light (430 nm)

– Color blindness results from missing cone type(s)

Physiology of Vision: The Physiology of Vision: The RetinaRetina

• Strangely, rods and cones are at the back of the retina, behind a mostly-transparent neural structure that collects their response.

• http://www.trueorigin.org/retina.asp

Perception: MetamersPerception: Metamers

• A given perceptual sensation of color derives from the stimulus of all three cone types

• Identical perceptions of color can thus be caused by very different spectra

Perception: Other GotchasPerception: Other Gotchas• Color perception is also difficult because:

– It varies from person to person– It is affected by adaptation (stare at a light

bulb… don’t)– It is affected by surrounding color:

Perception: Relative IntensityPerception: Relative Intensity

We are not good at judging absolute intensity• Let’s illuminate pixels with white light on scale of 0 - 1.0• Intensity difference of neighboring colored rectangles with

intensities:– 0.10 -> 0.11 (10% change)– 0.50 -> 0.55 (10% change)

will look the same• We perceive relative intensities, not absolute

Dynamic RangesDynamic Ranges Dynamic Range Max # of Display (max / min illum) PerceivedIntensities (r=1.01)

• CRT: 50-200 400-530• Photo (print) 100 465• Photo (slide) 1000 700• B/W printout 100 465• Color printout 50 400• Newspaper10 234

How well do we see color?How well do we see color?

• What color do we see the best?– Yellow-green at 550 nm

• What color do we see the worst?– Blue at 440 nm

• Can perceive color differences of 10 nm at extremes (violet and red) and 2 nm between blue and yellow

How well do we see color?How well do we see color?

• 128 fully saturated hues can be distinguished

• Cannot perceive hue differences with less saturated light.

• Sensitivity to changes in saturation for a fixed hue and brightness ranges from 16 to 23 depending on hue.

Combining ColorsCombining Colors

Additive (RGB) Subtractive (CMYK)

Color SpacesColor Spaces

• Three types of cones suggests color is a 3D quantity. How to define 3D color space?

• Idea: shine given wavelength () on a screen, and mix three other wavelengths (R,G,B) on same screen. Have user adjust intensity of RGB until colors are identical:

CIE Color SpaceCIE Color Space• The CIE (Commission Internationale d’Eclairage)

came up with three hypothetical lights X, Y, and Z with these spectra:

• Idea: any wavelength can be matched perceptually by positive combinations of X,Y,Z

Note that:X ~ RY ~ GZ ~ B

CIE Color SpaceCIE Color Space

• The gamut of all colors perceivable is thus a three-dimensional shape in X,Y,Z

• Color = X’X + Y’Y + Z’Z

CIE Chromaticity Diagram CIE Chromaticity Diagram (1931)(1931)

For simplicity, we often project to the 2D plane X’+Y’+Z’=1

X’ = X’ / (X’+Y’+Z’)Y’ = Y’ / (X’+Y’+Z’)Z’ = 1 – X’ – Y’

LAB SpaceLAB Space

Device Color GamutsDevice Color Gamuts

• Since X, Y, and Z are hypothetical light sources, no real device can produce the entire gamut of perceivable color

• Example: CRT monitor

RGB Color SpaceRGB Color Space

• Define colors with (r, g, b) amounts of red, green, and blue

Device Color GamutsDevice Color Gamuts

• The RGB color cube sits within CIE color space something like this:

Device Color GamutsDevice Color Gamuts

• We can use the CIE chromaticity diagram to compare the gamuts of various devices:

• Note, for example, that a color printercannot reproduceall shades availableon a color monitor

HSV Color SpaceHSV Color Space• A more intuitive color space

– H = Hue– S = Saturation– V = Value (or brightness)

ValueSaturation

Hue

HSI and HSL HSI and HSL

CMYK SpaceCMYK Space

Color WheelsColor Wheels

The Color WheelThe Color Wheel

The color wheel fits together like a puzzle - each color in a specific place.

Primary ColorsPrimary ColorsPrimary colors are not mixed from other

elements and they generate all other colors.

• Red• Yellow• Blue

Secondary ColorsSecondary ColorsBy mixing two primary colors, a

secondary color is created.

• Red + Yellow = Orange• Yellow + Blue = Green• Blue + Red = Purple

Intermediate, or Tertiary, colors are created by mixing a primary and a secondary.

•red-orange•yellow-orange•yellow-green

•blue-green•blue-purple•red-purple

Intermediate ColorsIntermediate Colors

The principles of color mixing let us describe a variety of colors, but there are still many colors to explore. The neutral colors contain equal parts of each of the three primary colors. Black, white, gray and sometimes brown are considered "neutral”.

Neutral ColorsNeutral Colors

Color values are the lights and darks of a color you create by using black and white (‘neutrals”) with a color. This makes hundreds of more colors from the basic 12 colors of the wheel.

• white + color = tint• color + black = shade

Tints are lightened colors. Always begin with white and add a bit of color to the white until the desired tint is obtained. This is an example of a value scale for the tints of blue.

TintsTints

Shades are darkened colors. Always begin with the color and add just a bit of black at a time to get the desired shade of a color. This is an example of a value scale for the shades of blue.

ShadesShades

Color Schemes are a systematic way of using the color wheel to put colors together… in your art work, putting together the clothes you wear, deciding what colors to paint your room…..

monochromatic, complementary, analogous,

warm and cool.

“Mono” means “one”, “chroma” means “color”… monochromatic color schemes have only one color and its values. The following slide shows a painting done in a monochromatic color scheme.

MonochromaticMonochromatic

This non-objective painting has a monochromatic color scheme - blue and the values (tints and shades) of blue.

Complementary colors are opposite on the color wheel provided a high contrast - if you want to be noticed wear complementary colors!

ComplementaryComplementary

This painting has complementary colors and their values - blues and oranges.

The analogous color scheme is 3-5 colors adjacent to each other on the color wheel. This combination of colors provides very little contrast.

AnalogousAnalogous

Analogous colors are illustrated here: yellow, yellow-green, green and blue-green.

Warm colors are found on the right side of the color wheel. They are colors found in fire and the sun. Warm colors make objects look closer in a painting or drawing.

WarmWarm

This is an illustration of the use of warm colors - reds, oranges and yellows.

Cool colors are found on the left side of the color wheel. They are the colors found in snow and ice and tend to recede in a composition.

CoolCool

Note the cool color scheme in this painting (greens, purples and blues).