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

Basics Of Color Elements of color:

Basics 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 Vision The eye: The retina –Rods –Cones Color!

Physiology 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: 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 Retina Strangely, rods and cones are at the back of the retina, behind a mostly- transparent neural structure that collects their response. rg/retina.asp

Perception: 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 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 Intensity We are not good at judging absolute intensity Let’s illuminate pixels with white light on scale of 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 Ranges Dynamic RangeMax # of Display(max / min illum)Perceived Intensities (r=1.01) CRT: Photo (print) Photo (slide) B/W printout Color printout50400 Newspaper10234

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? 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 Colors Additive (RGB)Subtractive (CMYK)

Color 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 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 ~ R Y ~ G Z ~ B

CIE 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 (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 Space

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

Device Color Gamuts The RGB color cube sits within CIE color space something like this:

Device Color Gamuts We can use the CIE chromaticity diagram to compare the gamuts of various devices: Note, for example, that a color printer cannot reproduce all shades available on a color monitor

HSV Color Space A more intuitive color space –H = Hue –S = Saturation –V = Value (or brightness) Value Saturation Hue

HSI and HSL

CMYK Space

Color Wheels

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

Primary Colors Primary colors are not mixed from other elements and they generate all other colors. Red Yellow Blue

Secondary Colors By 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 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 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. Tints

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. Shades

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. Monochromatic

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! Complementary

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. Analogous

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. Warm

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. Cool

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