Human Eye and Color

• Rays of light enter the pupil and strike the back of the eye (retina) – signals go to the optic nerve and eventually to the brain

• Retina has millions (about 6) of photosensitive cells- mostly concentrated in the macula

• Two types of cells: rods (night vision, movement) and three types of cones (red, blue, green) – there is some overlap of the responses – we are most sensitive to green and least sensitive to blue

Color Models and Color Perception

• Color perception can vary – different combinations can produce the same color

• Devices vary

• Many color models: RGB (monitors), CMYK (print), HSB

• RGB and HSB are additive color models and CMYK and mixing paints are subtractive color models

RGB Model• Pure red, pure green and pure blue are

mixed together in varying intensities to form one color

• All three together give pure white and absence of all of them is black

• Values are given as integers from 0 to 255 or from 0.0 to 1.0

• Note the difference: in RGB, mixing red and blue gives magenta (in paints you get purple)

HSB or HSV Model• Hue, saturation, brightness (or value)• Hue is the color name – generally in a circle: 0 is

red -> green -> blue and then back to red (360 or 255)

• Saturation is the purity or intensity of a color (how much white is mixed in)- pure blue has a saturation of 100% or 1.0 or 255 – white has a saturation of 0 as does black and shades of gray

• Brightness (or value) describes contrast from black (0) to white (100% or 1.0 or 255)

• Often represented as a cone with hue the circular cross-section; distance from the center the saturation and top to bottom the brightness

HSB Representation

Illusions

• Illusions from http://www.colorcube.com/illusions/illusion.htm and http://www.eyetricks.com/illusions.htm and http://illusionsetc.blogspot.com/2005/06/kanizsas-triangle-and-marketing-101.html

Kanizsa’s triangle

• http://ccl.northwestern.edu/netlogo/models/OpticalIllusions

Color Interactions

• Colors are influenced by what is around them:

• http://facweb.cs.depaul.edu/sgrais/color_context.htm

• Josef Albers: http://www.albersfoundation.org/Albers.php?inc=Galleries&i=J_1

Lighting and Illumination

• Lighting is the major problem in computer graphics, for either realism or real-time compositions- harder than modeling

• Consider local illumination (only light sources are used, not reflections from other objects), isotropic (not anisotropic where reflected light changes with rotation of object)

• No model can do it all

Lighting: Components

• Objects can emit light (eg. light bulbs, glowing objects, spot lights)

• Light can be diffused from an object

• Light can be reflected off one object to another

• Light can be absorbed (partially or fully) by an object

Light Rays: Which Direction?• We think of “seeing” as if the direction is

from our eyes to a position on the screen (or in the world)

• But rays of light come from sources and objects to our eyes

• Light sources emit light, which then can enter our eyes directly or first interact with an object through specular or diffuse reflection – or interact with several objects before hitting our eyes

Lighting: Realism• In order to accurately represent the effects of

lighting we would need to trace rays from all light sources, let them interact with objects before entering our eyes, and calculate the result using laws of physics

• Recursion could be overwhelming- calculations become impossible

• Some simplifying assumptions must be made• But the more of these calculations that take

place the more realistic the rendering; difference between real-time and realism

Lighting: Reflectance• Objects have different capabilities to

reflect light: bidirectional reflectance distribution function gives the amount of reflected radiance in a given direction for a given incoming direction (BRDF)

• Think of a mirror, a shiny table top, a glossy picture, a matte picture, a black shirt

• Most computer graphics models reduce this function to two properties of the material: specular and diffuse

Lighting: Reflectance (con’t)

• A material’s diffuse reflectance goes in all directions; dependent on position of light source; same for the entire object

• The specular reflectance (mirror-like) goes in one direction and the angle of incidence is equal to the angle of reflectance (with the normal)

• We look at combinations of these two from a light source

• Objects can emit light, reflect light or absorb light

• In a simplified version, reflected light can be thought of as a combination of diffuse and specular

• Objects can also be transparent, partially transparent or opaque

• Also have ambient light, indirect illumination

Shading

• Flat shading: calculate illumination once for each patch or triangle

• Gouraud shading: calculate for each vertex and then use interpolation for each edge and interpolation for each interior point

• Phong uses interpolated normals