CSCE 441 Lecture 16

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Midterm on Mar. 3

Color

Sharp produced two 4-color monitors: added yellow / cyan (respectively)

Adding a 4th color requires a new encoding standard.

Color Models

CIE XYZ is not very intuitive since the basis axes is not visible.

RGB

Red, Green, Blue

Additive color system:

• each phosphor adds a color
• add all 3 to get white

CMY

Cyan, Magenta, Yellow

Complementary to RGB: $\left\langle c,m,y\right\rangle =\left\langle 1,1,1\right\rangle -\left\langle r,g,b\right\rangle$

Commonly used in printing.

Subtractive color system:

• removes color from reflected light.
• add all 3 to get black

CMYK

Comes from printing process, since CMY combine to form black, we replace equal amounts of CMY with Black, saving ink:

${\begin{aligned}k&=\min {(c,m,y)}\\c&=c-k\\m&=m-k\\y&=y-k\end{aligned}}$

YIQ / YUV

NTSC, PAL standards for broadcast TV

Backward compatible to B&W TV

Y is luminance, only part picked up by B&W televisions; given most bandwidth in signal

I,Q (or U,V) channels contain chromaticity information

HSV

Hue, Saturation, Value

User-friendly way to specify color:

• hue: angle around cone
• saturation: how far from center (gray)
• value: how far up the cone (luminance) from the bottom (black) to the top (white)

Lab and Luv

Perceptually-based color spaces (CIE standards)

Idea: the distance between colors in the color space should correspond to intuitive notion of how "similar" colors are.

Not perfect, but better than XYZ and RGB

Representing Color

3 color channels in equal bits

(not necessary for YIQ)

Can sometimes get better mapping of color space for an application by adjusting bits. (e.g. 10 bits for R, 8 bits for G, and 6 bits for B)

Color Indexing:

• give each color a numerical identifier
• use identifier as reference to desired color
• discrete color palette
• useful for compression

Note: you can use dithering with color channels just like with black and white

Lighting

Color is a function of how light reflect sfrom surfaces to the eye

local illumination

only accounts for light that directly hits a surface and is transmitted to the eye

easy to generate real-time

global illumination

light hits one object, hits another object, and another, etc.

light from all sources as transmitted throughout the environment

responsible for soft shadows.

not done real-time

Reflection

Light incident on a surface interacts with the surface such that it leaves on the incident side without changing frequency.

Types of reflection functions:

• Ideal specular: reflection law; think of a mirror
• Ideal diffuse: lambert's law (scatter light equally in all directions; matte surfaces
• Specular: glossy; directional diffuse reflection; reflects mostly in incident direction, but some blurring occurs