Light
Light is fundamental for color vision
Unless there is a source of light, there is nothing to see! What do we see? We do not see objects, but the light that has been reflected by or transmitted through the objects
Light and EM waves
Light is an electromagnetic wave
If its wavelength is comprised between 400 and 700 nm (visible spectrum), the wave can be detected by the human eye and is called monochromatic light
What is color?
It is an attribute of objects (like texture, shape, smoothness, etc.)
It depends on: 1) spectral characteristics of the light source(s) (e.g., sunlight) illuminating the objects (relative spectral power distribution(s) SPD) 2) spectral properties of objects (reflectance)
3) spectral characteristics of the sensors of the imaging device (e.g., the human eye or a digital camera)
Primary and Secondary Colors Due to the different absorption curves of the cones, colors are seen as variable combinations of the socalled primary colors: red, green, and blue
Their wavelengths were standardized by the CIE in 1931: red=700 nm, green=546.1 nm, and blue=435.8 nm The primary colors can be added to produce the secondary colors of light, magenta (R+B), cyan (G+B), and yellow (R+G)
Colors in computer graphics and vision
How to specify a color? set of coordinates in a color space
Several Color spaces
Relation to the task/perception blue for hot water
Color Models
The purpose of a color model (or color space or color system) is to facilitate the specification of colors in some standard way A color model provides a coordinate system and a subspace in it where each color is represented by a single point
Color spaces
Device based color spaces:
color spaces based on the internal of the device: RGB, CMYK, YCbCr Perception based color spaces: color spaces made for interaction: HSV
Conversion between them?
Red-Green-Blue Most commonly known color space used (internally) in every monitor additive
The RGB Color Model If R,G, and B are represented with 8 bits (24-bit RGB image), the total number of colors is (28 )3=16,777,216
Cyan-Magenta-Yellow Used internally in color printers Substractive Complementary to RGB: C=1-R
M=1-G
Y=1-B Also CMYK (blacK)
mostly for printer use
CMYK
K is for blacK Save on color inks, by using black ink preferably
K = min(C,M,Y)
C = C-K M = M-K Y = Y-K
The RGB color cube
The HSI Color Model RGB, CMY, and the like are hardware-oriented color spaces (suited for image acquisition and display) The HSI (Hue, Saturation, Intensity) is a perceptive color space (suited for image description and interpretation) It allows the decoupling of chromatic signals (H+S) from the intensity signal (I)
Brightness, Hue, and Saturation
Brightness is a synonym of intensity
Hue represents the impression related to the dominant wavelength of the color stimulus
Saturation expresses the relative color
purity (amount of white light in the color) Hue and Saturation taken together are called
the chromaticity coordinates (polar system)
Matlab conversion function: rgb2hsv
Two HSI Color Models
Example Comparison: CMYK, RGB, and HSI
Class Y color spaces similar to HSI
YIQ, YUV, YCbCr Used in television sets and videos
Y is luminance
I and Q is chromaticity BW television sets display only Y Color TV sets convert to RGB YUV=PAL, YIQ=NTSC
Interests of Class Y
Sometimes you have to use it
video input/output Makes sense in image compression:
better compression ratio if changing class Y
before compression High bandwidth for Y Small bandwidth for chromaticity Lab is fine for that too
YCbCr Color Space is used in MPEG video compression standards
Y is luminance Cb is blue chromaticity Cr is red chromaticity Y = 0.257*R + 0.504*G + 0.098*B + 16 Cr = 0.439*R - 0.368*G - 0.071*B + 128 Cb = - 0.148*R - 0.291*G + 0.439*B + 128 YIQ color space (Matlab conversion function: rgb2ntsc):
