计算机图形学Colour-Anonymous

7
Color As Spectral Distributions
Spectral radiant power
λ
400
700 Wavelength nm Spectral Energy Distribution
8
Spectrum Efficiency Curve
9
Human Color Perception
Light Intensity and Brightness
Intensity describes the physical amount of energy, brightness describes our perception of this energy;
Our perception of light is function of our eyes, which performs numerous unconscious corrections and modifications. For example, the equal densities of colored light are perceived as being of different brightness depending on the color.
16
Infinite to Finite
Color space is infinite dimensional Visual system filters the energy distribution through a finite set of channels Constructs a finite signal space (retinal level) Through optic nerve to higher order processing (visual cortex).
Color Model
1
Outline
Introduction Spectral distributions Simple Model for the Visual System Simple Model for an Emitter System Generating Perceivable Colors CIE-RGB Color Matching Functions
C(λ) ≥ 0 all λ C(λ) > 0 some λ (at least one λ) .
(Cardinality of this space is 2c)
15
Perception and ‘The Sixth Sense’ Movie
We do not ‘see’ C(λ) directly but as filtered through visual system. Two different people/animals will ‘see’ C(λ) differently. Different C(λ)s can appear exactly the same to one individual (metamer). (Ignoring all ‘higher level’ processing, which basically indicates “we see what we expect to see”).
29
RGB Model
A color we can perceived can be synthesized by any three pure colors that meet certain requirements. These three basic colors are called primitives. The quite often used primitives are Red、Green and Blue。
22
Primaries (Basis) for an Emitter
CE(λ)=α1E1(λ)+ α2E2(λ) + α3E3 (λ) Ei are the primaries (form a basis) αi are called the intensities. CIE-RGB Primaries are:
L-cones M-cones S-cones
19
LMS Response Curves
l = ∫C(λ)L(λ)dλ m = ∫C(λ)M(λ)dλ s = ∫C(λ)S(λ)dλ C → (l,m,s) (trichromatic theory) LMS(C) = (l,m,s) LMS(Ca) = LMS(Cb) then Ca, Cb are metamers.
24
Color Matching Functions
Previous method relied on knowing L, M, and S response curves accurately. Better method based on color matching functions. Define how to get the color matching functions γi(λ) relative to a given system of primaries.
17
A Simple Model for Visual System
Human Eye Schematic
18
Photosensitive Receptors
Rods – 130,000,000 night vision + peripheral (scotopic) Cones – 5-7,000,000, daylight vision + acuity (one point only)(Photopic) Cones
20
2-degree cone normalised response curves
Spectral Sensitivity (L,M,S-cone) 1.20 1.00 0.80 0.60 0.40 0.20 0.00 300 400 500 600 700 800
L M S
wavelength (nm)
The Human Retina – The eye is basically just a camera – Each neuron is either a rod or a cone. – Rods are not sensitive to color, but the cones are!
10
3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 -0.50300 -1.00
intensity
r g b
350 400 450 500 550 600 650 700 750 800
wavelength nm
28
Color Metamer
A given color that we perceived matches with unlimited spectrum distributions. This phenomena is called metamer. So the spectrum can not be used as color metric. The strategy is to choose the simplist spectrum to represent a specific color.
5
Spectrum Character of Color
6
Color and Wave Length
Most light we see is not just a single wavelength, but a combination of many wavelengths like below. This profile is often referred to as a spectrum, or spectral power distribution.
11
Monochromatic Light (Pure Color)
δ(λ) = 0, λ≠0 ∫ δ(λ) d λ = 1 ∫ δ(t) f(x-t)d t = f(x) C(λ) = δ(λ-λ0) is spectral distribution for pure color with wavelength λ0
21
Simple Model for an Emitter System
Generates chromatic light by mixing streams of energy of light of different spectral distributions Finite number (>=3) and independent
3
Primitives of Color
The different visual light spectrum distribution stimulate the eyes and cause the color perception
4
Spectral Distributions
Radiometry (radiant power, radiance etc)
Measurement of light energy
Photometry (luminance etc)
Measurement including response of visual system
Φ(λ) = Kn(λ)/ λ spectral radiant power distribution Generally C(λ) defines spectral color distribution λ∈ [λa, λb] = Λ In computer graphics C is usually radiance.