LECTURE III: COLOR IN IMAGE & VIDEO DR. OUIEM BCHIR

Transcription

1 1 LECTURE III: COLOR IN IMAGE & VIDEO DR. OUIEM BCHIR

2 2 COLOR SCIENCE Light and Spectra Light is a narrow range of electromagnetic energy. Electromagnetic waves have the properties of frequency and wavelength. The distance from peak to peak of the wave is called the wavelength. Color is characterized by the wavelength content of the light. Laser light consists of a single wavelength: e.g., a ruby laser produces a bright, scarlet-red beam.

3 3 Most light sources produce contributions over many wavelengths. Human cannot detect all light, just contributions that fall in the visible wavelengths. Short wavelengths produce a blue sensation, long wavelengths produce a red one. The human eye responds to the band of wavelengths between 300nm and 700nm

4 4 Most sources used for illumination emit white, or nearly, white light. Using a prism, Newton showed that white light is made up of contributions from all of the visible wavelengths.

5 5 Spectrophotometer: device used to measure visible light, by reflecting light from a diffraction grating (a ruled surface) that spreads out the different wavelengths. White light contains all the colors of a rainbow Visible light is electromagnetic wave in [300nm 700nm]

6 HOW MATERIALS MODIFY LIGHT 6 Vision occurs when light from surrounding objects reaches the eye. Materials modify the light incident upon them in several ways: Light can be reflected from a surface Light can be absorbed by, or transmitted through a surface. In many cases, light is both absorbed by and reflected from a surfaces. The amount of absorption and reflection depends on the wavelength, resulting in some wavelengths being absorbed and others reflected to varying degrees. The wavelengths that are reflected from an object, and perceived by the human eye, is what gives an object its color.

7 7 In this example red light is reflected Blue and green are absorbed Object appears red.

8 Light can be transmitted, or passed through an object in varying degrees. The amount of transmission can also be dependent on the wavelength. 8

9 9 Light is passed through from a colorless, transparent object unchanged. Object will appear clear (e.g. window) Light is partially transmitted. Certain wavelengths are transmitted, others are absorbed. This type of objects appear translucent, and colored (e,g, colored glass) This is the principle behind absorptive filters, which are used to remove or pass certain wavelengths of light

10 10 Inks are formulated to absorb certain wavelengths while reflecting others A surface can create the perception of color by absorbing certain wavelengths and reflecting others.

11 11 Only wavelengths that are present can be reflected. An object will appear its usual color under white light because all wavelengths are present to be reflected.

12 12 A red apple appears red under white light because red is reflected and other wavelengths are absorbed. Apple is illuminated with light of a single color (green). No wavelength in the red region to be reflected All light is absorbed Apple appears dark!

13 13 The apparent color of an object is the product of 1. Object characteristics 2. Light source used for illumination of that object + Human vision

14 14 HUMAN VISION It seems likely that the brain makes use of differences R-G, G-B, and B-R, as well as combining all of R, G, and B into a high-lightlevel achromatic channel.

15 SPECTRAL SENSITIVITY OF THE EYE 15 The eye is most sensitive to light in the middle of the visible spectrum. The sensitivity of our receptors is also a function of wavelength. The Blue receptor sensitivity is much smaller than the Red and Green

16 16 Luminous-efficiency function= Overall sensitivity Denoted V(λ) Formed as the sum of the response curves for Red, Green and Blue. R, G, and B cones, and Luminance Efficiency curve

17 17 IMAGE FORMATION MODEL C(λ): Color signal C(λ ) = E(λ )S(λ )

18 18 CAMERA SYSTEMS Camera systems are made in a similar fashion: a studio-quality camera has three signals produced at each pixel location (corresponding to a retinal position) Analog signals are converted to digital, truncated to integers, and stored. If the precision used is 8-bit, then the maximum value for any of R, G, B is 255, and the minimum is 0. However, the light entering the eye of the computer user is that which is emitted by the screen. Therefore, we need to know the light E(λ) entering the eye.

19 19 GAMMA CORRECTION The light emitted is in fact roughly proportional to the voltage raised to a power, this power is called gamma (γ) If the value in the red channel is R The screen emits light proportional to R^γ The value of gamma is around 2.2.

20 EFFECT OF CRT ON LIGHT EMITTED FROM SCREEN 20 Light output with no gamma-correction applied: darker values are displayed too dark.

21 Effect of pre-correcting signals by applying the power law R1/γ 21