7.3 การประมวลผล ภาพสี Color Image Processing

Color is a perceptual manifestation of light which in turn is an electromagnetic signal. Color is a sensation produced by the human eye and nervous system.

Human Visual System Image formation Exposure Control DetectionProcessing Cornea Lens Iris/pupil Photoreceptor sensitivity Retina Rods Cones Brain

Human Eye Human eye is a complete imaging system. Ear side (Temporal) Nose side (Nasal) Cornea Aqueous Humor Pupil Iris Ciliary Muscle Sclera Fovea Retina Optic Nerve Vitreous Humor Eyelens Choroid Suspensory ligament

Overwhelming experimental evidence tells us that the perception of a color is related to the strength of three signals which are transmitted along the optic nerve to the brain.

7.3.1 Color Models

Color Model A "color model" is any method of associating names or numbers with colors. The importance of a color model is that it allows communication about what color should be produced.

Examples include CMYK dot densities as reproduced by an offset press, RGB voltages sent to the electron guns of a cathode ray tube display,

CIE LAB numbers as measured by a "colorimeter", or Pantone color numbers as seen on the "swatch books" published by that company.

Color Model The international Commission on Illumination (CIE, for Commison Internationale de l’Eclairage) developed a device-independent color model based on human perception. Color representation consists in a reduction of the spectral color space to a finite-dimensional space.

1) The RGB (CMYK) Color Model

RGB and its subset CMY form the most basic and well-known color model. This model bears closest resemblance to how we perceive color. It also corresponds to the principles of additive and subtractive colors.

RGB Red, green, and blue are the primary stimuli for human color perception and are the primary additive colors. The relationship between the colors can be seen in this illustration:

The secondary colors of RGB, cyan, magenta, and yellow, are formed by the mixture of two of the primaries and the exclusion of the third. Red and green combine to make yellow, green and blue make cyan, blue and red make magenta.

RGB(CYMK) Model

2) The HSV Color Model

The HSV (Hue, Saturation, and Value) color model is more intuitive than the RGB color model. The user specifies a color (hue) and then adds white or black.

There are 3 color parameters: Hue, Saturation, and Value. Changing the saturation parameter corresponds to adding or subtracting white and changing the value parameter corresponds to adding or subtracting black.

The HSB/HLS Color Model

In HSB, these are hue, saturation, and brightness; in HLS, they are defined by hue, lightness, and saturation.

Hue defines the color itself, for example, red in distinction to blue or yellow. The values for the hue axis run from 0– 360° beginning and ending with red and running through green, blue and all intermediary colors like greenish-blue, orange, purple, etc.

Saturation indicates the degree to which the hue differs from a neutral gray. The values run from 0%, which is no color saturation, to 100%, which is the fullest saturation of a given hue at a given percentage of illumination.

Lightness indicates the level of illumination. The values run as percentages; 0% appears black (no light) while 100% is full illumination, which washes out the color (it appears white). In this respect, the lightness axis is similar to Munsell's value axis. Colors at percentages less than 50% appear darker while colors at greater than 50% appear lighter.

The Munsell Color System

Munsell modeled his system as an orb around whose equator runs a band of colors. The axis of the orb is a scale of neutral gray values with white as the north pole and black as the south pole.

Extending horizontally from the axis at each gray value is a gradation of color progressing from neutral gray to full saturation. With these three defining aspects, any of thousands of colors could be fully described. Munsell named these aspects, or qualities, Hue, Value, and Chroma.

Hue Munsell defined hue as "the quality by which we distinguish one color from another." He selected five principle colors: red, yellow, green, blue, and purple; and five intermediate colors: yellow-red, green- yellow, blue-green, purple-blue, and red- purple; and he arranged these in a wheel measured off in 100 compass points:

Value Value was defined by Munsell defined value as "the quality by which we distinguish a light color from a dark one." Value is a neutral axis that refers to the grey level of the color. This ranges from white to black.

Chroma Chroma is the quality that distinguishes the difference from a pure hue to a gray shade. The chroma axis extends from the value axis at a right angle and the amount of chroma is noted after the value designation.

7.3.2 Color descriptors

1. Color histograms สามารถนิยามได้ในรูปแบบของ tuple โดย C คือ colar space, Ci เป็น color quantization ของเซลล์ที่ I, M เป็นจำนวนเซลล์ และ H(Ci) = Ni/N โดย Ni เป็นจำนวนจุดภาพทั้งหมดในภาพนั้น

2. Color feature descriptor 3. Color names 4. Color-induced sensations