Digital Image Processing 6. Color Image Processing Computer Engineering, Sejong University Category of Color Processing Algorithm Full-color processing Using Full color sensor, it can obtain the image Color TV camera Using in Color scanner Can be applied in Publishing, Internet, Display, etc, many various fields. 2/46
pseudo-color processing Using Monochrome sensor, that can obtain the image Until recently using black and white in most of Image Processing Black and White TV camera Color processing of the appropriate values for the gray-scale value Compared to Black and White image, the ablilty of Image Segmentation is Improved You can apply medical, observation, computer Vision, etc, it can apply many various fields. 3/46 The Discovery of Color Isaac Newton : Find a color separated sunlight through the prism In 1666 4/46
The Recognition of Color The stimulation of optic is delivered to the brain, so that you can recognize color Color recognition mechanism is not fully understood yet. Recognition of Color - The pressure of pupil - Dream, Hallucination - visible light of various wavelengths that stimulate optic 5/46 The sunlight of spectral power Violet Indigo Blue Green Yellow Orange Red 6/46
Characteristic of Various Objects Reflection Violet Indigo Blue Green Yellow Orange Red 7/46 Difference of Light D65 : natural light A : light bulb Violet Indigo Blue Green Yellow Orange Red 8/46
The Reason of Color Generation The Difference of Light The different power of light generate from the different wavelengths The Difference of Reflectivity The difference of reflectivity of visual light frequency, so that it represent different color in the same light Refraction The difference of refractive index of visual light frequency (Newton s prism) Fluorescence Absorb non visual light and then generate visual light 9/46 Human Photoreceptors 10/46
Color receptors The case of Cone, receipt three types of Color Type S : It response to small wave of visual light Type M : It response to middle wave of visual light Type L : It response to long wave of visual light There is a little difference with R,G,B component S M L 11/46 Color Appearance Color appearance is influenced by various factor Nearby Colors Adaptation to previous views State of mind Film color mode : the way to measure color accurately When you measure color, recognizing it through hole Make outer of hole to gray, so that get rid of the Nearby Colors influence 12/46
The Effect of Around color 13/46 14/46
15/46 Quantification of Color Need to represent Accuracy Color Many products represent specific color Limits of explain color as language Different expression in language from the same color For example: digital library different color expression from the same picture The principle of trichromacy When you measure color with Film color mode, can express almost every color with three primary color s weighted sum Make outer of hole to gray, so that get rid of the Nearby Colors influence 16/46
Film Color Mode 17/46 Primary Color Mixture of light Additive primaries Red, Green, Blue Color monitor Mixture of pigments Subtractive primaries Cyan, Magenta, Yellow Color printer 18/46
Additive color mixing 19/46 Subtractive color mixing 20/46
Color matching test 21/46 22/46
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29/46 Additive Matching Let s assume that Three primary color is A, B, C You can express every color use Three Primary color s sum M = aa + bb + cc When you can express color with this pormula, this is additvie matching Equal(=) is meaning matching(it recognize same color) In that case you can express color with (a, b, c) 30/46
Subtractive Matching Let s assume that Three primary color is A, B, C Some color can t express addictive matching Instead M + aa = bb + cc If you can express color with this formula, it can say subtractive matching Equal is meaning matching(it can recognize same color) In that case you can express color (-a, b, c) Case of Monitor You can t express all of color just using visual light If you install three primary color, you have to setting as possible as you can express color 31/46 Color matching test -using single color 645.2nm, 526.3nm, 444.4nm -The result of color matching experience, there is a minus -You need to have Subtractive matching 32/46
The Result of Color Matching Using Three Primary color to matching all of color You need Subtractive matching not only Addictive matching Most of Subject matching using Three Primary color But, some of Color-blindness matching using Two Primary Color Most of participant have same matching result 33/46 Linear Color Space When selecting the three primary colors, it is possible to configure a linear color space Coordinate of the color can be expressed as a percentage of the corresponding primary color The type of the three primary colors determines the type of the color space RGB color space A monochromatic light of three primary colors are configured at the 645.2nm, 526.3nm, 444.4nm Has a negative value When only positive implementation, there is a color that can not be expressed XYZ color space Use a hypothetical three primary colors. Color coordinate expression : (X,Y,Z) => The mathematical process is easy Generally, this is expressed by using the (x, y) x = X/(X+Y+Z), y = Y/(X+Y+Z) 34/46
RGB Color space Pixel depth: the number of bits used to represent each pixel in RGB space Full-color image: 24-bit RGB color image (R, G, B) = (8 bits, 8 bits, 8 bits) 35/46 RGB Color space 36/46
Safe RGB Colors Separately define the color component to be used in a lot of colors that can be represented in the RGB color space A subset of the full RGB color Using a total of about 256 colors Separating each of the RGB components to Step 6 The color 216(6x6x6), the operating system is used in common. To be defined as needed, the remaining 40 colors. 37/46 Safe RGB Colors 38/46
RGB to XYZ Eliminates the negative component of the RGB color space. RGB => XYZ conversion through a matrix operation X 2.7689 Y 1.0000 Z 0.0000 1.7528 4.5907 0.0565 1.1302 R 0.0601 G 5.5943 B 39/46 XYZ Color space The two-dimensional representation of XYZ color space When XYZ color space was developed(1931), there was a difficulty in the three-dimensional representation of the color space 개념전개의용이성 x X X Y Z, y X Y Y Z, z X Z Y Z x y z X X Y Y Z Z 1 In the plane of x+y+z = 1, x, y value is represented in the XYZ color space. - The value of z is determined automatically by x,y value 40/46
XYZ Color space 41/46 42/46
CIE 1931 Chromaticity diagram CIE( Commission Internationale de l Eclairage) 43/46 CIE Chromaticity diagram - The area is only visible inside the horseshoe-shaped area - In the coordinates x, y adding the value, that can obtain a value of z 44/46
Black body radiation Black body :The object that absorbs all light outside Heat the black body when It generate light The color of the light generated by the black body is expressed as a function of temperature(planck's radiation formula) 1 5 E 1 exp hc k T 1 Color temperature : Indicates the color of the light source to the color matching to the color temperature of the black body temperature that occurs in a specific When the color temperature is low, the red line. When the color temperature is high, the blue line Unit is the absolute temperature K Light bulbs : 2,800K, Fluorescent lamp : 4,500 ~ 6,500K 45/46 Relationship between the RGB and XYZ coordinate system By additivity of colors: Any color inside the triangle can be produced by combinations of the three initial colors RGB gamut of monitors Color gamut of printers 46/46