Ch 6 Color Image processing CS446 Instructor: Nada ALZaben

Color Model Primary colors: ◦ red (R) – green (G) – blue (B) ◦ In 8bit image each color range [0,255]

Color Model Secondary colors produced by adding primary colors together : ◦ Magenta (R+B) – Cyan (G+B) – Yellow (R+G)

Color Model Additive Model (RGB). ◦ RGB as primary, CMYK as secondary ◦ Display color as a result of light being added for that the total absence of color gives Black. ◦ Used in screens (eg. TV) Subtractive Model (CMYK) ◦ CMYK as primary, RGB as secondary ◦ Display color as a result of light being subtracted by the printers inks for that the total absence of color gives White. ◦ Used on paper (eg. Printers)

Color Model whiteBlack No light added No light subtracted RGB Color Model CMYK Color Model R=G=B=255 C=M=Y=100%

Colors characteristics The characteristics generally used to distinguish one color from another are: ◦ Brightness: embodies the chromatic notion of intensity. ◦ Hue: attribute associated with the dominant wavelength in mixture of light waves. ◦ Saturation: refers to the relative purity or the amount of white light mixed with a hue.

How are colors coded in RGB? Each pixel is represented by 3 values. ie. Light yellow = The color is represented in 3 channels In 8 bit image it will range for each color from (0-255) Computers generally display RGB using 24 bit color there for we have 16,777,216 possible colors..

RGB Code: ◦ Pure Red = ◦ Pure Green= ◦ Pure Blue= In a Hexadecimal Code: ◦ Consists of 16 unique symbols: 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E and F ◦ Color is represented by 6 digits (examble: #FFCC66)  Pure Red =  Pure Green=  Pure Blue= Q:Are they the same color (#660099) and ?? How are colors coded?

Converting between coloring codes From RGB to Hexadecimal: ◦ Same as converting from decimal number to hexadecimal number.  Color =#??????  24=( ) 2 = (18) 16  do the rest. From Hexadecimal to RGB: Same as converting from hexadecimal number to decimal number.  #1020FC =  (10) 16 = ( ) 2 = 16

Indexed color If R=G=B, then the color is shade of gray so grayscale colors can be represented by a single value. The more number of bits used the more colors are distinguished, BUT image will be large and for that we may use Color Look UP Table (CLUT) to save memory (eg. Mobile phone memory) ◦ Instead of storing (r,g,b) for each pixal store an index into a palette. ◦ To determine a color of a pixel look at the stored value use it to look up full 24 bit (R,G,B) in palette.

Safe colors