2.3 Image Representation

Image representation

	A digital image consists of many picture elements (pixels). The quality of the image is defined by the number
	of pixels which is called the image resolution. The higher resolution always yields a better image quality.
	The images can be classified by the number of available colors. The number of colors available to each pixel
	within an image is defined by the bit depth. Bit depth is the number of bits used to represent every pixel in a
	digital image.
	In 1-bit images, each pixel is stored as a single bit (1 or 0).
	In 8-bit images, each pixel is stored as a byte (value between 0 to 255).
	An 8-bit color image has 256 available colors for each pixel.
	In 24-bit color images, each pixel is stored as three bytes. These bytes represent the red, green and blue
	(RGB) values.
	To convert 8-bit color images to 24-bit color images, color look-up tables (LUTs) are used. LUT groups similar
	colors and assign each group to a single entry.

2.3 Image Representation

Image standard formats

	Images have several formats. Some of these formats are system dependent and the others are system
	independent. The standard system independent formats include:

		GIF: GIF stands for Graphics Interchange Format which developed by the UNISYS Corp. and
		Compuserve in 1987 (GIF87a). This format used lossless image compression dictionary technique
		called Lempel-Ziv Welch (LZW).
		TIFF: TIFF stands for Tagged Image File Format which developed by the Aldus Corp. in the 1980's.
		This format can be used to store many different image types.
		PNG: PNG stands for Portable Network Graphics which supersede GIF standard.
		JPEG: JPEG stands for Joint Photographic Experts Group. It used lossy compression technique which
		is based on the frequency transform. It takes the advantage that the human vision system cannot notice
		the small differences caused by removing information because of the lossy compression technique.
		Postscript/Encapsulated Postscript: this format is typesetting languages which includes text and
		and vector/structured graphics images.

2.3 Image Representation

The standard system dependent formats include:

		BMP: BMP stands for bit map format which used by Microsoft Windows. It stores 24-bit bitmap images.
		Macintosh PAINT: PAINT is used in Macintosh system. This format was originally used in MacDraw for
		storing structured graphics.
		XBM: XBM format is used in X-window system. It supports 24-bit color bitmap.