image_processing - 1 1. Introduction to image processing...

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1 1. Introduction to image processing 1.1 What is an image? An image is an array, or a matrix, of square pixels (picture elements) arranged in columns and rows. Figure 1: An image — an array or a matrix of pixels arranged in columns and rows. In a (8-bit) greyscale image each picture element has an assigned intensity that ranges from 0 to 255. A grey scale image is what people normally call a black and white image, but the name emphasizes that such an image will also include many shades of grey. Figure 2: Each pixel has a value from 0 (black) to 255 (white). The possible range of the pixel values depend on the colour depth of the image, here 8 bit = 256 tones or greyscales. A normal greyscale image has 8 bit colour depth = 256 greyscales. A “true colour” image has 24 bit colour depth = 8 x 8 x 8 bits = 256 x 256 x 256 colours = ~16 million colours.
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2 Figure 3: A true-colour image assembled from three greyscale images coloured red, green and blue. Such an image may contain up to 16 million different colours. Some greyscale images have more greyscales, for instance 16 bit = 65536 greyscales. In principle three greyscale images can be combined to form an image with 281,474,976,710,656 greyscales. There are two general groups of ‘images’: vector graphics (or line art) and bitmaps (pixel-based or ‘images’). Some of the most common file formats are: GIF — an 8-bit (256 colour), non-destructively compressed bitmap format. Mostly used for web. Has several sub-standards one of which is the animated GIF. JPEG — a very efficient (i.e. much information per byte) destructively compressed 24 bit (16 million colours) bitmap format. Widely used, especially for web and Internet (bandwidth-limited). TIFF — the standard 24 bit publication bitmap format. Compresses non- destructively with, for instance, Lempel-Ziv-Welch (LZW) compression. PS — Postscript, a standard vector format. Has numerous sub-standards and can be difficult to transport across platforms and operating systems. PSD – a dedicated Photoshop format that keeps all the information in an image including all the layers. 1.2 Colours For science communication, the two main colour spaces are RGB and CMYK. 1.2.1 RGB The RGB colour model relates very closely to the way we perceive colour with the r , g and b receptors in our retinas. RGB uses additive colour mixing and is the basic colour model used in television or any other medium that projects colour with light. It is the basic colour model used in computers and for web graphics, but it cannot be used for print production. The secondary colours of RGB – cyan, magenta, and yellow – are formed by mixing two of the primary colours (red, green or blue) and excluding the third colour. Red and green combine to make yellow, green and blue to make cyan, and blue and red form magenta. The combination of red, green, and blue in full intensity makes white.
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3 In Photoshop using the “screen” mode for the different layers in an image will make
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This note was uploaded on 03/01/2012 for the course I.T ICc 231 taught by Professor Ramon during the Fall '10 term at Institute of Management Technology.

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image_processing - 1 1. Introduction to image processing...

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