Graphics Input/Output. Types of Printer. Printers. Laser Printers. Inkjet Printers. Printers Digital cameras Scanners

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Frederica McDonald
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1 Printers Digital cameras Scanners Graphics Input/Output Considering How they work Production of quality colour input/output IT82: Multimedia 1 IT82: Multimedia 2 Printers Types of Printer Desirable features for printers: Print quality Speed Cost Compactness Quietness Type of paper used Characters, fonts, formatting Reliability Dot-matrix printers Inkjet printers Laser printers ubiquitous now! Speed from 4 to 40+ pages per minute High resolution at dpi (dots per inch) Colour not just black and white Other types of printer include Plotters Daisywheel printers Richardson, Section 2.4 IT82: Multimedia 3 IT82: Multimedia 4 Inkjet Printers Provides cheap printing (black & white or colour) Works by firing extremely small ink droplets onto paper ink droplets per mm The bubble jet is a popular method: The ink cartridges have tiny heating elements that cause microdrops of ink to heat up and expand (into bubbles) When they expand, they shoot out of the nozzle and onto the paper. Using several colours of inks gives colour printing Laser Printers A laser, bouncing off a mirror, creates a static charge on a rolling drum coated with a photoreactive film. Switching the laser on/off rapidly gives individual pixels. The static charge attracts the toner particles (dry ink) The paper (also charged) pulls the toner off the drum. The fusing rollers melt the toner onto the paper. Colour laser printers need to use 3 or 4 coloured inks Some pass each sheet of paper over 3/4 separate drums Some put the colours onto an intermediate belt, which then passes the colours once onto the paper IT82: Multimedia 5 IT82: Multimedia 6 1

2 Colour Printing Colour produced by full C+M+Y Very complicated to calculate a suitable mix of inks to produce a given colour The printed results are very printer-dependent Need to use a subtractive system - CMY will do, or will it? Printing cyan, magenta and yellow on top of one another doesn t give true black remember that it wasn t claimed the colour models are perfect! Compared to Black IT82: Multimedia 7 IT82: Multimedia 8 CMYK CMYK is CMY with Black (K) Cheaper to use black ink to produce black rather than wasting three times the amount of ink (coloured ink is expensive!) Printing inks causes the paper to become wet. If 1 ink will do the job of three, the process will be faster! Printing black with three inks causes coloured edges to black shapes, due to mechanical error, and that is very noticeable! Some systems for photo-quality printing use a CCMMYK system, e.g. PhotoSmart Not enough colours? Whether on a monitor, printer or other output device, there are techniques to achieve the impression of more colours than there actually are. These are the most common: Quantization Dithering Halftoning Richardson, Section 4.11 IT82: Multimedia 9 IT82: Multimedia 10 Quantization A technique used for analogue fi digital conversion (many different uses, not just colour) If all the values can t be represented, a representative subset is chosen, and each value is mapped onto the closest representative value The selection of the subset can be Uniform (evenly spread across the range) Selected according to frequency of use (more frequently used values remain intact, and less frequently used values get grouped together) In graphics, the values are colours Quantization Examples Digitizing pictures Scanning colour photographs into greyscale Windows 256-colour palette IT82: Multimedia 11 IT82: Multimedia 12 2

3 Dithering Displaying two or more colours close together (using some kind of a pattern) From a distance, the colours blend together to give the required colour Best results are when colours are close, and large areas dithered Examples: Used to display photographs in newspapers Pictures on web pages where the browser has a limited palette This is why black-and-white (greyscale) photographs look better on a colour monitor than in monochrome Half-toning Very fine pattern of dots of varying sizes Using different coloured dots produces colour images Typically used to represent photographs in a magazine (get very close and you may be able to make out the dots) Dots of one colour form a (dithered) pattern, but the individual patterns are laid at angles to reduce possible undesirable side-effects (Moiré patterns) IT82: Multimedia 13 IT82: Multimedia 14 Dithering / Half-toning Example These occur when interference between two patterns produces yet another pattern Moiré Patterns IT82: Multimedia 15 IT82: Multimedia 16 Halftone Angles Human visual perception is very good at spotting patterns 90 degree dot angle (left) appears more regimented than the 45 degree dot angle (right) Conventional Halftone Angles IT82: Multimedia 17 IT82: Multimedia 18 3

4 Digital Cameras These have a 2D array of light sensors The most common are charge coupled devices (CCDs). A CCD is a collection of tiny light-sensitive diodes, where the bright the light that hits the diode, the greater the electrical charge Some cheaper cameras use complementary metal oxide semiconductor (CMOS) technology. CMOS sensors cheaper but lower quality. Richardson, Section Digital Cameras Resolution of a digital camera today might be 640 x 480, certainly enough for an image for a web page Digital cameras have improved a lot but are still lower quality than film A 35mm negative has about 80,000 million photo-sensitive crystals A good digital camera may have about 6 million Scanning a photograph at high resolution (drum scanners can do more than 10,000 dpi) gives more resolution in a 6-inch photograph IT82: Multimedia 19 IT82: Multimedia 20 Scanners Flatbed scanners item lies on clear glass plate whilst head moves beneath it uses reflected light, light generated beneath the glass plate can even scan small 3D items need transparency adapter to scan transparent materials (e.g. slides) user has to place each page individually on the glass Sheetfed scanners item moves past the scanning head less exact because of movement precision takes up less desk space Drum scanners Richardson, Section Scanner Characteristics Detectors used: CCDs are the most common drum scanners use PMTs (photo-multiplier tubes), more expensive CIS (contact image sensors) used sometimes Points to take into consideration when buying: Dynamic Range Bit Depth Scanning Area Speed Resolution IT82: Multimedia 21 IT82: Multimedia 22 Resolution Optical resolution is the physical number of pixels it can see (i.e. the density of sensors on the head) Measured in dpi (dots per inch) Only the smallest number matters (e.g. a 300 dpi x 600 dpi scanner is really just 300 dpi) Interpolated resolution results in lower quality scans Choose a scanner which gives you sufficient resolution for the output you require If you require a higher magnification, the resolution you scan at must be adjusted accordingly Resolution and Output Photos to be printed in a magazine 266 dpi is sufficient (133dpi printing, halftones used) Images for desktop laser printers 300 dpi or 600 dpi is usually sufficient Web graphics, usually 72dpi is sufficient most monitors are dpi Converting images to text, use 300dpi dpi OCR (optical character recognition) systems will usually accept either, but 400dpi gives better results Line art illustrations if being printed, use 1200 dpi (or as much as possible) if for onscreen, 72 dpi IT82: Multimedia 23 IT82: Multimedia 24 4

5 Half-tone scanning Scanned printed material (e.g. from books, magazines) is problematic Sampling from printed halftones results in Moiré patterns and thus lower quality Solutions: scan at higher resolution and then reduce in size use scanning software that takes half-toning into consideration IT82: Multimedia 25 IT82: Multimedia 26 Example Scans magnified from original End of Lecture IT82: Multimedia 27 IT82: Multimedia 28 5

Desktop Publishing 5N0785 Learning Outcome 2 Monaghan Institute Level 5 Module

Desktop Publishing 5N0785 Learning Outcome 2 Monaghan Institute Level 5 Module Contents Learners will be able to describe the characteristics of key components of DTP systems including those used for input,