Image display Display is the final stage in the image processing pipeline: Continuous scenes are acquired and digitally processed. The display process essentially converts the discrete image back to continuous space. Display medium CRT monitors, LCD displays, printers, etc. Other than when used to convey information (as text), objective is to provide a faithful/pleasing representation of a scene. Design is heavily influenced by the properties of the human visual system (HVS).
Display devices Screens; media that use additive primaries Cathode ray tube (CRT) Liquid crystal display (LCD) Gas plasma display Organic light emitting diode (OLED), Surface-conduction electron-emitter display (SED), etc. Printers; media that use subtractive primaries Impact printers (dot-matrix) almost obsolete Inkjet printers Laser printers
CRT 1. Electron guns 2. Electron beams 3. Deflection guns 4. Anode connection 5. Mask for separating beams for red, green, and blue part of displayed image 6. Phosphor layer with red, green, and blue zones 7. Phosphor-coated inner side of screen From Wikipedia.org. Image & details available under the terms of GNU free documentation license.
CRT Shadow mask A metal plate with holes is placed in the layer behind the front glass of screen Most early TVs and computer monitors Curved screens Aperture-grill Fine metallic stripes separate phosphors into strips First introduced by Sony (Trinitron) Flat screens
CRT phosphors Spectral transmittances of common CRT phosphors CRT phosphors have been used to define the srgb color space to ensure consistent description of color across display devices
LCD I I I I I Layer of LC molecules aligned between two transparent electrodes and two polarizing filters Polarizers are orthogonal to each other such that all light is blocked An electric field twists the LC to allow light to pass through Color filters placed in front of the pixel assembly A set of three sub-pixels (red, green, and blue) constitutes a pixel
Subpixel rendering The capability to address subpixels on LCD devices is exploited to render clearer text
Plasma displays Subpixels containing xenon/neon placed between glass plates Transparent electrodes form a grid that has points that intersect at subpixels A subpixel is turned on by charging intersecting electrodes The ensuing UV photons strike colored phosphors Uses PWM to control brightness Since each subpixel is excited individually, it allows for large screen sizes
Dot-matrix printer Print head acts like a typewriter. Print head travels across page and prints by impact on ink-soaked cloth ribbon. Color is achieved by a multi-pass printing process. Sections of the ribbon have different primary colors. During each pass, the head strikes a different primary.
Inkjet printer Liquid ink is deposited directly onto paper. Two main technologies: Thermal bubble Ink is in a cartridge that has multiple nozzles. Heat is used to create an ink bubble. Bubble expands through the nozzle onto paper. Piezoelectric crystal Piezoelectric crystals at the base of the ink cartridge vibrate to push ink out of cartridge and onto paper.
Laser printer Laser/Xerographic printers work as follows: Charge is deposited uniformly on a drum. The rasterized image is scanned onto the drum using a laser and system of lenses. The laser reverses the charge where it strikes. Toner particles are attracted to the drum according to the charge pattern. The patterned toner on the drum is then pressed onto the paper. A fusing stage binds the plastic toner onto the paper using heat and pressure.
Halftones Printers have only 1 bit of grayscale resolution. The trade-off between spatial and grayscale resolution is exploited to give the effect of high grayscale resolution. A group of 4 pixels may be used to represent five different intensities. Larger areas can be used to represent higher grayscale resolutions.
Dithering 8-bit grayscale image 1-bit dithered image
Color rendering On screens that use additive primaries (CRTs, LCDs, plasma displays), colors are formed by the combination of the additve primaries red, green, and blue. Printers use layers of subtractive primaries on paper to render different colors. In addition to the subtractive primaries cyan, magenta, and yellow, typical printers employ a black partition (CMYK printers) to save ink and improve device color gamut.