Raster scan
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A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for
In most modern graphics cards the data to be drawn is stored internally in an area of semiconductor memory called the framebuffer. This memory area holds the values for each pixel on the screen. These values are retrieved from the refresh buffer and painted onto the screen one row at a time.
Description
Scan lines
In a raster scan, an image is subdivided into a sequence of (usually horizontal) strips known as "
Scanning pattern
In raster scanning, the beam sweeps horizontally left-to-right at a steady rate, then blanks and rapidly moves back to the left, where it turns back on and sweeps out the next line. During this time, the vertical position is also steadily increasing (downward), but much more slowly – there is one vertical sweep per image frame, but one horizontal sweep per line of resolution. Thus each scan line is sloped slightly "downhill" (towards the lower right), with a slope of approximately –1/horizontal resolution, while the sweep back to the left (retrace) is significantly faster than the forward scan, and essentially horizontal. The resulting tilt in the scan lines is very small, and is dwarfed in effect by screen convexity and other modest geometrical imperfections.
There is a misconception that once a scan line is complete, a
In fact, spikes do occur, both horizontally and vertically, and the corresponding
In electronics, these (usually steady-rate) movements of the beam[s] are called "sweeps", and the circuits that create the currents for the deflection yoke (or voltages for the horizontal deflection plates in an oscilloscope) are called the sweep circuits. These create a sawtooth wave: steady movement across the screen, then a typically rapid move back to the other side, and likewise for the vertical sweep.
Furthermore, wide-deflection-angle CRTs need horizontal sweeps with current that changes proportionally faster toward the center, because the center of the screen is closer to the deflection yoke than the edges. A linear change in current would swing the beams at a constant rate angularly; this would cause horizontal compression toward the center.
Printers
Computer printers create their images basically by raster scanning. Laser printers use a spinning polygonal mirror (or an optical equivalent) to scan across the photosensitive drum, and paper movement provides the other scan axis. Considering typical printer resolution, the "downhill" effect is minuscule. Inkjet printers have multiple nozzles in their printheads, so many (dozens to hundreds) of "scan lines" are written together, and paper advance prepares for the next batch of scan lines. Transforming vector-based data into the form required by a display, or printer, requires a Raster Image Processor (RIP).
Fonts
Computer text is mostly created from font files that describe the outlines of each printable character or symbol (glyph). (A minority are "bit maps".) These outlines have to be converted into what are effectively little rasters, one per character, before being rendered (displayed or printed) as text, in effect merging their little rasters into that for the page.
Video timing
In detail, each line (horizontal frame or HFrame) consists of:
- scanline, when beam is unblanked, and moving steadily to the right
- front porch, when beam is blanked, and moving steadily to the right
- sync pulse, when beam is blanked, and moves rapidly back to the left
- back porch, when beam is blanked, and again moving steadily to the right.
The porches and associated blanking are to provide
Perception
Raster scan on CRTs produces both the impression of a steady image from a single scanning point (only one point is being drawn at a time) through several technical and psychological processes. These images then produce the impression of motion in largely the same way as film – a high enough frame rate of still images yields the impression of motion – though raster scans differ in a few respects, particularly interlacing.
Firstly, due to
Second, by persistence of vision, the viewed image persists for a moment on the retina, and is perceived as relatively steady. By the related flicker fusion threshold, these pulsating pixels appear steady.
These perceptually steady still images are then pieced together to produce a moving picture, similar to a movie projector. However, one must bear in mind that in film projectors, the full image is projected at once (not in a raster scan), uninterlaced, based on a frame rate of 24 frames per second. By contrast, a raster scanned interlaced video produces an image 50 or 60 fields per second (a field being every other line, thus corresponding to a frame rate of 25 or 30 frames per second), with each field being drawn a pixel at a time, rather than the entire image at once. These both produce a video, but yield somewhat different perceptions or "feel"[citation needed].
Theory and history
In a CRT display, when the electron beams are unblanked, the horizontal deflection component of the magnetic field created by the deflection yoke makes the beams scan "forward" from left to right at a constant rate. The data for consecutive pixels goes (at the pixel clock rate) to the digital-to-analog converters for each of the three primary colors (for modern flat-panel displays, however, the pixel data remains digital). As the scan line is drawn, at the right edge of the display, all beams are blanked, but the magnetic field continues to increase in magnitude for a short while after blanking.
To clear up possible confusion: Referring to the magnetic deflection fields, if there were none, all beams would hit the screen near the center. The farther away from the center, the greater the strength of the field needed. Fields of one polarity move the beam up and left, and those of the opposite polarity move it down and right. At some point near the center, the magnetic deflection field is zero. Therefore, a scan begins as the field decreases. Midway, it passes through zero, and smoothly increases again to complete the scan.
After one line has been created on the screen and the beams are blanked, the magnetic field reaches its designed maximum. Relative to the time required for a forward scan, it then changes back relatively quickly to what's required to position the beam beyond the left edge of the visible (unblanked) area. This process occurs with all beams blanked, and is called the retrace. At the left edge, the field steadily decreases in magnitude to start another forward scan, and soon after the start, the beams unblank to start a new visible scan line.
A similar process occurs for the vertical scan, but at the display refresh rate (typically 50 to 75 Hz). A complete field starts with a polarity that would place the beams beyond the top of the visible area, with the vertical component of the deflection field at maximum. After some tens of horizontal scans (but with the beams blanked), the vertical component of the unblank, combined with the horizontal unblank, permits the beams to show the first scan line. Once the last scan line is written, the vertical component of the magnetic field continues to increase by the equivalent of a few percent of the total height before the vertical retrace takes place. Vertical retrace is comparatively slow, occurring over a span of time required for several tens of horizontal scans. In analog CRT TVs, setting brightness to maximum typically made the vertical retrace visible as zigzag lines on the picture.
In analog TV, originally it was too costly to create a simple sequential raster scan of the type just described with a fast-enough refresh rate and sufficient horizontal resolution, although the French 819-line system had better definition than other standards of its time. To obtain a flicker-free display, analog TV used a variant of the scheme in moving-picture film projectors, in which each frame of the film is shown twice or three times. To do that, the shutter closes and opens again to increase the flicker rate, but not the data update rate.
Interlaced scanning
To reduce flicker, analog CRT TVs write only odd-numbered scan lines on the first vertical scan; then, the even-numbered lines follow, placed ("interlaced") between the odd-numbered lines. This is called
Radar
Raster scans have been used in (naval gun) fire-control radar, although they were typically narrow rectangles. They were used in pairs (for bearing, and for elevation). In each display, one axis was angular offset from the line of sight, and the other, range. Radar returns brightened the video. Search and weather radars have a circular display (
Television
The use of raster scanning in television was proposed in 1880 by French engineer
An early use of the term raster with respect to image scanning via a rotating drum is Arthur Korn's 1907 book which says (in German):[5] "...als Rasterbild auf Metall in solcher Weise aufgetragen, dass die hellen Töne metallisch rein sind, oder umgekehrt" (...as a raster image laid out on metal in such way that the bright tones are metallically pure, and vice versa). Korn was applying the terminology and techniques of halftone printing, where a "Rasterbild" was a halftone-screened printing plate. There were more scanning-relevant uses of Raster by German authors Eichhorn in 1926:[6] "die Tönung der Bildelemente bei diesen Rasterbildern" and "Die Bildpunkte des Rasterbildes" ("the tone of the picture elements of this raster image" and "the picture points of the raster image"); and Schröter in 1932:[7] "Rasterelementen," "Rasterzahl," and "Zellenraster" ("raster elements," "raster count," and "cell raster").
The first use of raster specifically for a television scanning pattern is often credited to Baron Manfred von Ardenne who wrote in 1933:[8] "In einem Vortrag im Januar 1930 konnte durch Vorführungen nachgewiesen werden, daß die Braunsche Röhre hinsichtlich Punktschärfe und Punkthelligkeit zur Herstellung eines präzisen, lichtstarken Rasters laboratoriumsmäßig durchgebildet war" (In a lecture in January 1930 it was proven by demonstrations that the Braun tube was prototyped in the laboratory with point sharpness and point brightness for the production of a precise, bright raster). Raster was adopted into English television literature at least by 1936, in the title of an article in Electrician.[9] The mathematical theory of image scanning was developed in detail using Fourier transform techniques in a classic paper by Mertz and Gray of Bell Labs in 1934.[10]
CRT components
- Electronic gun:-
- Primary gun: used to store the picture pattern.
- Flood gun: used to maintain the picture display.
- Phosphor coated screen: coated with phosphors that emit light when an electron beam strikes them.
- Focusing system: focusing system causes the electron beam to converge into a small spot as it strikes the phosphor screen.
- Deflection system: used to change the direction of electron beam so it can be made to strike at different locations on the phosphor screen.
See also
- Broadcast television systems
- Cathode-ray tube
- Computer display standard
- Counter-scanning
- Image resolution
- Raster graphics
- Rasterisation
References
- ^ Leblanc, Maurice, "Etude sur la transmission électrique des impressions lumineuses" (Study on electrical transmission of luminous impressions), La Lumière électrique (Electric light), December 1, 1880
- ^ "Half-Tone Photo-Engraving". The Photographic Times. 25. Scoville Manufacturing Co.: 121–123 1894.
- ^ Josef Maria Eder, Ausführliches Handbuch der Photographie Halle: Druck und Verlag von Wilhelm Knapp, 1897
- ISBN 0-8240-7782-2.
- ^ Arthur Korn, Elektrisches Fernphotograhie und Ähnliches, Leipzig: Verl. v. S. Hirzel, 1907
- ^ Gustav Eichhorn, Wetterfunk Bildfunk Television (Drahtloses Fernsehen), Zürich: Teubner, 1926
- ^ Fritz Schröter, Handbuch der Bildtelegraphie und des Fernsehens, Berlin: Verl. v. Julius Springer, 1932
- ^ Manfred von Ardenne, Die Kathodenstrahlröhre und ihre Anwendung in der Schwachstromtechnik, Berlin: Verl. v. Julius Springer, 1933.
- ^ Hughes, L. E. C., "Telecommunications XX-IV: The Raster," Electrician 116 (Mar. 13):351–352, 1936.
- ^ Pierre Mertz and Frank Gray, "A Theory of Scanning and Its Relation to the Characteristics of the Transmitted Signal in Telephotography and Television," Bell System Technical Journal, Vol. 13, pp. 464-515, July, 1934