raster scan rasterscan display sample a raster scan or raster scanning is the rectangular pattern of image capture and reconstruction in television by analogy the term is used for raster graphics the pattern of image storage and transmission used in most computer bitmap image systems the word raster comes from the latin word rastrum a rake which is derived from radere to scrape see also rastrum an instrument for drawing musical staff lines the pattern left by the lines of a rake when drawn straight resembles the parallel lines of a raster this linebyline scanning is what creates a raster it is a systematic process of covering the area progressively one line at a time although often a great deal faster it is similar in the mostgeneral sense to how ones gaze travels when one reads lines of text description scan lines in a raster scan an image is subdivided into a sequence of usually horizontal strips known as scan lines each scan line can be transmitted in the form of an analog signal as it is read from the video source as in television systems or can be further divided into discrete pixels for processing in a computer system this ordering of pixels by rows is known as raster order or raster scan order analog television has discrete scan lines discrete vertical resolution but does not have discrete pixels horizontal resolution it instead varies the signal continuously over the scan line thus while the number of scan lines vertical resolution is unambiguously defined the horizontal resolution is more approximate according to how quickly the signal can change over the course of the scan line scanning pattern the beam position sweeps follow roughly a sawtooth wave in raster scanning the beam sweeps horizontally lefttoright 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 1horizontalresolution 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 crt display in effect suddenly jumps internally by analogy with a typewriter or printers paper advance or line feed before creating the next scan line as discussed above this does not exactly happen the vertical sweep continues at a steady rate over a scan line creating a small tilt steadyrate sweep is done instead of a stairstep of advancing every row because steps are hard to implement technically while steadyrate is much easier the resulting tilt is compensated in most crts by the tilt and parallelogram adjustments which impose a small vertical deflection as the beam sweeps across the screen when properly adjusted this deflection exactly cancels the downward slope of the scanlines the horizontal retrace in turn slants smoothly downward as the tilt deflection is removed theres no jump at either end of the retrace in detail scanning of crts is performed by magnetic deflection by changing the current in the coils of the deflection yoke rapidly changing the deflection a jump requires a voltage spike to be applied to the yoke and the deflection can only react as fast as the inductance and spike magnitude permit electronically the inductance of the deflection yokes vertical windings is relatively high and thus the current in the yoke and therefore the vertical part of the magnetic deflection field can change only slowly in fact spikes do occur both horizontally and vertically and the corresponding horizontal blanking interval and vertical blanking interval give the deflection currents settle time to retrace and settle to their new value this happens during the blanking interval in electronics these usually steadyrate movements of the beam 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 widedeflectionangle 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 vectorbased 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 fall time and settle time for the beam to move back to the left the voltage to decrease and for ringing to die down the vertical frame vframe consists of exactly the same components but only occurs once per image frame and the times are considerably longer the details of these intervals are called the video timing see video timing details revealed for a diagram of these these are mostly not visible to end users but were visible in the case of xfree86 modelines where users of xfree86 could and sometimes needed to manually adjust these timings particularly to achieve certain resolutions or refresh rates 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 phosphor persistence even though only one pixel is being drawn at a time recall that on an analog display pixel is illdefined as there are no fixed horizontal divisions rather there is a flying spot by the time the whole screen has been painted the initial pixel is still relatively illuminated its brightness will have dropped some which can cause a perception of flicker this is one reason for the use of interlacing since only every other line is drawn in a single field of broadcast video the bright newlydrawn lines interlaced with the somewhat dimmed older drawn lines create relatively more even illumination 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 theory and history in a cathode ray tube 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 digitaltoanalog converters for each of the three primary colors for modern flatpanel 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 so 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 whats 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 75hz 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 fastenough refresh rate and sufficient horizontal resolution although the french 819line system had better definition than other standards of its time to obtain a flickerfree display analog tv used a variant of the scheme in movingpicture 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 interlaced video to reduce flicker analog crt tvs write only oddnumbered scan lines on the first vertical scan then the evennumbered lines follow placed interlaced between the oddnumbered lines this is called interlaced scanning in this case positioning the evennumbered lines does require precise position control in old analog tvs trimming the vertical hold adjustment made scan lines space properly if slightly misadjusted the scan lines would appear in pairs with spaces between modern highdefinition tv displays use data formats like progressive scan in computer monitors such as 1080p 1080 lines progressive or interlaced such as 1080i radar raster scans have been used in naval gun firecontrol 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 plan position indicator ppi that covers a round screen but this is not technically a raster analog ppis have sweeps that move outward from the center and the angle of the sweep matches antenna rotation up being north or the bow of the ship history further information history of television the use of raster scanning in television was proposed in 1880 by french engineer maurice leblanc it has not been determined whether they used the word raster in their patent or other writings an early use of the term raster with respect to image scanning via a rotating drum is arthur korns 1907 book which says in german 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 broadcast television system cathoderay tube computer display standard counterscanning image resolution raster graphics rasterisation look up raster in wiktionary the free dictionary vteanalog television broadcasting topicssystems 180line 405line system a 441line 525line system j system m 625line system b system c system d system g system h system i system k system l system n 819line system e system f color systems ntsc pal palm pals palplus secam video back porch and front porch black level blanking level chrominance chrominance subcarrier colorburst color killer color tv composite video frame video horizontal scan rate horizontal blanking interval luma nominal analogue blanking overscan raster scan safe area television lines vertical blanking interval white clipper sound multichannel television sound nicam soundinsyncs zweikanalton modulation frequency modulation quadrature amplitude modulation vestigial sideband modulation vsb transmission amplifiers antenna radio broadcast transmittertransmitter station cavity amplifier differential gain differential phase diplexer dipole antenna dummy load frequency mixer intercarrier method intermediate frequency output power of an analog tv transmitter preemphasis residual carrier split sound system superheterodyne transmitter television receiveonly directbroadcast satellite television television transmitter terrestrial television transposer digital television transition frequencies bands frequency offset microwave transmission television channel frequencies uhf vhf propagation beam tilt distortion earth bulge field strength in free space knifeedge effect noise electronics null fill path loss radiation pattern skew television interference testing distortionmeter field strength meter vectorscope vit signals zero reference pulse artifacts dot crawl ghosting hanover bars sparklies 