question: Why do photographs of screens look like they’re “moving” when you zoom into them?
comment: > Photographs of a [TV]( URL_0 ) screen taken with a [digital camera]( URL_2 ) often exhibit moiré patterns. Since both the TV screen and the digital camera use a scanning technique to produce or to capture pictures with horizontal scan lines, the conflicting sets of lines cause the moiré patterns. To avoid the effect, the digital camera can be aimed at an angle of 30 degrees to the TV screen. [Moiré]( URL_1 )
passages: The "continuous" display can be driven at any integer multiple of the capture rate - it won't matter for the viewer, nor can it be visually discriminated. However, in general, "continuous" displays show noticeable smear over quickly-moving objects in 50 and ~60 Hz video material (even if their response time is instant). However, there are two emerging techniques to combat smearing of the video-based material in LCD display: it can be effectively converted into the "flashing" display by appropriately modulating its backlight; and/or it can be driven at double the capture rate while calculating intermediate frames using the motion analysis (see LCD television).
   The image change rate fundamentally affects how "fluid" the motion it captures will look on the screen. Moving image material, based on this, is sometimes divided into two groups: "film-based" material, where the image of the scene is captured by camera 24 times a second (24 Hz), and "video-based" material, where the image is captured roughly 50 or 60 times a second.
   Conversely, extra motion blur can unavoidably occur on displays when it is not desired. This occurs with some video displays (especially LCD) that exhibits motion blur during fast motion. This can lead to more perceived motion blurring above and beyond the preexisting motion blur in the video material. See display motion blur.
  
  Sometimes, motion blur can be removed from images with the help of deconvolution.
   For these displays, the 24–30 Hz material is usually displayed at 2x, 3x, or 4x the capture rate. 50 and ~60 Hz material is usually displayed at its native rate, where it delivers a very accurate motion without any smearing. It can also be displayed at twice the capture rate, although moving objects will look smeared or trailed, unless intermediate frames are calculated using the motion analysis and are not just simply duplicated.
   Both OLED and Sony's Crystal LED displays use an independent light source for every pixel, without a traditional CCFL or LED backlight used in LCD. Sony's Crystal LED uses individual light emitting diodes for each pixel, instead of using LED as a backlight. Several displays demonstrated at the CES 2012 have been the first modern high-definition television sets to overcome the motion artifacts by selectively blanking parts of the screen. Both OLED and "Crystal LED" technologies also have response times far shorter than LCD technology, and can reduce motion blur significantly. However, all consumer OLED Displays are sample-and-hold, which leads to the same amount of motion blur as a traditional LCD Display.
   There has also been Microvision's Laser MEMS Based Pico Projector Pro, which has no display lag, no input lag and no persistence or motion blur 
  
  Section::::Fixes.:LED and OLED.
   A 1993 article by George Fitzmaurice studied spatially aware palmtop computers. These devices had a 3D sensor, and moving the device caused the contents to move as if the contents were fixed in place. This interaction could be referred to as “moving to scroll.” Also, if the user moved the device away from their body, they would zoom in; conversely, the device would zoom out if the user pulled the device closer to them. Smartphone cameras and “optical flow” image analysis utilize this technique nowadays. 
   At the receiving end, a digital display is free to independently present the image sequence at a multiple of its capture rate, thus reducing visible flicker. Most modern displays are "multisync," meaning that they can refresh the image display at a rate most suitable for the image sequence being presented. For example, a multisync display may support a range of vertical refresh rates from 50 to 72 Hz, or from 96 to 120 Hz, so that it can display all standard capture rates by means of an integer rate conversion.
  
  Section::::Presentation.
   BULLET::::- BenQ developed SPD (Simulated Pulse Drive), also more commonly known as "black frame insertion", and claim that their images are as stable and clear as CRTs. This is conceptually similar to a strobing backlight.
  
  BULLET::::- Sharp Corporation use a "scanning backlight" which rapidly flashes the backlight in a sequence from the top to the bottom of the screen, during every frame.
   To create the illusion of movement, an image is displayed on the computer screen then quickly replaced by a new image that is similar to the previous image, but shifted slightly. This technique is identical to the illusion of movement in television and motion pictures.
  
  Section::::Concepts and principles.
  
  Images are typically created by devices such as cameras, mirrors, lenses, telescopes, microscopes, etc.
  
  Digital images include both vector images and raster images, but raster images are more commonly used.
  
  Section::::Concepts and principles.:Pixel.
   In interlaced video, the horizontal scan lines of each complete frame are treated as if numbered consecutively, and captured as two "fields": an "odd field" (upper field) consisting of the odd-numbered lines and an "even field" (lower field) consisting of the even-numbered lines. Analog display devices reproduce each frame, effectively doubling the frame rate as far as perceptible overall flicker is concerned. When the image capture device acquires the fields one at a time, rather than dividing up a complete frame after it is captured, the frame rate for motion is effectively doubled as well, resulting in smoother, more lifelike reproduction of rapidly moving parts of the image when viewed on an interlaced CRT display.
   Section::::3D viewers.
  
  There are two categories of 3D viewer technology, active and passive. Active viewers have electronics which interact with a display. Passive viewers filter constant streams of binocular input to the appropriate eye.
  
  Section::::3D viewers.:Active.
  
  Section::::3D viewers.:Active.:Shutter systems.
   To create the illusion of movement, an image is displayed on the computer screen and repeatedly replaced by a new image which is similar to the previous image, but advanced slightly in the time domain (usually at a rate of 24 or 30 frames/second). This technique is identical to how the illusion of movement is achieved with television and motion pictures. This is not real.
  
  Section::::Virtual worlds.
   This way of proceeding is suitable only when the display device does not exhibit "loading effects", which means that the luminance of the test pattern is varying with the size of the test pattern. Such loading effects can be found in CRT-displays and in PDPs. A small test pattern (e.g. 4% window pattern) displayed on these devices can have significantly higher luminance than the corresponding full-screen pattern because the supply current may be limited by special electronic circuits.
  
  Section::::Full-swing contrast.
   Formerly, LCDs were not very suitable for stereoscopic 3D due to slow pixel response time. Liquid crystal displays have traditionally been slow to change from one polarization state to another. Users of early 1990s laptops are familiar with the smearing and blurring that occurs when something moves too fast for the LCD to keep up.
   This optical illusion is caused by the fact that the human optic nerve responds to changes in light at about 10 cycles per second, so changes about double of this are registered as motion instead of being separate distinct images.
  
  Section::::Examples of use.
  
  One example of the beta movement effect would be a set of LEDs, as shown at the adjacent picture. The LEDs, electronically, are individually controlled, but our eyes and brains perceive them as a snake running clockwise around the four edges of the square picture. This is also seen commonly on LED displays.
  
  Section::::Experiment.
   Currently, most flat-panel displays employ lenticular lenses or parallax barriers that redirect imagery to several viewing regions; however, this manipulation requires reduced image resolutions. When the viewer's head is in a certain position, a different image is seen with each eye, giving a convincing illusion of 3D. Such displays can have multiple viewing zones, thereby allowing multiple users to view the image at the same time, though they may also exhibit dead zones where only a non-stereoscopic or pseudoscopic image can be seen, if at all.
  
  Section::::Technology.:Parallax barrier.
   Since a CMOS sensor typically captures a row at a time within approximately 1/60th or 1/50th of a second (depending on refresh rate) it may result in a "rolling shutter" effect, where the image is skewed (tilted to the left or right, depending on the direction of camera or subject movement). For example, when tracking a car moving at high speed, the car will not be distorted but the background will appear to be tilted. A frame-transfer CCD sensor or "global shutter" CMOS sensor does not have this problem, instead captures the entire image at once into a frame store.
   BULLET::::- Flicker can be noticed except at very high refresh rates, as each eye is effectively receiving only half of the monitor's actual refresh rate. However, modern LC glasses generally work in higher refresh rates and eliminate this problem for most people.
  
  BULLET::::- Until recently, the method only worked with CRT monitors; some modern flat-panel monitors now support high-enough refresh rates to work with some LC shutter systems. Many projectors, especially DLP-based ones, support 3D out of the box.
   The input must be held steady in a period around the rising edge of the clock known as the aperture. Imagine taking a picture of a frog on a lily-pad. Suppose the frog then jumps into the water. If you take a picture of the frog as it jumps into the water, you will get a blurry picture of the frog jumping into the water—it's not clear which state the frog was in. But if you take a picture while the frog sits steadily on the pad (or is steadily in the water), you will get a clear picture. In the same way, the input to a flip-flop must be held steady during the aperture of the flip-flop.
   Chewing something crunchy such as tortilla chips or granola can induce flicker perception due to the vibrations from chewing synchronizing with the flicker rate of the display.
  
  Section::::Software artifacts.
  
  Software can cause flicker effects by directly displaying an unintended intermediate image for a short time. For example, drawing a page of text by blanking the area to white first in the frame buffer, then drawing 'on top' of it, makes it possible for the blank region to appear momentarily onscreen. Usually this is much faster and easier to program than to directly set each pixel to its final value.
   Many motion blur factors have existed for a long time in film and video (e.g. slow camera shutter speed). The emergence of digital video, and HDTV display technologies, introduced many additional factors that now contribute to motion blur. The following factors are generally the primary or secondary causes of perceived motion blur in video. In many cases, multiple factors can occur at the same time within the entire chain, from the original media or broadcast, all the way to the receiver end.
  
  BULLET::::- Pixel response time on LCD displays (motion blur caused by slow pixel response time)
   Different manufacturers use many names for their strobed backlight technologies for reducing motion blur on sample-and-hold LCD displays. Generic names include "black frame insertion" and "scanning backlight".
  
  BULLET::::- Philips created Aptura, also known as ClearLCD, to strobe the backlight in order to reduce the sample time and thus the retinal blurring due to sample-and-hold.
  
  BULLET::::- Samsung uses strobed backlighting as part of their "Clear Motion Rate" technology. This was also called "LED Motion Plus" in some previous Samsung displays.
   25 Hz material, for all practical purposes, looks and feels the same as 24 Hz material. 30 Hz material is in the middle, between 24 and 50 Hz material, in terms of "fluidity" of the motion it captures; but, in TV systems, it is handled similarly to 24 Hz material (i.e. displayed at least twice the capture rate).
  
  Section::::Capture.
   These artifacts appear in the form of sharp horizontal and vertical edges which are easily spotted by the human eye and produce false edges and ringing effects (large coefficients in high frequency sub-bands) due to quantization of coefficients of the Fourier-related transform used for transform coding of the residual frames
  
  Block motion compensation divides up the "current" frame into non-overlapping blocks, and the motion compensation vector tells where those blocks come "from"
  
  (a common misconception is that the "previous frame" is divided up into non-overlapping blocks, and the motion compensation vectors tell where those blocks move "to").
  