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Why take a camera to take a picture of a computer screen, the picture will appear ripples?
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This is the legendary Moire Pattern. In short, two groups of patterns with similar spatial frequencies interfere with each other, resulting in patterns with lower frequencies (and wider spacing). The spatial frequency refers to the reciprocal of the spacing between the characteristic fringes.
Said so mysterious, in fact, the truth is very simple! For example, in two pieces of transparent plastic paper respectively draw a row of vertical lines, the top one every 1 mm draw a, the bottom one every 1.1 mm draw a, it is easy to find that the vertical lines will overlap every 11 mm. Near the overlapping position of thin lines, the gap exposed is larger and appears bright; And the position near the thin line does not overlap, the gap exposed is smaller, appears gloomy.
Thus, the distribution of light and shade with a period of 11 mm is formed. The overall appearance is a thick stripe with larger spacing, which is easy to be perceived by the eyes, as shown in Figure 1. When there is a certain Angle between the two, the fringe is inclined, as shown in Figure 2.
That’s just the case with one-dimensional periodic patterns. What about two dimensions?
I bet you’ve seen two overlapping layers of window screens or cut-out seatbacks in your life? Or… Leggings?
Careful you can discover certainly, on the foundation of original fine close stripe indistinctly have the thick stripe with wider spacing about to appear. When the two layers of window screens are not completely parallel or have their own fluctuations, the stripes will bend and twist.
The same is true with a camera on a TV screen: the horizontal and horizontal grid of pixels on a computer screen acts as a first screen, the CMOS or CCD sensor array in a mobile camera acts as a second screen, and even the mobile screen acts as a third screen, resulting in patterns with Moire stripes.
Coupled with the perspective caused by Angle deviation, the lens distortion, and the small deformation of the screen itself, the moire stripes photographed can also be bent to varying degrees. In one of them, with perspective effect, the stripes are curved, as shown in figure 3.
Note:
In fact, image interference can be spatial or temporal. The former is the moire fringe, while the latter is the bright and dark scintillation fringe caused by limited refresh rate and scanning process. Looking at the statement of the question itself, I have a subjective feeling that the questioner should mean the former, so I answered so.
In addition, from the perspective of digital signal processing, the above phenomenon can also be roughly understood as frequency aliasing, and the relationship between signal frequency and sampling rate needs to be analyzed. But this a little complicated the simple question, the topic since the phenomenon of the question, should not have learned this knowledge, take a similar vernier caliper two vertical line examples enough to illustrate the principle.
I always believe that answering questions or popularizing science can be deep or shallow, and different interpretation methods are needed for different readers. For example, the mathematical derivation of Moire’s fringe can be explained by the concept of the least common multiple for pupils. For middle school students, trigonometric functions can be deduced by summation and difference product, product and difference formula; For undergraduate readers, you can analyze the discrete Fourier transform from the perspective of digital signal processing.
Clarification of several common concerns in the comments section:
1. Many friends mentioned that the screen refresh rate caused by light and dark stripes, this has nothing to do with Moire stripes.
The stripes caused by refresh rate are the result of time out of sync, whereas moire’s stripes are simply the effect of periodic patterns in space.
This can be clearly illustrated in attached Figures 4 and 5. For example, in figure 5, there are both vertical light and dark stripes related to refresh rate and curved color Moire stripes, while when the screen refresh rate is adjusted to the appropriate frequency, the stripes related to refresh disappear, leaving only the pure Moire stripes in Figure 4.
2. Moire fringe is directly related to the spatial frequency of the grid line, that is to say, it is sensitive to the change of the spacing of the grid line.
Because of this, camera shooting distance, zoom in and out, focal length adjustment, hand shaking, etc., will lead to the change of screen grid line spacing in the corresponding projection pattern on the camera sensor array, so that the stripe deformation, or even disappear. Don’t forget the words “similar spatial frequencies” emphasized at the beginning of the text.
3. The current Retina screen resolution is greater than 300 Dpi, that is, there are more than 300 grid lines per inch, which is too dense and moire stripes are not obvious. But with careful camera spacing, you can still see it, as shown in my attached picture 4. In addition, printers currently generally have distribution rates greater than 300 dpi, which is similar.
4. In order to eliminate Moire stripe, many cameras and mobile phones will automatically process it in hardware or software, so the stripe will be weakened. A typical example is when a camera is taking a picture, it is likely to automatically deal with weak streaks; However, in the photo preview, the stripes are more obvious without the algorithm processing, and the result of the screen shot is also more obvious than the direct photo.
5. The picture itself is a digital image, which is also composed of discrete points, so there will be Moire stripes when zooming in, zooming out and rotating on the mobile phone. However, the elimination algorithm in this aspect has been mature, so it is not so obvious. However, for large images, it sometimes takes a certain amount of time to do enlargement preview on mobile phones. During this process, Moire stripes can be seen when the image is just enlarged, and then disappear after algorithm adjustment for about 1s.
6. Some screens also have another stripe that nobody seems to have mentioned, but I might add. As shown in the attached figure 6, concentric ring color fringe has nothing to do with the above. It is an optical phenomenon caused by the structure of the screen itself and related to polarization, which will not be described here. Unfrosted LCDS are easier to see, especially when viewed sideways. Its characteristic is to turn off the computer can also see, and the naked eye has nothing to do with the camera. It’s easy to tell.
Acquaintances buzz
Answers questions I’ve had for years.
Cui ray
The answer is very impressive, from a question to extend a lot of knowledge. And the answer is given from different angles.
surfaces
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