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How does JPEG format work?

Knowing some of the principles reimburse ignorance of many facts. Today we talk about the JPEG compression technology and the fact due to that achieved such a strong degree of compression.

The process of JPEG compression scheme consists of several steps. The first step is the transformation of images from the RGB color space in the space of YUV, based on the characteristics of brightness and color. All further work done with this particular color space, which thanks to some of its features allows us to obtain such high compression ratios.

What is so unusual in YUV color representation, compared with the RGB? And the fact that it is closest to the "natural", that which unknowingly executes man. Y-component, or brightness, is closely linked to the quality of the picture. More precisely Y - this is a picture, just black and white. Components U and V provide information about color and allow us to paint Y-image.

The next step after the transformation of the image is divided into square plots of 8x8 pixels. Then on each plot is called discrete cosine transform (DCT). It analyzes each block, its decomposition into component colors and counting the frequency of occurrence of each color.

The human eye is designed in such a way that is most sensitive to the brightness component image (Y-component) and the least to the color. The reason for this phenomenon lies in the physiology. You probably remember that the pupil is an optical lens which focuses the image on the bottom of the ocular, covered with rods and cones. Well, sticks - it sensors, perceiving it was the brightness component, and cones - color. And stick to the order of magnitude greater than the cones, and they are much more sensitive to light. Remember the saying "At night all cats are gray." Why? Why all the evening loses color? It is because of the fact that the amount of light falling on the pupil is not enough to cause a reaction cones. But the sensitivity of the human eye to different colors, too, value is not constant. Pupil is more sensitive to the bottom of the color spectrum than to the top. JPG format as time and takes into account these features.

Analyzing the frequency information about the appearance of colors, can get rid of some of the information already in the process of quantization. In this color in the upper part of the spectrum are excluded, that virtually no effect on the visual perception of the image. Also excluded part of the brightness information. Roughly speaking, JPG simply discards from the brightness is half of the useful signal, but from the color 3 / 4. This, of course, roughly, as There are gradations and more sophisticated compression schemes.

The amount of information excluded under compression depends on the desired image quality. When possible, high levels of compression components completely obliterated, and the block becomes gray. At medium and low levels of compression in the file is _primernaya_ information about the color of the site. The magnitude of this "exemplary" depends on the degree of compression. And it must be understood that, in contrast to the usual format, preserving the image pixel by pixel, JPG preserves the approximate color. If you speak the language of science, the JPG is used to store Fourier series and at high degrees of compression simply discards the terms of the series of higher order. And every time when playing back images on a computer screen produces a synthesis. And, quite resource-intensive and noticeable on slow computers. It follows from this one point - if you saved any picture in the JPG, then restore it back to the last pixel is impossible! Because of this format is called "lossy format", and it is therefore not recommended reconvert JPG-images, because they will become worse. And if you do it 10 times?

Information about the brightness and color then it is encoded so that there remain only difference between adjacent blocks. As a result, blocks represent lines of numbers that can be compressed further. As a result of processing units contain many zeros, the last stage of encoding (performed by Huffman - similar to that used in archives) gives good results. Hence, even a small remark - compress JPG-file archive makes no sense, since he already compressed. The resulting archive will certainly be larger than the original picture.

Thus, the initial 24 bits per picture element, or 1536 bits (192 bytes) to block converted into a handful of bits that describe the visual characteristics of the entire area of the image.

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