JPEG (Joint Photographic Experts Group) is a product of the JPEG standard, which was formulated by the International Organization for Standardization (ISO) and is a compression standard for continuous-tone still images. The JPEG format is the most commonly used image file format, with a suffix of .jpg or .jpeg.
JPEG (Joint Photographic Experts Group), or Joint Photographic Experts Group, is a standard for continuous-tone still image compression. The file suffix name is jpg or jpeg, which is the most commonly used image file format. It mainly uses the combined coding method of predictive coding (DPCM), discrete cosine transform (DCT) and entropy coding to remove redundant images and color data, which is a lossy compression format, which can compress images in a small storage Space will cause damage to image data to a certain extent. In particular, the use of an excessively high compression ratio will reduce the quality of the image recovered after the final decompression. If a high-quality image is pursued, an excessively high compression ratio should not be adopted.
However, JPEG compression technology is very advanced, it can remove redundant image data using lossy compression, in other words, it can use less disk space to get better image quality. And JPEG is a very flexible format, with the function of adjusting the image quality, it allows to compress files with different compression ratios, supports multiple compression levels, the compression ratio is usually 10; 1 to 40; 1, the more the compression ratio The larger, the lower the image quality; conversely, the smaller the compression ratio, the higher the image quality. For the same image, the files stored in the JPEG format are 1/10 to 1/20 of other types of files, usually only a few tens of KB, and the quality loss is small, which is basically not visible. The compression of JPEG format is mainly high-frequency information, which retains good color information and is suitable for use on the Internet; it can reduce the transmission time of images and support 24-bit true color; it is also commonly used in images that require continuous tones.
JPEG format can be divided into standard JPEG, progressive JPEG and JPEG2000 three formats.
1. Standard JPEG format; this type can only display images in order from top to bottom when downloading webpages, and the full picture can only be seen until all the image data is downloaded.
2. Progressive JPEG; this type presents the rough appearance of the image when downloading from the webpage, and then slowly presents the complete content, and the document saved in the progressive JPG format is more important than the document saved in the standard JPG format Comes small, so if you want to use images on a web page, you can use this format more.
3. JPEG2000; it is a new generation of image compression method, with higher compression quality, and can improve the mosaic phenomenon and position disorder caused by unstable signal during wireless transmission, and improve the transmission quality.
The performance of JPEG, measured by the ratio of quality to bit rate, is quite superior. Its advantages are:
It supports extremely high compression rates, so the download speed of JPEG images is greatly accelerated.
It can easily handle 16.8M colors and can reproduce full-color images well.
In the process of compressing the image, the image format can allow the freedom to choose between the minimum file size (lowest image quality) and the maximum file size (highest image quality).
The file size of this format is relatively small, and the download speed is fast, which is conducive to transmission when the bandwidth is not "rich".
The disadvantages of JPEG are:
Not all browsers support inserting various JPEG images into web pages.
When compressing, the quality of the image may be lost, so this format is not suitable for displaying high-definition images.
The low complexity of JPEG and the long usage time are impressive. The following is an image of 8-bit/pixel medium-complexity screen. Several levels given by JPEG are used as a criterion to measure the effect of compression encoding:
0.25 bit/pixel ~ 0.5 bit/pixel; medium ~ good, enough to meet some applications.
0.5 bit/pixel ~ 0.75 bit/pixel; good ~ very good, enough to meet many applications.
0.75 bit/pixel ~ 1.5 bit/pixel; excellent, enough to meet most applications.
1.5 bits/pixel ~ 2.0 bits/pixel; difficult to distinguish from the original image, enough to meet most applications.
>2.0 bits/pixel; almost perfect, satisfy almost all applications.
The bit/pixel is defined as the total number of bits of the compressed image (including chrominance components) divided by the number of samples of the luminance component.
JPEG was established in 1986 and the standard was officially adopted in 1992. Its official name is "Digital Compression Coding of Information Technology Continuous Tone Still Images." In the JPEG algorithm, there are four operating modes, one of which is a lossless compression algorithm based on DPCM, and the other three are lossy compression algorithms based on DCT. The main points are as follows:
· Lossless compression coding mode. The prediction method and Huffman coding (or arithmetic coding) are used to ensure that the reconstructed image is exactly the same as the original image (set the mean square error to zero), and there is no distortion.
· Sequential coding mode based on DCT. According to the principle of DCT transformation, the image data is compression-encoded in order from top to bottom and from left to right. When the information is transmitted to the receiving end, it is first decoded according to the above-mentioned rules, thereby restoring the image. There is information loss during this process, so this is a lossy image compression encoding.
Progressive coding mode based on DCT. It is also based on DCT transformation, but its scanning process is different. It uses multiple scanning methods to compress an image. The description process is carried out gradually from coarse to fine. When the image is restored, the first thing you can see on the screen is the general situation of the image, and then gradually refine it until all are restored.
Layered coding mode based on DCT. This mode encodes the image based on the image resolution. It starts with a low resolution and gradually increases the resolution until it is the same as the resolution of the original image. The same is true during image reconstruction. It can be seen that the effect is similar to the DCT-based progressive coding mode, but its processing is more complicated and the compression ratio obtained is higher.
Since the JPEG standard was adopted in 1992, it has achieved great success in just a few years because of its excellent quality. However, with the continuous expansion of the multimedia application field, the traditional JPEG compression technology has been unable to meet people's requirements for multimedia image data. JPEG uses discrete cosine transform to compress the image into 8×8 small blocks, and then put it into the file in turn. This algorithm achieves compression by discarding the frequency information. Therefore, the higher the image compression rate, the more the frequency information is discarded. In extreme cases, JFEG images only retain the basic information that reflects the appearance, and fine image details are lost. For this reason, JPEG has developed a new generation of still image compression standard JPEG2000.
The biggest difference between JPEG2000 and traditional JPEG is that it abandons the block coding method based on discrete cosine transform (DCT) adopted by JPEG, and adopts the multi-analysis coding method based on wavelet transform. Its main purpose is to The frequency components of the image are extracted. Wavelet transform transforms and encodes an image as a whole line, which saves the correlation in the image information and achieves a better compression and encoding effect.
JPEG2000 has the following characteristics:
1. High compression ratio
In the discrete wavelet transform algorithm, the image can be converted into a series of "wavelets" that can more effectively store the pixel module. Therefore, the image compression ratio of the JPEG2000 format can be further increased by 10% to 30% on the basis of the current JPEG, and after compression The image is more delicate and smooth, this feature has a wide range of applications in the field of image transmission such as Internet and remote sensing.
2. Lossless compression and lossy compression
JPEG2000 provides both lossless and lossy compression methods. Lossless compression is necessary in many fields, such as medical images and archive images, which require high image quality. At the same time, JPEG2000 provides an embedded code stream, allowing progressive decompression from lossy to lossless.
3. Progressive transmission
JPEG images on the Internet are now transmitted in "blocks" when downloaded, so they can only be displayed line by line. Images in the JPEG2000 format support progressive transmission, first transmitting image outline data, and then gradually transmitting other data to continuously improve the image quality. The Internet, printers and image files are the main applications of this feature.
4. Region of interest compression
This feature can specify areas of interest on the picture, and then specify the compression quality for these areas during compression, or specify the decompression requirements for certain areas during recovery. This is because the wavelet transform is localized in the spatial and frequency domains. To completely restore a certain part of the image, it is not necessary that all codes are accurately retained, as long as there is no error in part of the codes corresponding to it. This way we can easily highlight the key points.
5. Random access and processing of code stream
This feature allows the user to randomly define the region of interest in the image, so that the image quality of this region is higher than other image regions. The random processing of the code stream allows the user to perform operations such as rotation, movement, filtering, and feature extraction.
6. Fault tolerance
JPEG2000 provides fault-tolerant measures in the code stream. To transmit images in communication channels with high transmission errors such as wireless, fault-tolerant measures must be taken to achieve a certain reconstruction image quality.
7. Content-based description
Image files, image indexing and searching are an important field in image processing. MPEG7 is an international standard that supports users to quickly and efficiently retrieve various "materials" that interest them. Content-based description is one of the characteristics of the compression system in JPEG2000.
The compression ratio of the JPEG format is currently the highest among various image file formats. It uses lossy compression to remove the redundant data of the image, but there is a certain distortion. Because of its high efficiency compression efficiency and standardization requirements, it has been widely used in color fax, still image, teleconference, printing and news photo transmission. Since various browsers support the JPEG image format, it is also widely used for image preview and making HTM web pages.
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