What Is a JPEG Compressor?

JPEG was established in 1986, and its full name is Joint Photopaphic Experts Groop. Its standards are formulated for still image compression, including image encoding, decoding process, and encoding requirements for compressed image data. It was formally submitted for approval in March 1991. JPEG includes three algorithms: Baseline System, Extended System, and Distortionless System.The basic system is based on DCT transform and variable length coding (VLC) compression technology.On the premise of ensuring the quality of image restoration, It can provide a compression ratio of up to 100: 1, but due to distortion in the encoding process, the reconstructed image cannot accurately reproduce the original image. Its distortion is directly related to the compression ratio. All JPEG encoders and decoders support the basic system. The other two algorithms are used more in specific places.

Digitally Compressed JPEG

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JPEG was established in 1986, and its full name is Joint Photopaphic Experts Groop. Its standards are formulated for still image compression, including image encoding, decoding process, and encoding requirements for compressed image data. It was formally submitted for approval in March 1991. JPEG includes three algorithms: Baseline System, Extended System, and Distortionless System.The basic system is based on DCT transform and variable length coding (VLC) compression technology.On the premise of ensuring the quality of image restoration, It can provide a compression ratio of up to 100: 1, but due to distortion in the encoding process, the reconstructed image cannot accurately reproduce the original image. Its distortion is directly related to the compression ratio. All JPEG encoders and decoders support the basic system. The other two algorithms are used more in specific places.

Chinese name

Joint Photographic Experts Group

Foreign name

JPEG

Digital compression JPEG (digital compression JPEG) is a universal international standard. JPEG is the acronym for Joint PhotographicExpert Group, which is the Joint Photographic Experts Group. It is the international ISO / ITU-T joint technical committee on continuous tone (gray (Degrees and color) General international standard for digital compression of still images. The international standard file number is ISOCD 10918. Its application fields include storage and transmission of still images such as computer graphics, color photography, color facsimile, and color printing. The compressed JPEG standard is for digital compression of still color images, so the basic compression processing method it applies is compression of redundant information in the frame, that is, two-dimensional spatial information compression in the picture, without using adjacent frames of moving images. Inter-frame redundant information compression, that is, without reducing the temporal correlation redundancy in the previous and subsequent frames. However, this is exactly what the digital compression JPEG standard applies to. If it is applied to TV images, Digital compression, the compression algorithm is limited to the data information in the frame, and the compression ratio can be determined according to the image quality requirements.

With proper adjustment, it can be flexibly selected in the range of 2: 1 to 50: 1 according to different applications. In the non-linear editing system of commonly used equipment in TV studios, in order to obtain frame-accurate editing effects, JPEG compression is generally used Technology, but the compression speed must meet the TV frame rate of 25 frames per second (the digital compression JPEG standard originally did not require compression speed), so this compression technology is specifically called M-JPEG (Motion-JPEG).

Digital compression JPEG specifies two types of compression algorithms, one is DCT (Discrete Cosine Transform) as the main body, and the other is differential pulse code modulation (DPCM). The first compression algorithm is used in the non-linear editing system. Get images with a large range of compression ratios and different quality levels. There are three basic steps in the DCT algorithm:

1. Perform DCT transformation on 8X8 image blocks to transform spatial pixel values into spatial frequency coefficient values;

2. Weighted quantization of 64 spatial frequency coefficients (DCT coefficients) according to human visual characteristics;

3. After the quantized DCT coefficients are read out in the "meta" glyph, encoding is performed.

By changing the quantization step size during weighted quantization, you can adjust the compression ratio. When the compression ratio is 2: 1, there is no quality difference between the decoded image and the original image; when 6: 1 to 8: 1, it can Meet the requirements of good image quality; 30: 1 to 50: 1, suitable for offline editing occasions. The decoding algorithm of digital compression JPEG is an inverse processing process that is commensurate with the encoding, and requires the same digital processing power and processing time This point is significantly different from the asymmetric compression method of the MPEG (Moving Picture Experts Group) standard (the decoding algorithm is much simpler than the encoding algorithm). ^[1]