What is JPEG?

Specification of Joint Photographic Expert Group

JPEG is an expert group composed of two organizations, ISO and IEC, which is responsible for formulating the compression and coding standard of static digital image data, so it is also called JPEG standard. JPEG is a widely used still image data compression standard, which can be used for gray-scale images and color images. JPEG Expert Group has developed two basic compression algorithms, one is a lossy compression algorithm based on DCT (Discrete Cosine Transform), and the other is a lossless compression algorithm based on prediction technology. When using lossy compression algorithm, when the compression ratio is 25: 1, it is difficult for non-image experts to find the difference between the restored image and the original image, so it has been widely used. For example, in V-CD and DVD-Video TV image compression technology, JPEG lossy compression algorithm is used to eliminate spatial redundant data. In order to further improve the compression ratio under the premise of ensuring the image quality, in recent years, JPEG expert group is working out JPEG2000 (JP2000 for short) standard, which will adopt wavelet transform algorithm.

JPEG is a lossy image compression algorithm, which usually reduces the size of bitmap image by 10 times or more, and it produces little or no distinguishable image loss. The working principle of JPEG compression is to filter out the high-frequency information of the image, reduce the amount of data, and then compress the obtained data through lossless compression algorithm. Low-frequency information is more helpful to define the characteristics of the image, so losing some high-frequency information may not affect the image quality.

JPEG is the abbreviation of Joint Photographic Experts Group (the name of the organization that developed this format), and the file name is ". Jpg "or" jpeg”。 JPEG format is one of the most widely used formats at present, and almost all image programs in Mac and Windows systems can open and save JPEG images. JPEG is also one of the two main file formats used for image processing on the World Wide Web (JPEG and GIF, JPEG files are generally smaller than GIF files).

One advantage of JPEG format is that it can compress image data. JPEG is a lossy compression format, which can compress images in a small storage space. Repeated or unimportant data in images will be lost, so it is easy to damage image data. Especially, if the compression ratio is too high, the quality of the restored image after final decompression will obviously decrease. If you pursue high-quality images, you should not use too high compression ratio. But JPEG compression technology is very advanced. It uses lossy compression to remove redundant image data, and can display very rich and vivid images at the same time, in other words, it can obtain better image quality with the least disk space. Moreover, JPEG is a very flexible format with the function of adjusting image quality. It allows files to be compressed at different compression ratios and supports multiple compression levels. The compression ratio is usually between 10: 1 and 40: 1. The higher the compression ratio, the lower the mass. On the contrary, the smaller the compression ratio, the better the quality.

Therefore, JPEG image files are relatively small. The smaller the image file, the more disk space will be saved, and the more time will be saved when downloading from the Internet. The problem is that JPEG uses a lossy compression scheme. This means that some image data will be lost in the compression process. Every time an image is opened, edited and saved, it is repeatedly compressed, resulting in more losses. JPEG supports extremely high compression ratio. JPEG format saves storage space by accurately recording the brightness of each pixel and averaging its hue, so it has little impact on image quality, so it can obtain better image quality with relatively small disk space. You can choose different compression ratios to compress JPEG files, that is, both compression ratio and image quality are optional. JPEG format can support 16M color, can reproduce full-color images well, and is suitable for storing photographic images. Because the compression algorithm of JPEG format is to balance the brightness and color between pixels, it is more beneficial to express the image with gradual color change and unclear outline. JPEG (Still Image Compression Standard) image format can support 24-bit full color. It accurately records the brightness of each pixel, but it compresses the image in a balanced tone, so that our naked eyes can't clearly distinguish it. In fact, it is recording the description of the image, rather than compressing it when it appears. The web browser or image editing software used by the viewer will interpret the recorded description as a dot matrix image, so that it looks very similar to the original image.