What is image compression?
The image is made up of a lot of information. Image compression consists of reducing the total amount of information by reducing the extra part of that information.
Image compression is similar to compressing clothes and futons.
Newlywed Atsuko is worried about storing her new home. When he was single, Mr. Atsuko, who was fashionable, had a lot of clothes. However, the new room has a small amount of storage and there is no room for take-out clothes. In addition, you have to put a futon for the couple and the client in the storage room.
That I have to do?
Therefore, I decided to ask Mr. Chijimi, a senior who is good at storing.
Mr. Shumi noted that many thick futons and garments are stored after losing weight with a “compression garment bag (futon)”.
Immediately, Mr. Atsuko tried to follow Mr. Shumi’s advice and found that he could put away all the clothes and futons that couldn’t be put in the closet until now.
Once compressed, getting around is easy. Mr. Atsuko’s luggage, which was estimated at three trucks, could be transported by a truck by packing clothes and futons in a compression bag.
Real image compression method
Clothes and futons remove the air that is in the spaces, but the compression of the image extracts information that can be eliminated by a fixed calculation.
In other words, there is less information.
(Still image) Compression method
To send a lot of information at once, the sender must compress the information (still image). We will explain how to compress using the commonly used JPEG method as an example.
(JPEG is an abbreviation for Joint Photographic Experts Group, which is one of the methods for compressing still images.)
JPEG compression method
The image is decomposed into square blocks consisting of 8 x 8 dots, and the brightness information and color information are converted to frequencies for each block (DCT calculation: Discrete Cosine Transform). The high-frequency component and the low-frequency component are separated, and the high-frequency component is a number close to “0”, so it is not read as information. Only low-frequency components are converted to signal 01.
JPEG features (progressive method)
The entire image is displayed from the beginning with a rough image. And have you ever seen a gradual, finer and cleaner image? That is the characteristic of the progressive method.
(Video) Compression method
The information of the video is greater than that of the still image. In other words, moving images need to be compressed more than still images. For example, MPEG-2 is a standard for compressing video. This standard is a standard established by the Moving Picture Experts Group and is used in digital terrestrial broadcasting (abbreviation, digital terrestrial broadcasting).
Video compression
Since there is a lot of information, in addition to the method of compressing still images, the amount of information is reduced by the “send only the information that has changed” method.
For example, if you have a video of a cat wagging its tail, it will only send information about the tail change. So all the information is less.
Video compression standard
Video compression standard stream
As the amount of information we handle increases, new standards are being developed (standards are also called formats). Each standard has its own characteristics and the fields of application are different. (Researcher Akira Nakagawa was awarded the Purple Ribbon Medal in the Spring 2016 Medal for H.264 / MPEG-4 AVC “Development of Digital Video Coding Technology and Transmission Equipment”)
