Digital Video Compression Part 4
Lossy compression
(2) Compression can be lossy if information is lost during the compression process. However, from the point of view of human perception, lossy compression should be considered only that compression in which it is possible to distinguish with the naked eye the result of compression from the original.
Thus, despite the fact that the two images, the original and the result of compression with one or the other compressor, may not coincide little by little, however, the difference between them may be completely imperceptible. Examples include the JPEG algorithm for compressing static graphics and the M-JPEG algorithm for compressing video.
Compression without loss of perception
(3) Although technically lossy compression, a compression scheme can at the same time appear to be lossless in terms of human perception. Most lossy compression technologies have a so-called Compression Quality Factor (QF), which characterizes the perceived quality side and ranges from 0 to 100. With a compression quality factor of 100, the perceived quality characteristics of the compressed video are indistinguishable from the original. .
Lossy compression
(4) JPEG and MPEG and other lossy compression technologies are sometimes compressed, losslessly, exceeding the limits of compression in terms of perception of video information. However, compressed video and still images are quite acceptable for proper human perception. In other words, in this case the so-called natural degradation of the image is observed, in which some small details of the scene are lost. Something similar can happen in natural conditions, for example, rain or fog. The image in such conditions is usually distinguishable, but its detail is reduced.
Unnatural quality loss compression
(5) Poor compression quality, which greatly distorts the image and introduces artificial details (not present in the original) of the scene, is called unnatural lossy compression. An example of this is true “blocking” in highly compressed MPEG and other compressors using BDKP technology. Unnaturalness lies mainly in the violation of the most important features of the image from the point of view of human perception – the outlines. Experience shows that it is precisely the contours that allow a person’s perceptual apparatus to correctly identify a particular visual object.
In conclusion, we note that all widely used video compressors use lossy compression technologies. At high enough compression ratios, they will all compress with an unnatural loss of quality.
Thus, when choosing one or the other compressor to compress digital video, it is necessary to achieve compression, at least with a natural loss of quality.
Video compression algorithms and technologies overview
Run length encoding
The compressors that use RLE technology are:
Microsoft RLE (MRLE) RLE is also used to encode coefficients in the BDCP used in MPEG-1234, H.261, H.263, and JPEG.
Advantages and disadvantages
It works exclusively with 8-bit images.
Not suitable for full color video compression.
General description
RLE encodes a sequence of repeating picture elements or monochrome elements with one code word. For example, a sequence of pixels 77 77 77 77 77 77 77 can be encoded as 7 77 (for seven 77-rock). RLE is good for compressing images in which there is a repetition of the outlines or colors of individual elements. There is much less color repetition in full color images, so compressing full color video with RLE technology is pointless.
Vector quantization (VQ)
The compressors that use VQ technology are Indeo 3.2 and Cinepak. They both use the YUV (not RGB) color scheme.
Advantages and disadvantages
The encoding process is very time consuming and practically impossible without special additional equipment.
The decoding process is very fast.
Locks out distortion at high compression ratios.
With technologies that use BDKP algorithms, Fibreboard can achieve higher compression levels.
