Digital Video Compression Part 2
Individual images are made up of macroblocks. The macroblock is the main structural unit of image fragmentation.
Corresponds to an area of the image with a size of 16 * 16 pixels. It is for them that the displacement vectors are determined relative to the I or P images. The total number of macroblocks in the image is 396. To increase the resistance of the image recovery process to possible data transmission errors, the Consecutive macroblocks are combined into independent sections (slices), with a maximum number of 396. In the limiting case of “clean transmission”, only one section of the image falls on the image. 396 macroblocks. In turn, each macroblock consists of six blocks, four of which contain information about the brightness Y, and one at a time determine the U and V components of color. Each block is an 8 * 8 matrix.
In conclusion, we would like to emphasize an important feature of MPEG streams: they do not allow frame-by-frame editing. In fact, due to the complex dependency on the images described above, making changes to one of them will inevitably lead to the need for complex calculation of many parameters across the entire group.
Motion JPEG or M-JPEG (MJPG)
Most desktop video capture and video editing systems use this method when recording video to AVI files. In Motion JPEG, each video frame in an image is compressed separately using the JPEG standard. In this case no other additional algorithms are used. The undoubted advantage of this method is the possibility of editing video without loss of quality, since the frames are independent. This, in fact, determines the use of this method precisely as a video storage mechanism that serves for editing, and not for distribution. Motion JPEG uses the Block Discrete Cosine Transform (DCT) algorithm to compress images.
Editable MPEG (XMPG. MPGI)
Editable MPEG, such as M-JPEG, used for digital video editing is an AVI file consisting only of MPEG type i frames. However, all other MPEG compression mechanisms are involved here. It is included in the standard delivery of Microsoft Video for Windows 1.1. and is used by desktop digital video editing systems like Adobe Premiere.
Although the above compressors are quite popular, they are far from all standards for compressing AVI files. In characterizing this group of compressors, it can be seen that they were designed and created primarily as a means of compressing video and audio data stored on hard drives and CDs, and this, in turn, indicates their low compression capacity and relatively high quality. high during playback. .
With the advent of the Internet, methods and tools for compressing video and audio data are becoming increasingly popular, allowing, using advanced technologies (sophisticated motion estimation and compensation, wavelets, fractals, and others) to achieve kilobits. / second smaller. ratios that allowed, for example, videoconference sessions over the Internet. It is clear that such compression methods provide significantly higher compression ratios with relatively low quality.
Compressor generation – H.XXX
More recently (5/18/97), a new series of digital video compressors was officially registered, defining trends in the development of video compression mechanisms. Some compressors in the H.XXX family, such as H.261, are quite popular, others are little known and use advanced and improved technologies such as wavelets.
A distinguishing feature of the H.XXX family compressors is their focus on reducing the flow of digital video over the Internet, which naturally leads to the quality factor fading into the background. Microsoft is improving some of the products in the H.XXX family, and some are already included in teleconferencing packages such as NetShow and NetMeeting.
