Video Basics and Compression Part 3
Drawing method
The drawing method is a method of scanning video signals.
There are “progressives” and “interlaced”.
Progressive performs one scan drawing at a time. Interlacing, on the other hand, is a gradual and clear drawing method.
Main video compression methods and functions
From here, I will explain the following three main video compression methods.
MPEG
H.264
Motion JPEG (MJPEG)
MPEG
MPEG is read as “Mpeg”. It is one of the standards for compressing video and audio.
MPEG Features
High degree of freedom in encoder control.
Not suitable for taking a frame clearly.
Originally, it was an abbreviation taken by the acronym for “Moving Picture Experts Group”, an international standardization organization for video encoding, but it has come to be used as the name of the encoding method (compression method) standardized by that organization. . .
MPEG processes the differences frame by frame block by block.
The “block” here refers to a group of adjacent pixels.
In video compression, groups of adjacent pixels in a square range are generally treated as a group, and this is called a macroblock. This block is compared to the block at the same position in the next frame, and only the difference is sent as data.
Quoted from Wikipedia “Data compression” (last consultation: 05/21/2020)
A frame is a “one-by-one still image”.
If multiple still images (frames) continue, “shifting portions” and “unchanging portions” will appear.
The part that does not change is the “duplicate information”, so it can be compressed.
Thus, in MPEG, the part that changes and the part that does not change for each frame are processed in block units.
H.264
H.264 is read as “H.dot Nirokuyon”. This is also the standard method for video compression.
H.264 Features
High compression rate. (More than twice that of MPEG-2)
The bit rate is low.
It is sometimes called “MPEG-4 AVC” because it is standardized as “MPEG-4 Part 10 Advanced Video Coding” in the MPEG-4 standard. (MPEG-4 AVC is read as “Mpeg for AVC”. AVC is an abbreviation for “Advanced Video Coding”).
Motion JPEG (MJPEG)
Motion JPEG is read as “Motion Jepeg”. Also written as “MJPEG”.
As the name suggests, the still images (frames) that make up a movie are “JPEG images.” (To be exact, the frame is compressed in JPEG format.)
These JPEG images are made continuous like a flip book.
So to speak, “a set of continuous JPEG images” is the video data.
Motion JPEG (MJPEG) features
It is beautiful even if you look at it in a painting.
The video capacity is great.
The compression ratio is not as high as MPEG.
However, due to the characteristics of MPEG processing in “block units”, there is a drawback that “block noise” occurs when the bit rate is low.
Also, in cases where there is a lot of movement (the change is large for each frame), the image tends to be rough, but in the case of Motion JPEG, it can be handled even in cases where there is a lot of movement. .
In addition, Motion JPEG has a powerful advantage that MPEG and H.264 do not have, ie “you can extract a frame clearly”.
