MPEG and its applications. video art

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MPEG and its application

video art

Concept

Video data, by its nature, is extremely large. Experts in this field have been struggling with the task of making video compression as efficient as possible for many years. At the beginning of the third millennium, due to the urgent need to transmit large volumes of video over various networks, the task of an optimal video encoding method in terms of quality / volume ratio became even more urgent.

The concept of video compression in MPEG is very simple - to determine what kind of information in the stream is repeated at least during some period of time and take measures to avoid duplication of this information. The most valuable advantage of MPEG encoding, which is especially convenient for transmission over various networks, is the ability to flexibly adjust the image quality depending on the network bandwidth. This made MPEG-2 the de facto standard for receiving / transmitting digital television over various networks.

Unfortunately, there is no way to unambiguously evaluate the quality of coding using certain instruments and measurements. The only criterion here is the person and how he perceives the compressed information. Therefore, the video data compression rules for MPEG encoding were developed on the basis of the human visual perception model (HVS - Human Visual Sense).

Image redundancy according to HVS is defined by three main criteria:

Image details invisible to the human eye are vertical and horizontal blanking points. Removing this information does not affect the image at all.
Statistical redundancy. It is divided into spatial and temporal. Spatial redundancy refers to areas of the image in which adjacent pixels are almost the same. Under the temporary - not changing in time fragments of the image.
Redundancy in color and brightness - calculated based on the limited human sensitivity to small changes in the colors and brightness of image details.

How is it implemented

For the convenience of encoding video data, the entire video stream is divided into groups called GOPs (Group of Pictures). Such a group is built as follows:

MPEG and its application

Here:

I - Intra frames, which are usually called anchor and contain all information about the image. MPEG sequences without these frames cannot exist in principle. When I frames are compressed, only spatial redundancy is removed. It is from this frame that the decoding of the image in the sequence begins.

P - Predictive frames. "Predicted" frames, in the formation of which the method of predicting the image on the next frame is used, taking into account motion compensation from the last I or P frame before the one being formed. The P frame also serves to further predict the image. The P frame is created using interframe compression, which reduces both spatial and temporal redundancy. The picture P of the frame is subtracted from the next picture and this difference is encoded and added to the compressed data along with the motion vector.

B - Bi-directional, "bidirectional" frames. They are so named because they store the most essential information from the surrounding I and P frames. B frames have the highest degree of compression, but require a previous and subsequent image to compensate for the movement of objects in the image.

Such an MPEG stream structure is usually described as a fraction M/N, for which M indicates the total number of frames in the GOP, and N indicates what the next P frame after the previous one will be. So the GOP sequence shown in the figure above can be written as 12/3.

The actual MPEG data stream consists of 6 hierarchical levels:

Block - brightness and color data for 8x8 image blocks. Blocks are analyzed for Y (brightness), CB, and CR (color difference) values.
Macroblock - as the name suggests, it consists of 4 simple blocks in a 16x16 pixel window, respectively. In the 4:2:0 format, a macroblock contains 4 blocks of Y luminance data and one each of CB and CR.
Layer - contains several adjacent macroblocks.
Frame - consists of a group of layers containing an image, which, in turn, can be either I, P or B.
Group of images (aka GOP) - contains a sequence of frames. It can include up to 15 frames and must necessarily begin with frame I.
Video sequence - must contain at least one GOP, as well as a title at the beginning of the sequence and an end code for the sequence.

MPEG levels and profiles

An MPEG profile is a subset of the compressed video bitstream structure. Within such a subset, a wide spread of stream parameters and, accordingly, encoders and decoders for them is possible.

Level refers to a number of restrictions applied to MPEG stream parameters, for example, output image resolution, frame rate, etc.

The table below illustrates the maximum limits for MPEG levels and profiles:

This table is needed in the first place, to understand the frequently encountered designations of the format of a particular record or descriptions of the capabilities of any encoder. For example, this is how an MPEG recording can be characterized:

** In fact, it would be more correct to write here: PAL - the name of the analog television standard in which the original video was created. Actually, MPEG film cannot be attributed to any color television system, since frames in MPEG are simply a digital representation of a previously analog image and have nothing to do even with the original television system for the original film.

Practical use

A picky reader will ask - how will this information help me in my work? For those who do not work with video, of course, this information can give little, but the rest can help in their practical work. Many users who started working with DV camcorders and non-linear video editing programs began to think about ways to save their creations, if not for centuries, then at least for the next few decades. Strange as it may seem, most beginner DV video editors, still not knowing MPEG thoroughly, plan to store their projects on CDs in MPEG format, not even realizing how difficult this task is to solve if the user wants to preserve the quality of the original DV movie in MPEG version. So, the most frequently asked questions on this topic are:

Why, after saving my movie in MPEG-2 format, do I see that its quality has noticeably deteriorated?

There are several reasons for this:

An already compressed, compressed image is dropped from a digital video camera to a computer. DV is essentially a special case of JPEG compression - it is a stream consisting of frames compressed according to a slightly modified JPEG algorithm. From the idea (but not the compression principles) of MPEG in DV, the only thing used is that when compressing in DV, information from two fields of one frame is taken into account - the so-called intrafield compression. Therefore, to compress in MPEG DV, the image is first decompressed and then recompressed, which will never improve the quality.
The compression settings were set incorrectly in terms of image quality.
A fast, but not very high-quality MPEG encoder is used.

Why is the image quality on DVD discs very high, although it is also MPEG-2 - is it possible to get such quality at home?

The video quality on DVD discs is really very high. It is explained by two main factors - uncompressed professional-quality video is used as the source video material for creating an MPEG-2 version of the film, and, in addition, hardware encoders with very high image coding quality are used for encoding in MPEG-2. The cost of such encoders reaches several tens of thousands of US dollars. Therefore, it is impossible to obtain such quality from a DV movie at home.

What do the parameters that you are asked to specify before encoding a movie into MPEG mean?

There can be many such parameters, but we should dwell on the most important of them:

Profile ID and Level ID - see table above.
Frame Intervals - to understand what is meant, it should be noted that the interval between I frames is usually set, which actually means the size of the GOP, and also what the next P frame will be after the previous one. The longer the interval between I frames, the worse the image quality will be, but the size of the resulting file will be smaller. In addition, the interval between I frames is not recommended to be greater than 15.
Motion Estimation - motion compensation. The parameter determines how correctly the motion of the object will be predicted and is very important both for the quality of the output image and for estimating the encoding time. A larger value will give better quality, but will significantly increase the encoder's running time. When setting this parameter, you need to understand that the maximum value can be "unbearable" for the computer processor and the desired movie can be created for several days.
Frame sequence is a parameter similar to Frame Intervals in many respects. Usually it offers to choose the type of encoding - only I frames, I + P frames and I, P, B frames, as well as intervals for P and B frames. As for how to encode - the best quality, but the maximum amount is encoded in only I frames, then, respectively, I + P and I, P, B. The classic proven values for the interval values are P=3 and B=2. Strictly speaking, it is customary for DVDs to use the 15/3 structure, or IBBBPBBPBBPBBPBB.
Video Data Rate - video stream speed. In many encoders designed for the mass user, this is the main parameter, relative to which the encoder himself sets all the others. It is usually measured in megabits per second. By setting this value and multiplying it by the duration of the movie in seconds, it is easy to get the size of the total file.

I have a DV camcorder with progressive scan capability. Then I want to save the finished movie in MPEG format. How do I shoot better, normal interlaced or progressive?

For MPEG-1, the answer is unequivocal - shooting with progressive scan is better, so when compressing in MPEG-1, one field of the television image is simply discarded. For MPEG-2, the answer to this question depends on how you plan to use the generated MPEG-2 clip. If you plan to view it only on a computer and / or there is no hardware decoder, then it is better to use progressive scan shooting - the image will be clearer, without motion artifacts. If there is a hardware MPEG-2 decoder and you plan to watch a movie on a TV, then smoother and more natural movements of objects in the movie will be when shooting in normal mode. Many modern MPEG-2 encoders allow you to select the type of the original movie (progressive or interlaced), when you select interlaced scan, fields are used, and when output from a hardware decoder to the video output, the original image will be restored with the same fields as before compression.

I want to immediately save movies in MPEG format and edit in it. Is it possible and right to do so?

Of course, you can save, but editing MPEG video is very difficult and inconvenient. The fact is that when editing MPEG, real accurate cutting of fragments to the frame is impossible, since fragments can be separated from each other only by I frames, otherwise, when the reference frame is deleted and the frames following it are saved, the image will lose readability. MPEG-enabled video editors allow you to sort of edit with frame accuracy, but you need to understand that with any changes not to the first frame, the entire GOP will be recompressed, which will affect the quality of the film by no means for the better. The imposition of effects, transitions, filters will also be impossible to do with an accuracy of a real frame, which may lead to the impossibility of a high-quality embodiment of artistic ideas. In addition, image recompression in the case of MPEG-2 will take much more time compared to DV and require more powerful processors for this.

Now MPEG-4 format is very popular in Russia. Does it make sense to save your movies in this format?

The MPEG-4 format was originally created for video transmission over low-speed channels and was not intended for video storage. Image quality in MPEG-4 is definitely worse than MPEG-2, but slightly better than MPEG-1. Compressing/decompressing in MPEG-4 requires a lot of CPU time and viewing, and even more so creating video in MPEG-4 format on a computer with a processor below the Celeron II 700 is very difficult. Basically, MPEG-4 is used to recompress original MPEG-2 movies recorded on DVD discs into it. MPEG-4 allows you to burn an hour-long movie on a regular CD. There is no point in saving your own films in this format for two main reasons:

The quality is much worse than the original.
There is no standard for recording MPEG-4 to disc, similar to the DVD standard.

Which MPEG encoders support the DV format and provide good quality at the same time?

There are several popular MPEG encoders that support the DV format:

TMPEnc Encoder is a program that is popular not so much for its MPEG encoder itself, but for its tools for multiplexing, demultiplexing and merging/cutting MPEG files.
Main Concept MPEG Encoder is a good encoder built into the popular video editors Adobe Premiere, Ulead MSP and Vegas Video.
Canopus ProCoder is arguably the best DV encoder out there.

Author: Spline Company; Publication: pctuner.ru

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