What determines the quality of MPEG-4 movies?
The quality of MPEG-4 movies depends on many factors; they can be roughly divided into three groups.
The quality of the source material. For example, if a two VideoCD (MPEG-1) CD movie with a resolution of 352 * 288 is compressed onto a disc in MPEG-4 or worse, from a pirated videotape, then there can be no doubt of acceptable quality. …
Source video compression parameters: bit rate (data flow that goes through the decoder), image size and others, less significant. The value of these parameters determines, in the first place, the duration of the movie. So on a CD, you can put a movie that is one and a half hours long, or you can come up with a three-hour movie. It is clear that in the first case the data flow turns out to be wider and the required compression ratio is lower. Consequently, the movie will be compressed and burned to CD with less loss of image quality. The optimal choice of encoding parameters in MPEG-4 is strictly individual and depends on a particular movie. Therefore, without sufficient experience in this matter, it is difficult to achieve a good result. It is no secret that all MPEG-4 movies are made by hand. Image quality is often very poor, and the impression of watching a movie can be completely ruined by artifacts and constant image shake. In contrast, in the DVD format, virtually all discs have excellent studio quality picture and sound.
Compressed video decompression settings, video card settings, monitor / TV settings and the speed of the computer used for display. The lower the speed, the more choppy images and frame drops are noticeable (especially in dynamic scenes).
For obvious reasons, the viewer cannot have any influence on the factors of the first two groups, so we will not consider them. The only thing left to do here is to advise you to choose your discs carefully. The factors of the third group depend entirely on the user and his computer, we will consider them in more detail below.
Overlay – what is it?
A video stream in Windows can be played through a special DirectDraw mode called Overlay, while the video information is not sent to the video buffer, but to a separate area of the video card’s local memory, where it is further processed by the hardware. from the video card itself (YUV to RGB color space conversion, hardware scaling and filtering). The size of the overlay frame and the depth of the color are independent of the desktop. After processing, the overlay buffer can be displayed on the desktop or through a separate channel, for example, through the video output to the TV, and many video cards allow you to perform gamma correction on the content of this buffer, adjust the brightness, contrast etc. regardless of the desktop. The overlay is displayed on the desktop using “chromakey” technology. Windows draws the window where the overlay should be displayed, and fills it with “key color”; When a video controller encounters this color, by sending it to a DAC (digital-to-analog converter), it replaces it with data from the overlay buffer, having previously scaled the image to window size or full screen. It is very simple to check if the overlay mode works or not when playing a video: just try to take a screenshot of the screen; if a black rectangle appears in place of the video image, then the video is displayed through the overlay.
What is DivX and what is it for?
DivX (divx.ctw.cc) is a codec that performs compression / decompression of compressed images in the MPEG-4 standard. In an effort to drive competitors out of the video streaming market (eg Apple with QuickTime), Microsoft has developed a codec that allows a video stream to be compressed in MPEG-4 format. In one of the debugging stages of the new product, a beta version of this codec was released to the masses, which, after minor modifications by a group of hackers, was renamed the DivX codec. For “political” reasons, Microsoft then slowed development in this direction, releasing only a module to play already encoded video.
