In most video sequences, most of the scenes are fixed or change very little; this is what is called the temporal redundancy.
When only the lips of the actor move, it is almost only the pixels of the mouth that will be modified from one image to another; it is thus sufficient to just describe the change from one image to another. This is the main difference between MPEG (Moving Pictures Experts Group) and M-JPEG. However, this method will have much less impact on an action scene.
The MPEG Group was established in 1988 with the aim of developing international standards for compression, decompression, processing and coding of animated images and audio data.
There are several MPEG standards:
The MPEG-1 standard represents each image as a set of 16 X 16 blocks. It makes it possible to obtain a resolution of:
MPEG-1 makes it possible to achieve rates of around 1.2 Mbps (readable on a CD-ROM).
MPEG-1 allows videos to be encoded using several techniques:
These images are coded only by using JPEG, coding, without worrying about the images which surround them. Such images are necessary in a MPEG video because it is these that ensure image cohesion (since the others images are described relative to their surrounding images); they are particularly useful for video streams which can be tuned in at will at any time (television), and are essential in the event of any error in the reception. There is thus one or two of these per second in a MPEG video.
These images are defined by their difference relative to the preceding image. The encoder seeks the differences of the image compared to the preceding one and defines blocks, called macroblocks (16x16 pixels) which will be superimposed on the preceding image.
The algorithm compares both images block per block and starting from a certain difference threshold it considering the preceding image area to be different from that of the image in progress and applies a JPEG compression to it.
It is the search for the macroblocks which will determine the speed of the encoding, because the more the algorithm seeks “good” blocks, the more time it wastes â€¦
Compared to I frames (directly compressed), P frames require the preceding image to always be in memory.
Like P frames, B frames work based on differences relative to a reference image, except that in the case of B frames this difference can be carried out either on the preceding one (as in the case of P frames) or on the following one, which allows a better compression, but induces a delay (since the following image needs to be known) and makes it necessary to keep three images in the memory (the preceding one, the current one and the following one).
These images offer a very low resolution quality but allow a very fast decompression, which is particularly useful during fast forward viewing because “normal” decoding would require too many processor resources.
In order to optimize MPEG coding, in practice the image sequences are coded according to a succession of I, B, and P images (D being, as was mentioned above, reserved for fast forward viewing), the order of these having been determined experimentally. The sequence known as GOP (Group Of Pictures is the following: