How the MPEG Process Works
MPEG
compression dramatically decreases the amount of storage space required
to record motion picture sequences by eliminating redundant and non-essential
image information from the stored data. Less total bits means that motion
pictures can be transferred more rapidly, so rapidly in fact, that slow,
but inexpensive, communication lines and storage devices have entirely
new moving picture applications. The following set of pictures are actual
screen captures of a frame undergoing MPEG compression using the PixelTools'
MPEG Expert-1.
Two adjacent
frames in a motion picture sequence are usually very nearly identical.
Often the only difference is that some parts of the picture have shifted
slightly between the frames. MPEG compression exploits this temporal redundancy
by carving each new frame into convenient pieces and searching the previous
frame to determine where each piece came from. If the content of the current
frame was mostly sent in the previous frame, why send it all again? Just
send the instructions for shifting pieces of the previous frame to their
new positions in the current frame.
Within a single frame many patches, such as regions of sky or walls are
almost entirely the same color. Again, MPEG compression exploits this
spatial redundancy by carving images into convenient pieces and reducing
such patches to a single color. If several pixel points in the same area
are all very nearly the same color, why send the same color over and over?
Just send the color for the whole area once.
The human eye is forgiving of approximation and outright elimination of
the finest details in images. This is fortunate because there is proportionally
a lot more fine detail than coarse detail in images. MPEG compression
approximates the intensity of fine detail with just a few shades (and
progressively more shades for coarser detail) saving many bits over full
representation. The eye also sees less color changes per inch than it
does brightness variations. MPEG compression exploits this reduction by
eliminating non- essential color detail that the human eye tends to gloss
over anyway.
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This image represents an image that has been carved up into about 300 squares to perform the compressions mentioned above. This image is part of a well known test motion picture sequence that depicts passage from left to right along a row of houses fronted by a strip of colorful flowers. The gray patches represent areas that were found in the previous frame and shifted to new positions to form this frame. A uniform gray patch indicates an exactly matching area was found. Mottled gray patches indicate a near match. This operation is performed to compress temporal redundancy, the nearly complete replication of the same picture in two adjacent frames of a sequence. |
Note that close matches for some squares could not be found in the previous frame. There was no close match for the new image detail that shifted into view on the right edge or for the tree that is passing by rapidly in the foreground. The process of finding the squares in another picture is called motion compensation and is very computationally intensive. Applying motion compensation to reconstruct an compressed MPEG sequence is, by contrast, a much easier task. This characteristic is called asymmetrical compression or encoding: hard to compress but easy to decompress or decode.
Image 2 A blowup of a section of Image 1 with the blue lines (called motion vectors) indicating the exact location of the best matching group of pixels in the previous frame used in motion compensation. This motion vector (maybe 4 bits of data) is recorded in the MPEG stream instead of the group of pixels (2048 bits). |
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The original picture with a tremendous amount of detail and color has been transformed to the representation shown in Image 1 with the first compression step. It is important to notice that resultant picture is now nearly one uniform color (the gray) with some finely mottled detail.
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Image
3 |
Image
4 |
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The final representation is nearly all black which is nearly all zeros or very small (short) numbers. These short numbers require very few bits to be stored or transmitted. The formerly, complex and colorful picture sequences have been reduced to the minimum essential representation required for storage or transmission in highly compressed MPEG format.
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