Information Technology Reference
In-Depth Information
Figure 3.28:
Warping is a process which allows the appearance of a non-flat surface to be computed.
Figure 3.29
shows another example. Here a simple rectangular object such as a bus is turning a corner. In one
picture, the outline of the bus may be a rectangle, whereas in the next it has become a trapezoid. In MPEG-2 it
would be necessary to send a considerable amount of data to convert the first picture into the second. In MPEG-4
the facility exists to transmit warping codes so that the texture of the side of the bus is warped in the decoder to the
new shape. This requires fewer data to be transmitted.
Figure 3.29:
An example of warping in compression. The change in perspective as the vehicle turns would require
a large residual in MPEG-2, but with MPEG-4 much of the new shape can be obtained by warping the pixels from
an earlier picture.
The principle of warping is the same as the technique used by cartographers for centuries. Cartographers are
faced with the problem that the earth is round, and paper is flat. In order to produce flat maps, it is necessary to
