Transform coding is used to convert spatial image pixel values to transform
coefficient values. Since this is a linear process and no information is lost, the
number of coefficients produced is equal to the number of pixels transformed.
The desired effect is that most of the energy in the image will be contained in a
few large transform coefficients. If it is generally the same few coefficients that
contain most of the energy in most pictures, then the coefficients may be further
coded by lossless entropy coding. In addition, it is likely that the smaller
coefficients can be coarsely quantized or deleted (lossy coding) without doing
visible damage to the reproduced image.
