HP L 5 manual Set Compression Method Command

Set Compression Method Command

The Set Compression Method command allows you to encode the data in a given raster transfer, either by plane or by row. These methods achieve data compression by exploiting byte-to-byte redundancies, either within a row or between rows, without regard to the pixel encoding mode of the data. While these methods reduce the amount of data transferred from the host to the printer, they do not reduce the memory requirements within the printer.

While PCL 5 compression methods are based on redundancies at the byte level, the pixel encoding mode must be considered when choosing a compression method. The direct by pixel mode transfers the three bytes which make up a pixel, one after the other. Therefore, compression formats that depend upon reducing redundancy within a row will not do well, since the pixel to pixel redundancy is disguised by the format. This is because, while two adjacent pixels have a fairly high probability of being identical, the probability that the components of a pixel are identical is very small. For example, consider a raster image whose background is all one color. While there are a large number of pixels that are identical, the only time the bytes within the pixel will be identical are if the color is a gray somewhere between white and black: such as (0, 0, 0), (128, 128, 128), (255, 255, 255). If the pixel represents a non-gray color, the bytes within the pixel will not be identical. Therefore, the only successful compression mode that exploits redundancy between the rows is delta row compression. Since the probability that a pixel is the same in the horizontal direction is usually equivalent to the probability that it is the same in the vertical direction, the bytes within a pixel will be redundant and therefore compressible from row to row.

The remaining pixel encoding modes: index by plane, index by pixel, and direct by plane, can be redundant within a row and are, therefore, compressed well by compression methods such as run-length encoding, and TIFF rev 4.0.

6-20 Raster Graphics EN