I've always believed that the compression software creates a table. So for example in a line of 3000 pixels there may be 967 particular ones (in blue sky there might be more) the table lists them and their positions and stores the data. The program reconstructs the image from the data in the table. Blue skies compress very well as do any large areas of the same hue and density. Try this with 'lossless' compression of a TIFF with plenty of sky, then one with a lot of detail trees, buildings, etc. The files will be very different in size. But this is an oversimplification. What do our software experts say? JPEG files are quite complicated and I may be totally wrong. Its also possible to drop the least significant bits in each byte without messing the images up too much. You can do all kinds of arcane things with these bits.
Don W Bob W wrote: >> -----Original Message----- >> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On >> Behalf Of keith_w >> >> Perhaps you'll pardon my ignorance, but, how does the >> software know what to >> add back, and where to put it? >> >> > > In some cases it doesn't, because jpeg is a lossy algorithm. However, > in general compression techniques find patterns in the source data, > and identify the frequency with which they occur. They store the > pattern itself & the frequency once in the destination and replace > other occurrences with references to the pattern. To reconstruct the > original they replace the references with the actual pattern. > > This is a much simplified explanation! > > Bob > > > > -- Dr E D F Williams www.kolumbus.fi/mimosa/ http://personal.inet.fi/cool/don.williams/ 41660 TOIVAKKA – Finland - +358400706616 -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
