Delta encoding
Delta encoding is a way of storing data in form of differences (deltas) between sequential data rather than data themselves. It is sometimes called delta compression because some instances of the encoding can make encoded data shorter than non-encoded data.Perhaps the simplest example is storing values of bytes as differences (deltas) between sequential values, rather than the values themselves. So, instead of 2, 4, 6, 9, 7, we would store 2, 2, 2, 3, -2. This is not very useful when used alone, but it can help further compression of data in which sequential values occur often. IFF 8SVX sound format applies this encoding to raw sound data before applying compression to it. Unfortunately, not even all 8-bit sound samples compress better when delta encoded, and the usability of delta encoding is even smaller for 16-bit and better samples. Therefore, compression algorithms often choose to delta encode only when the compression is better than without.
Example C/C++ algorithms for this encoding:
void delta_encode(char *buffer, int length)
{
char t = 0;
int i;
for(i = 0; i < length; i++)
{
buffer[i] -= t;
t = buffer[i];
}
}
void delta_decode(char *buffer, int length)
{
char t = 0;
int i;
for(i = 0; i < length; i++)
{
buffer[i] += t;
t = buffer[i];
}
}As another example, entries in a dictionary can be stored in such a way. Consider the following word list (abbreviations and names are not in the list for the sake of simplicity), on the left stored normally and on the right in form "#letters" where '#' is the number of letters that should be subtracted from previous word and 'letters' are letters that are to be added to the so created base in order to form the next word:
|
a aardvark aardwolf abaca abaciscus aback abacus abaft abandon abandoned abandonee abandonment abase abasement abash |
0a 0ardvark 4wolf 7baca 1iscus 5k 1us 3ft 2ndon 0ed 1e 2ment 7se 0ment 5sh |
As could be seen, list of word deltas is much shorter. But it would probably be longer for unsorted lists of words!
RFC 3229 - Delta encoding in HTTP proposes that HTTP should be able to send updated Web pages in form of differences between versions (deltas), which should decrease Internet traffic, as most pages change slowly over time, rather than being completely overwritten repeatedly:
- This document describes how delta encoding can be supported as a compatible extension to HTTP/1.1.
- Many HTTP (Hypertext Transport Protocol) requests cause the retrieval of slightly modified instances of resources for which the client already has a cache entry. Research has shown that such modifying updates are frequent, and that the modifications are typically much smaller than the actual entity. In such cases, HTTP would make more efficient use of network bandwidth if it could transfer a minimal description of the changes, rather than the entire new instance of the resource. This is called "delta encoding."
External links