Delta encoding
|
|
Delta encoding is a way of storing or transmitting 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. However, in video compression delta frames can considerably reduce frame size, and are used in virtually every video compression codec.
A variation of delta encoding which encodes differences between the prefixes or suffixes of strings is called incremental encoding. It is particularly effective for sorted lists with small differences between strings, such as a list of words from a dictionary.
In delta encoded transmission over a network where only a single copy of the file is available at each end of the communication channel special error control codes are used to detect which parts of the file has changed since its previous version.
The nature of the data to be encoded influences the effectiveness of a particular compression algorithm. Delta encoding performs best when data has small or constant variation; for an unsorted data set, there may be little to no compression possible with this method.
The following C code performs a simple form of delta encoding and decoding:
void delta_encode(char *buffer, int length)
{
char t = 0;
char original;
int i;
for(i = 0; i < length; i++)
{
original = buffer[i];
buffer[i] -= t;
t = original;
}
}
void delta_decode(char *buffer, int length)
{
char t = 0;
int i;
for(i = 0; i < length; i++)
{
buffer[i] += t;
t = buffer[i];
}
}
Another instance of use of delta encoding is RFC 3229, "Delta encoding in HTTP," which proposes that HTTP servers should be able to send updated Web pages in the form of differences between versions (deltas), which should decrease Internet traffic, as most pages change slowly over time, rather than being completely rewritten 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."