Package loci.formats.codec

Class LZWCodec

java.lang.Object
loci.formats.codec.BaseCodec
loci.formats.codec.LZWCodec
All Implemented Interfaces:
loci.formats.codec.Codec
public class LZWCodec extends BaseCodec
This is an optimized LZW codec for use with TIFF files. Most of the code is inlined, and specifics of TIFF usage of LZW (known size of decompressor output; possible lengths of LZW codes; specified values for CLEAR and END_OF_INFORMATION codes) are taken in account.

Estimating the worst-case size of compressor output:

  • The worst case means that there is no compression at all, and every input byte generates code to output.
  • This means that the LZW table will be full (and reset) after reading each portion of 4096-256-2-1=3837 bytes of input (first 256 codes are preallocated, 2 codes are used for CLEAR and END_OF_IFORMATION, 1 code is lost due to original bug in TIFF library that now is a feature).
  • Each full portion of 3837 byte will produce in output:
    • 9 bits for CLEAR code;
    • 9*253 + 10*512 + 11*1024 + 12*2048 = 43237 bits for character codes.
  • Let n=3837, m=(number of bytes in the last incomplete portion), N=(number of bytes in compressed complete portion with CLEAR code), M=(number of bytes in compressed last incomplete portion). We have inequalities:
    • N <= 1.41 * n
    • M <= 1.41 * m
    • The last incomplete portion should also include CLEAR and END_OF_INFORMATION codes; they occupy less than 3 bytes.
    Thus, we can claim than the number of bytes in compressed output never exceeds 1.41*(number of input bytes)+3.