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Happy New Year

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Enigma

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Not really much to talk about what with Christmas and the new year. I've spent a little free time looking further into Jpeg2000 over the last couple of weeks. I'm now trying to get my head round entropy decoding and the MQ arithmetic decoder. I printed out 22 pages of source code to take away with me over Christmas. I think I understand enough about the arithmetic decoder, which was about three of those 22 pages. The entropy decoder which took up the remainder of the space appears to be a very complicated "optimised" implementation of a relatively simple algorithm. I put the word optimised in quotes because I'm pretty confident that it was a bad choice of optimisation strategy. I shall find out if I'm right in the new year.

I thought I'd leave you with a couple of snippets from the JJ2000 source which made me laugh, when they didn't make me cry.

Check the JavaDoc comments:
/**
* Returns the reversibility of the filter. A filter is considered
* reversible if it is suitable for lossless coding.
*
* @return true since the 9x7 is reversible, provided the appropriate
* rounding is performed.
* */
public boolean isReversible() {
return false;
}
Tricky stuff, that dyadic decomposition:
for (rl=0; rl <= maxrl; rl++) { // Loop on resolution levels
// Find the number of subbands in the resolution level
if (rl == 0) { // Only the LL subband
minb = 0;
maxb = 1;
}
else {
// Dyadic decomposition
hpd = 1;
// Adapt hpd to resolution level
if (hpd > maxrl-rl) {
hpd -= maxrl-rl;
}
else {
hpd = 1;
}
// Determine max and min subband index
minb = 1<<((hpd-1)<<1); // minb = 4^(hpd-1)
maxb = 1<<(hpd<<1); // maxb = 4^hpd
}
// Allocate array for subbands in resolution level
exp[rl] = new int[maxb];
for(j=minb; j tmp = ehs.readUnsignedByte();
exp[rl][j] = (tmp>>SQCX_EXP_SHIFT)&SQCX_EXP_MASK;
}// end for j
}// end for rl
I've also decided, after reading far too much bad C++ code on the internet and far too many bad tutorials that come 2007 I shall try to find time to write my own modern C++ tutorial. Watch this space.

All that remains now is for me to wish you a Happy New Year. Farewell 2006, I hardly knew you.

?nigma
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