masksize.m

jpg compression and decompression~

function fsize=maskSize(imageMask, quantLevels)
%
% maskSize - returns the size of the mask if RLE compression is used.
%    imageMask assumed to vary between -1 and 1
%


% linearize imageMask
imMask=(imageMask+1)/2;
imMask=round((quantLevels-1).*imMask(:));

% keep an array for length of zero runs
zeroRuns=zeros(length(imageMask(:)), 1);

% keep an array for grey levels
greys=zeros(quantLevels, 1);

% loop through whole image, finding zero runs and grey values
zeroRunLength=0;
numGreys=0;
numZeros=0;
for i=1:length(imMask)
  
  if imMask(i)==round(0.5*(quantLevels-1))
    zeroRunLength=zeroRunLength+1;
  else
    zeroRuns(zeroRunLength+1)=zeroRuns(zeroRunLength+1)+1;
    zeroRunLength=0;
    numZeros=numZeros+1;
    greys(imMask(i)+1)=greys(imMask(i)+1)+1;
    numGreys=numGreys+1;
  end;
  
end;

% calculate average rate of zero chains
symbolProbs=zeroRuns/numZeros;
codeLengths=-log(symbolProbs)/log(2);

% Replace infinite code lengths with very long ones
for j=1:size(codeLengths(:),1)
  if codeLengths(j)== Inf
    codeLengths(j)=100000;
  end;  
end;

zeroRate=sum(symbolProbs.*codeLengths)

% calculate average rate of greys
symbolProbs=greys/numGreys;
codeLengths=-log(symbolProbs)/log(2);

% Replace infinite code lengths with very long ones
for j=1:size(codeLengths(:),1)
  if codeLengths(j)== Inf
    codeLengths(j)=100000;
  end;  
end;

greyRate=sum(symbolProbs.*codeLengths)

fsize=(greyRate*numGreys+zeroRate*numZeros)/8;