createjpeg.m

jpg compression and decompression~

function size=createJpeg(filename, r, g, b, fsize)
%
% size=createJpeg(filename, r, g, b, fsize)
%
% Creates a JPEG of the image represented by r, g, and b that is of
% size fsize in bytes.  It uses the jpegwrite function, and iterates
% until the proper size (within 500 bytes) is reached.  If the size
% can't be reached, it will get as close as possible.  
%

% set new compression value
q=100;
continue=1;

% upper and lower bounds
highQ=100;
lowQ=0;

% loop until done (old and new q same, or q out of range)
while continue 
  
  % write the file, then open it to check the size
  jpgwrite(filename, r, g, b, q);
  jFile=fopen(filename, 'r');
  fseek(jFile, 0, 'eof');
  size=ftell(jFile);
  fclose(jFile);
  
  % show q
  q
  
  % are we too big or too small?
  if size > fsize
    
    % see if our q lower than highQ
    if q < highQ
      highQ=q;
    end;
    
    % too big, need to lower quality
    q=(q+lowQ)/2;
      
  else
    
    % see if our q bigger than lowQ
    if q > lowQ
      lowQ=q;
    end;
    
    % too small, need to increase quality
    q=(q+highQ)/2;
    	      
  end;
  
  if abs(size-fsize) < 500 
    continue=0;
  end;
  
  if highQ < 1
    continue=0;
  end;
  
  if highQ-lowQ < 1
    continue=0;
  end;
  
end;