scieerror.m

jpg compression and decompression~

function [aveError, maxError, minError]=scieError(originalTIFF, compJPEG)
%
% [aveError, maxError]=scieError(originalTIFF, compJPEG)
%
% scieError returns the average (aveError), maximum (maxError) and
% minimum error between the originalTIFF (a filename) and the
% compressed JPEG (also a filename).  The error is in CIELAB error
% units, and is corrected for spacial viewing  on a 72 DPI monitor at
% 18 inches.
%

% load original
[rOrig gOrig bOrig]=tiffread(originalTIFF);

% load compressed version
[rComp gComp bComp]=jpgread(compJPEG);

% samples per degree at 18 inches, 72 DPI monitor
sampPerDeg = 23;

% Load in spectral power distributions of a sample monitor.
%  (these can be obtained from Wandell's S-CIELAB package)
%	wavelength: a vector describing the wavelengths of the cone
%  		     sensitivities (370:73))
%	displaySPD: a 361x3 matrix of the spectral power distributions
%		     of the red, green, and blue primaries
%
load displaySPD

% Cone information also from Wandell 
% We usually compute with respect to the human cone estimates from
%  Smith and Pokorny (19XX).  These are represented in the file named
%  SmithPokornyCones.mat 
%
%  The file contains the following variables:
%	
%       wavelength:  a vector describing the wavelengths of the cone
%  		     sensitivities (370:73))
%	cones:       a 361 x 3 matrix matrix describing the cone
%		     sensitivities at the corresponding wavelengths
%
load SmithPokornyCones

%  Now, we can compute the 3 x 3 transformation that maps the linear
%  intensity of the display r,g,b signals into the cone absorptions
%  (l,m,s). 
rgb2lms = cones'* displaySPD;

% Load in gamma correction info - also from Wandell's S-CIELAB package
%
%  The file displayGamma.mat contains the variables
%	scieGamma:  is a look-up table that maps 
%		framebuffer values -> relative linear display intensity
%	where the maximum display intensity is 1.0.
%
%	scieInvGamma: is the inverse of gamma and maps
%		relative lin. disp. intensity -> frame-buffer value
%		This table has been interpolated to 1024 intensity levels.
%
load displayGamma

% Set the whitepoint to that of the monitor (should be image dependent
% though)
rgbWhite = [1 1 1]';
whitepoint = rgb2lms* rgbWhite;

% Convert the RGB data to LMS.
img = [ rOrig gOrig bOrig];
imgRGB = dac2rgb(img,scieGamma);
origLMS = changeColorSpace(imgRGB,rgb2lms);

img = [ rComp gComp bComp];
imgRGB = dac2rgb(img,scieGamma);
compLMS = changeColorSpace(imgRGB,rgb2lms);

% Specify the color space
imageformat = 'lms';

% Run the scielab function.
errorImage = scielab(sampPerDeg, origLMS, compLMS, whitepoint, imageformat);

aveError=sum(sum(errorImage))/length(errorImage(:));
maxError=max(max(errorImage));
minError=min(min(errorImage));