The spatial-spectral illumination representation

ISET scenes represent the scene spectral radiance and the scene illuminant. The spectral radiance is a hypercube of data, that is a spectral power distribution at each scene point.

The scene illuminant can be a single spectral power distribution, in which case the illuminant is assumed to be the same (constant) across the entire scene.

Alternatively, the scene illuminant can also be a hypercube of the same size as the scene spectral radiance. In that case, the illuminant, which we call spatial-spectral, is allowed to vary across the scene.

See also: s_sceneIlluminant, s_sceneIlluminantMixtures, sceneIlluminantSS, sceneAdjustIlluminant, sceneAdjustReflectance

Contents

ieInit

Create a test scene

scene = sceneCreate('frequency orientation');

% Store this for later use when transforming the illuminant
illP = sceneGet(scene,'illuminant photons');

Make the scene illuminant spatial spectral

% This converts the illuminant representation to a hypercube that
% matches the spectral radiance.
scene = sceneIlluminantSS(scene);

Have a look at the scene illuminant - it will look uniform and white

illEnergy       = sceneGet(scene,'illuminant energy');
[illEnergy,r,c] = RGB2XWFormat(illEnergy);
wave = sceneGet(scene,'wave');

% This is how we show the any SPD (energy) as an image.
XYZ  = ieXYZFromEnergy(illEnergy,wave);
srgb = xyz2srgb(XW2RGBFormat(XYZ,r,c));
ieNewGraphWin; imagesc(srgb);  % Looks white, I guess

Adjust the SPD along the rows to be blackbody radiators from 6500 to 3000 K

illPhotons = sceneGet(scene,'illuminant photons');
[r,~,~] = size(illPhotons);
cTemp   = linspace(6500,3000,r);
spd     = blackbody(wave,cTemp,'photons');

% Apply the blackbody illuminants down the rows, scaling by the relative
% energy in the original.  Each illPhotons row starts out as illP,
% which we saved above.  We divide out by illP and multiply by the new
% spd for the blackbody calculated just above.  So the new illuminant
% in photons varies down the rows.
for rr=1:r
    illPhotons(rr,:,:) = squeeze(illPhotons(rr,:,:)) * diag((spd(:,rr)./illP(:)));
end

% Adjust the scene radiance to preserve the reflectance.
% When we divide to obtain the reflectance it should be the same
reflectance = sceneGet(scene,'reflectance');
p = reflectance .* illPhotons;
scene = sceneSet(scene,'photons',p);
scene = sceneSet(scene,'illuminant photons',illPhotons);

scene = sceneSet(scene,'Name','Temp varies along rows');
sceneWindow(scene);

Show the illuminant energy as a mesh

illEnergy = sceneGet(scene,'illuminant energy');

vcNewGraphWin;
mesh(wave,1:r,squeeze(illEnergy(:,1,:)))
xlabel('wavelength')
ylabel('pos')
view([-70,44]);

Show the illuminant as an image

% This code is equivalent to
% scenePlot(scene,'illuminant image')

[illEnergy,r,c] = RGB2XWFormat(illEnergy);
wave = sceneGet(scene,'wave');

XYZ = ieXYZFromEnergy(illEnergy,wave);
XYZ = XW2RGBFormat(XYZ,r,c);
srgb = xyz2srgb(XYZ);
ieNewGraphWin; imagesc(srgb);  % Looks white

Make an intensity varying illuminant, starting with spatial spectral

% Initial illuminant photons
illPhotons = sceneGet(scene,'illuminant photons');
[r,c,w] = size(illPhotons);

% We will scale across the columns
cc = 1:c;
illScale = 1 + 0.5*sin(2*pi*(cc/c));
% vcNewGraphWin; plot(wave,spd);

% Scale the illuminant intensity along the cols
for cc=1:c
    illPhotons(:,cc,:) = squeeze(illPhotons(:,cc,:)) * illScale(cc);
end

% Correct the scene photons to preserve the reflectance
% When we divide to obtain the reflectance it should be the same
reflectance = sceneGet(scene,'reflectance');
p = reflectance .* illPhotons;
scene = sceneSet(scene,'photons',p);
scene = sceneSet(scene,'illuminant photons',illPhotons);

% Have a look
scene = sceneSet(scene,'name','Col harmonic');
ieAddObject(scene); sceneWindow;

Now scale across rows

rr       = 1:r;
freq     = 1;
illScale = 1 + 0.5*sin(2*pi*freq*(rr/r));

vcNewGraphWin;
plot(illScale); grid on
title('Scale along the rows');

% Scale across the columns
for ii=1:r
    illPhotons(ii,:,:) = squeeze(illPhotons(ii,:,:)) * illScale(cc);
end

% Correct the energy for this illuminant
reflectance = sceneGet(scene,'reflectance');
scene = sceneSet(scene,'illuminant photons', illPhotons);
scene = sceneSet(scene,'photons',illPhotons .* reflectance);
scene = sceneSet(scene,'name','Row/col harmonic');

ieAddObject(scene); sceneWindow;

Show the scene illuminant image

scenePlot(scene,'illuminant image');

Published with MATLAB® R2024a