Lab gradient filter

gistfile1.matlab

% Convert an image to the Lab color space
colorTransform = makecform('srgb2lab');
img = applycform(rgbImg, colorTransform);

% Make it double to improve representation
img = double(img);

% Find x and y derivative of a 9x9 gaussian
[hx, hy] = gradient(fspecial('gaussian',[9 9],sigma));

% Apply filters    
gx = double(imfilter(img,hx,'replicate'));
gy = double(imfilter(img,hy,'replicate'));

% Find absolute value
gSquared = sqrt(gx .* gx) + (gy .* gy);

% Apply non-maxima suppression (find best points for edges) 
[mag, ] = max(gSquared, [], 3);