    init;
    seti.contrast = 'corner2D';
    seti.rotation = 20;
    seti = setGeomSim(seti);
    qROI = seti.qROIexact;
    [JA,JB] = derivative(seti,qROI);

    DFFq = @(h) JA*diag(h)*JB;
    h = ones(size(qROI))+1i*ones(size(qROI));
    DFFqh = DFFq(h); % Evaluate F'(q)[h].
    figure(1); imagesc(real(DFFqh)); axis xy; colorbar;
    % set(gca,'FontSize',20); axis square; print(1,'-depsc','guideDerivative.eps');