function opt2dot(opt, filename)
% Produce a GraphViz .dot file from an Optickle model.

% The .dot format is documented here:
% http://www.graphviz.org/Documentation/dotguide.pdf

isPDF = 0;

if nargin < 2
    fid = 1;
else
    if strcmpi(filename(end-3:end),'.pdf')
        isPDF = 1;
        pdffilename = filename;
        filename = [filename(1:end-4),'.dot'];
    end
    fid = fopen(filename, 'w');
end

% suppress optickle's warnings, if optickle set a warning id, 
% we wouldn't need to set all off
warning('off','all');

fprintf(fid, 'digraph G {\n');
% output the optics (graph nodes)
for snOptic=1:opt.Noptic,
    optic = opt.optic{snOptic};
    fprintf(fid, '    %s', optic.name);
    switch class(optic)
        case 'Source',
            P0=(optic.vArf((length(optic.vArf)+1)/2))^2;
            fprintf(fid, ' [shape=box,style=filled,fillcolor=grey,label="%s\\nP0=%.1f W"]',optic.name,P0);
        case 'RFmodulator',
            fprintf(fid, ' [shape=box,style=filled,fillcolor=orange,label="%s\\nfMod=%.2f MHz\\naMod=%.2f+%.2fi"]',optic.name,optic.fMod/1e6,real(optic.aMod),imag(optic.aMod));
        case 'Mirror',
            fprintf(fid, ' [shape=box,style=filled,fillcolor=lightblue,label="%s\\nChr=1/%.1f m\\nThr=%.2f %% "]',optic.name,1/optic.Chr,optic.Thr*100);
        case 'BeamSplitter',
            fprintf(fid, ' [shape=box,style=filled,fillcolor=lightskyblue,label="%s\\nChr=1/%.1f m\\nThr=%.2f %%"]',optic.name,1/optic.Chr,optic.Thr*100);
        case 'Telescope',
            fprintf(fid, ' [shape=diamond,style=filled,fillcolor=violet,label="%s\\nf=%.1f m"]',optic.name,optic.f);
        case 'GouyPhase',
            fprintf(fid, ' [shape=hexagon,style=filled,fillcolor=yellow,label="%s"]',optic.name);
        case 'Sink',
            fprintf(fid, ' [shape=ellipse,style=filled,fillcolor=limegreen,label="%s\\nloss=%.2f"]',optic.name,optic.loss);
    end
    fprintf(fid, ';\n');
end
% output the links (graph edges)
for snLink=1:opt.Nlink,
    [src_name, src_sn, src_port] = getSourceName(opt, snLink);
    [snk_name, snk_sn, snk_port] = getSinkName(opt, snLink);
    fprintf(fid, '     %s -> %s [label="%s&rarr;%s\\n%.4f m",color=red];\n', ...
        getOpticName(opt, src_sn), ...
        getOpticName(opt, snk_sn), ...
        opt.optic{src_sn}.outNames{src_port}{1}, ...
        opt.optic{snk_sn}.inNames{snk_port}{1}, ...
        opt.link(snLink).len);
end
% output the probes
for snProbe=1:opt.Nprobe,
    probe = opt.probe(snProbe);
    if probe.freq==0,
        fprintf(fid, '    %s [color=blue,label="%s\\nDC"]', probe.name,probe.name);
    else
        fprintf(fid, '    %s [color=blue,label="%s\\nfreq=%.2f MHz\\nphase=%.1f deg"]', probe.name,probe.name,probe.freq/1e6,probe.phase);
    end
    fprintf(fid, ';\n');
    for snOptic=1:opt.Noptic,
        optic = opt.optic{snOptic};
        try
            if optic.in(1)==probe.nField,
                fprintf(fid, '     %s -> %s [label="%s",color=red];\n',optic.name,probe.name,optic.inNames{1}{1});
                break
            elseif optic.in(3)==probe.nField,
                fprintf(fid, '     %s -> %s [label="%s",color=red];\n',optic.name,probe.name,optic.inNames{3}{1});
                break
            end    
        end
    end
end
fprintf(fid, '}\n');

% now turn warnings back on
warning('on','all');

if fid ~= 1
    fclose(fid);
end

if isPDF
    system(['dot -Tpdf ',filename,' > ',pdffilename]);
end