title: Mask overlay with matlab
body: <p>I have the following code which shows an image called <code>zero</code>.  </p>
  
  <p>I would like just to overlay a mask image (only <code>0</code> and <code>1</code> values) of the same dimensions so that <code>0</code> values are transparent (and then we can see the values of the <code>zero</code> image) and <code>1</code> values are shown in black.  </p>
  
  <p>What could I do?</p>
  
  <pre><code>hFig = figure(1);    
  set(hFig, 'Position', [11 11 27 7.5]);    
  set( gca, 'Units', 'normalized', 'Position', [0.05 0.05 0.93 1.2] );    
  set(0,'defaultFigureUnits','centimeters');    
  m_proj('equidistant cylindrical','longitudes',[-15 21],'latitudes',[11.5 18]);    
  m_gridF('ytick',4,'tickdir','out','ticklen',0.005,'linestyle','none');
  
  hold; Plat=18-[0:13]*0.5;    
  Plon=-15+[0:72]*0.5;    
  [Plg,Plt]=meshgrid(Plon,Plat);    
  m_pcolor(Plg,Plt,zero);shading flat;
  
  hold on;    
  polarmap(flipud(polarmap),0)    
  M=m_shaperead('clip');
  
  for k=1:length(M.ncst),    
      m_line(M.ncst{k}(:,1),M.ncst{k}(:,2),'color','k'); 
  end    
  hold;
  
  h=colorbar('SouthOutside');    
  set(h, 'Position', [0.13 .1 .78 .05]);    
  set(get(h,'title'),'string','test','fontsize',14)    
  hold;
  </code></pre>