Color a mesh using `ListDensityPlot`

Question

I have a list of data and I plot using ListDensityPlot:

lista2 = Drop[
   Import["/home/mateus/LaminarSeparationBubble/RD/Dados/loop_sol_\
sist.dat"], 1];

contorno = 
  Table[{lista2[[i, 1]], lista2[[i, 2]], lista2[[i, 4]]}, {i, 1, 
    Length[lista2]}];


sol = ListDensityPlot[contorno, 
  FrameLabel -> {"\!\(\*SubscriptBox[SuperscriptBox[\(y\), \(*\)], \
\(d\)]\)", 
    "\!\(\*SubscriptBox[SuperscriptBox[\(y\), \(*\)], \(inf\)]\)"}, 
  RotateLabel -> False, FrameStyle -> FontSize -> 14, Mesh -> 25, 
  MeshStyle -> Opacity[0.5]]

Data file:

https://www.dropbox.com/s/s8r4bqbavz6exvt/loop_sol_sist.dat?dl=0

The result:

Is there a way to color the mesh just in the region where is not blue (the blue region is the value of data which is zero)?

Something like that:

Answer 1

You can use a second ListDensityPlot with option RegionFunction ->(#3 > 0 &) to show only the region of interest and combine the two plots using Show:

contorno = lista2[[All, {1, 2, 4}]];
Show[ListDensityPlot[contorno], 
 ListDensityPlot[contorno, RegionFunction -> (#3 > 0 &), Mesh -> 24, MeshStyle -> Red]]

With this method you can restrict the mesh lines to arbitrary regions. For example, replace (#3 > 0& above with (Sin[#] + Cos[#2] <= .2 && # #2 <= 10 &) to get

 data = Table[Sin[x] Cos[y], {y, -Pi, Pi, Pi/64}, {x, -Pi, Pi, Pi/64}]; 
 Show[ ListDensityPlot[data], ListDensityPlot[data, Mesh -> 24, 
  MeshStyle -> Red, RegionFunction ->(-2/4 < #3 < 1/4 ||#3>3/4&)]]