Categorical colormaps for meshes/poly or contour plots on meshes

[baxmittens]

Feature description

Hello,
first of all: Makie.jl is a great package. Thank you guys for your efforts.

What I would like to do is plot contours on a mesh or a poly.

using CairoMakie
f = Figure();
ax = Axis(f[1, 1]);
cmap = cgrad(:viridis, 2, categorical = true)
verts = Makie.GeometryBasics.Point2f[(0,0),(0,1),(1,0)]
colrs = [0,0,1]
poly!(ax, verts, color = colrs, colorrange=(0,1), colormap=cmap)
Colorbar(f[2, 1], limits = (0,1), colormap = cmap, vertical = false)
f

It seems like Makie is converting the color = colrs-values to colors first and then do an interpolation of the colors for each pixel in the triangle.

I would like to have it the other way around. First do an interpolation of the color = colrs-values and then transform it to a color according to the colormap.

Is there a way to do this in Makie right now?

Thx in advance.

Graetz max

[asinghvi17]

What you ask for will work in GLMakie but not in CairoMakie, since vector graphics standards only support interpolation over colours in meshes and not values.

[baxmittens]

Thank you for your answer @asinghvi17 .

Can you tell me how this would work or guide me to an example somewhere?

I want to achieve something like that (which is, I think, a standard way to do color plots for field results)

For the latter, I refined the mesh to 3_066_880 elements and provided a constant color for each element, which looks okay but is not perfect.

[asinghvi17]

Hmm, in this case I would use tricontourf instead, if you have a known mesh. You can create a DelaunayTriangulation.Triangulation from that mesh, and pass it to tricontourf.

As you saw, GLMakie will do the interpolation correctly, but if you need a vector renderer then tricontourf should do the job!

[baxmittens]

Hey @asinghvi17

thanks again for your answer.
tricontourf is exactly what I was looking for.
But as the name is tricontourf, I am a bit irritated that I cannot provide a vector with vertices and a vector with indices to that method like

vertices = Point2f[ ... ]
indices = Tuple{Int,Int,Int}[(1,2,3),(2,3,4),...]
vertexcolors = rand(Float64, length(vertices))
tricontourf(vertices, indices, color=vertexcolors)

That would be so easy and convenient. I looked in the recipe, but it would be hard for me to alter.
DelaunayTriangulation.Triangulation is such a high-level construct. Since my domain is non-convex, I would have to compute the boundary of the domain first and provide it as a closed loop (or am I wrong here?). But I know the triangulation so I would rather want to avoid that.

Maybe there is a different solution?
But if not, I think it would maybe be a useful addition to that method to support a triangulation in the notation described above (or some triangle mesh from Meshes.jl).

[jkrumbiegel]

Maybe @DanielVandH could clarify

[DanielVandH]

The tricontourf docstring mentions this case:

  triangulation = DelaunayTriangulation() — The mode with which the points in xs and ys are triangulated. Passing DelaunayTriangulation() performs a Delaunay triangulation. You can also pass a preexisting triangulation as an AbstractMatrix{<:Int} with size (3, n), where each column specifies the vertex    
  indices of one triangle, or as a Triangulation from DelaunayTriangulation.jl.

Just make your indices a matrix and use triangulation = indices.

[baxmittens]

Thank you very much. This is what I was looking for. Closing this for now...

[baxmittens]

@asinghvi17 @jkrumbiegel @DanielVandH

Got figured it out.
The result is an 80KB PDF graphic that looks ridiculously beautiful.
This is plain nuts. Thank you for being so supportive.

temp.pdf