Color each face of a mesh with a colormap gradient depending on external data

Question

Motivation: Post-process a finite element solver answer file (in my case FEMM).

I'm creating a mesh with Python with the following code (well, my actual code is a bit more complex, but it doesn't matter).

import bpy

mesh = bpy.data.meshes.new("NewMesh")  # add the new mesh
obj = bpy.data.objects.new(mesh.name, mesh)
col = bpy.data.collections.get("Collection")
col.objects.link(obj)
bpy.context.view_layer.objects.active = obj

verts = [(0, 0, 0),
         (1,  1,  0), 
         (1, -1,  0),
         (-1, -1,  0),
         (-1, 1,  0),
         ]

edges = []

faces = [[0, 1, 2], [0, 2, 3], [0, 3, 4], [0, 4, 1]]

face_values = [-1.5, 10, -5, 3]

mesh.from_pydata(verts, edges, faces)

With Python (I have 10000+ faces to colorize), how can I affect a separate color to each face depending on the corresponding value in face_values (based on a color map)?

I'd like to achieve something like that (except that all faces should have a uniform color).

Here is an example that I faked using texture painting:

with [-1.5, 10, -5, 3] corresponding to [red, blue, green, white].

Have a look into vertex colors: https://blender.stackexchange.com/a/60730/31447 — brockmann, May 12 '20 at 16:51
@brockmann Thanks for your comment. As far as I understand, with vertex colors, face colors won't be uniform on each face. Then, if I'm not mistaken, this is not what I'm looking for (I must admit that the image in my question does not properly show this requirement). Please correct me if I'm wrong. — cjorssen, May 13 '20 at 08:02
Sure. You can assign the same color to each vertex of a triangle/quad which makes it a "uniform color" per face component. Does that answer your q? — brockmann, May 13 '20 at 08:20
So a given vertex can have multiple colors corresponding to the different faces it belongs to? — cjorssen, May 13 '20 at 09:08
Nope, only one value. However your can blend them of course, see https://blender.stackexchange.com/a/911/31447 — brockmann, May 13 '20 at 09:11
@brockmann Ah. Ok. Understood. Thanks! But then, I must find the blending algorithm that will allow me to go from face color data to vertex blended color data. I'm wondering if the texture painting way is not simpler (see my edit). — cjorssen, May 13 '20 at 09:25
Updated the script in the linked answer above for 2.8x, run it and you'll see. Not exactly sure what you're talking about. All I can say is that the given example can be achieved using vertex colors as well, see my artistic statement of the day: https://i.stack.imgur.com/6xPKA.png (it is a face attribute so...). — brockmann, May 13 '20 at 09:42
@brockmann Thanks for all. Now I need to study all that. If you can spare the time, would you mind turning your comments (and your artistic statement of the day) into an answer so that I can accept it? — cjorssen, May 13 '20 at 10:12

score 1 · Answer 1 · answered Jan 10 '23 at 21:31

for those stuck on this struggle, this is the code to add arbitrary point centered OR cell centered data to a blender mesh. This is valid for Blender 3.3+ that I've tested, maybe more.

this allows you to store the actual data (floats, float vectors, ints, strings), and then use shader nodes to color it later.

assuming you have an object in your scene called 'Object':

#the object    
newobj = bpy.data.objects['Object']
#convert to bmesh
bm = bmesh.new()
bm.from_mesh(newobj.data)
#these will be out of date, update so you can index
#you only need to update the one you need
bm.verts.ensure_lookup_table()
bm.faces.ensure_lookup_table()
#add point data
pkey = bm.verts.layers.float.new('pointdata')
for j,v in enumerate(bm.verts):
    v[pkey] = np.random.random() #the value for the point
#add cell data
ckey = bm.faces.layers.float.new('celldata')
for j,f in enumerate(bm.faces):
    f[ckey] = np.random.random() #the value for the face
#return data to blender
bm.to_mesh(newobj.data)
bm.free()

you can now see these in your mesh attributes when you click on the object and can set up some simple shader nodes to color by point/cell data:

not a glamorous example by any means, but you get the idea

Color each face of a mesh with a colormap gradient depending on external data

1 Answers1