Blender: assign a unique vertex color for each material to the mesh (id map)

material_to_vertexcolor.py

import bpy
import random
import colorsys
import math

RANDOM_COLORMAP = False

VERTEX_COLOR_LAYERNAME = "_IDMAP_"
NO_MATERIAL_COLOR = (0,0,0,1)

def material_to_random_color(mat):
    seed = hash(mat.name)
    random.seed(seed)    
    r, g, b = [random.random() for i in range(3)]
    color = (r, g, b, 1.0)    
    return color

def get_material_random_colormap():
    map = {}
    
    for mat in bpy.data.materials:
        color = material_to_random_color(mat)
        map[mat.name] = color
        
    return map

def sin01(r):
    return (math.sin(r) + 1.0) * 0.5
    
def get_material_colormap():
    
    if RANDOM_COLORMAP:
        return get_material_random_colormap()
    else:
        map = {}
        
        material_count = len(bpy.data.materials)
        reciprocal = 1.0 / material_count
        
        i = 0
        
        for mat in bpy.data.materials:
            
            h = i * reciprocal
            s = 0.4 + sin01(math.pi * 2 * h * 191) * 0.6
            v = 0.4 + sin01(math.pi * 2 * h * 7 * 191) * 0.6            
            color = colorsys.hsv_to_rgb(h, s, v)
            color = (*color, 1.0)
            
            print(i, mat.name, h,s,v)
            map[mat.name] = color
            
            i += 1
            
        return map
    

def get_vertex_colors(obj):
    vertex_colors = obj.data.vertex_colors.get(VERTEX_COLOR_LAYERNAME)
    
    if vertex_colors == None:
        vertex_colors = obj.data.vertex_colors.new(name = VERTEX_COLOR_LAYERNAME)
        
    return vertex_colors


def main():
    # make random color for each material

    colormap = get_material_colormap()

    # for each selected object

    for obj in bpy.context.selected_objects:
        
        if obj.type != 'MESH':
            continue
        
        if len(obj.material_slots) == 0:
            print(obj.name + ": No materials assigned")
        
        # ensure vertex color layer
        
        vertex_colors = get_vertex_colors(obj)

        # https://blender.stackexchange.com/questions/51782/get-material-and-color-of-material-on-a-face-of-an-object-in-python
        # https://blender.stackexchange.com/questions/909/how-can-i-set-and-get-the-vertex-color-property
        
        # for each object.data.polygon
        
        i = 0
        
        for polygon in obj.data.polygons:
            
            # get object.material_slots[polygon.material_index]
            
            vertex_color = NO_MATERIAL_COLOR
            
            if len(obj.material_slots) > 0:
                slot_index = polygon.material_index
                mat = obj.material_slots[slot_index]
            
                # get color by slot.material.name
                        
                vertex_color = colormap[mat.name]
                
            # assign color to polygon
            
            for loop_index in polygon.loop_indices:
                vertex_colors.data[i].color = vertex_color    
                i += 1
        pass


if __name__ == '__main__':
    main()