Add different material to top face of mesh with varying CSG subtractions

[jmeng]

r145

Hi, these forums and SO have been incredibly helpful so far, but I’m stuck on the final piece of a personal project.

I start with a mesh using THREE.BoxGeometry where I use an array of materials (one for the top face and another for all other faces).

I then use manthrax’s THREE-CSGMesh to create a new non-indexed mesh by subtracting a smaller mesh from my starter mesh.

Below are the desired results that I have so far, but I’ve hardcoded (through trial and error) the vertex indices when using geometry.addGroup().

const vertices = csg_result.geometry.getAttribute('position').array
const vert_count = vertices.length

//successful trial and error vertex indices for one cube subtraction
const start = 84
const mid = 30
const end = 114

//vertex indices for two cube subtractions
//const start = 150
//const mid = 60
//const end = 210

csg_result.geometry.clearGroups()
csg_result.geometry.addGroup( 0, start, 0 )
csg_result.geometry.addGroup( start, mid, 1 )
csg_result.geometry.addGroup( end, vert_count, 0 )

const materials = [
        dirt_material,
        pdf_material,
    ]

csg_result.material = materials

With constant variables, the top material will not be applied correctly if I were to make more cube subtractions or subtractions using different geometry types (which makes sense).

I’m using the y-up coordinate system and I assumed the vertices that create the triangles on the top face would always have a local position.y value of 7.5. However, the log below shows the vertex data at the current working indices (84 - 114) and that doesn’t seem to be the case.

Sorry for the novel…Does anyone have any ideas on how I can parse geometry.getAttribute('position').array to identify the correct range of vertices for only the top face? There’s a good chance I’m missing something here or doing something wrong, so let me know if that’s the case.

Thanks.

[jmeng]

I figured it out. I realized geometry.getAttribute('position').array provides the XYZ coordinates in a one-dimensional array. My mistake was not iterating in sets of three for the vertices and then again for the triangles to test the y-values.

const vertex_data = csg_result.geometry.getAttribute('position').array
const data_count = vertex_data.length
let group_start_index = null
let group_end_index = null
let start_status = false

for (let i = 8; i <= data_count; i+=9) {
    const v1 = [vertex_data[i - 8], vertex_data[i - 7], vertex_data[i - 6]]
    const v2 = [vertex_data[i - 5], vertex_data[i - 4], vertex_data[i - 3]]
    const v3 = [vertex_data[i - 2], vertex_data[i - 1], vertex_data[i - 0]]

    if (group_start_index == null && v1[2] == 7.5 && v2[2] == 7.5 && v3[2] == 7.5) { // top of mesh always at position.y 7.5
        group_start_index = (i - 8) / 3
        start_status = true
    }

    if (start_status && group_end_index == null) {
        if (v1[2] != 7.5 || v2[2] != 7.5 || v3[2] != 7.5) {
            group_end_index = (i - 8) / 3
        }
    }
}

csg_result.geometry.clearGroups()
csg_result.geometry.addGroup( 0, group_start_index, 0 )
csg_result.geometry.addGroup( group_start_index, group_end_index - group_start_index, 1 )
csg_result.geometry.addGroup( group_end_index, data_count / 3, 0 )

const materials = [
    dirt_material,
    pdf_material,
];

csg_result.material = materials