Gets default bone positions from glTF files (for Minetest)

default_bones.lua

-- helper function
local function quaternion_to_rotation(q)
    local rotation = {}

    local sinr_cosp = 2 * (q[4] * q[1] + q[2] * q[3])
    local cosr_cosp = 1 - 2 * (q[1] * q[1] + q[2] * q[2])
    rotation.x = math.atan2(sinr_cosp, cosr_cosp)

    local sinp = 2 * (q[4] * q[2] - q[3] * q[1])
    if sinp <= -1 then
        rotation.y = -math.pi/2
    elseif sinp >= 1 then
        rotation.y = math.pi/2
    else
        rotation.y = math.asin(sinp)
    end

    local siny_cosp = 2 * (q[4] * q[3] + q[1] * q[2])
    local cosy_cosp = 1 - 2 * (q[2] * q[2] + q[3] * q[3])
    rotation.z = math.atan2(siny_cosp, cosy_cosp)
end

-- call with glTF path & set of bones {bonename = true}
function bone_data(path, bones)
    local file = io.open(path, "r")
    local nodes = minetest.parse_json(file:read("*a")).nodes
    file:close()
    for _, node in ipairs(nodes) do
        if bones[node.name] then
            bones[node.name] = {
                position = vector.new(unpack(node.translation)),
                rotation = (node.rotation and quaternion_to_rotation(node.rotation)) or {x=0,y=0,z=0}
            }
        end
    end
end

-- example call:
-- bone_data(minetest.get_modpath("this_mod").."/models/model.gltf", {Head = true})