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Add Minetest .obj model reader
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Lars Mueller
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76
doc/irr_obj_spec.md
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76
doc/irr_obj_spec.md
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# Minetest Wavefront `.obj` file format specification
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Minetest Wavefront `.obj` is a subset of [Wavefront `.obj`](http://paulbourke.net/dataformats/obj/).
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It is inferred from the [Minetest Irrlicht `.obj` reader](https://github.com/minetest/irrlicht/blob/master/source/Irrlicht/COBJMeshFileLoader.cpp).
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`.mtl` files are not supported since Minetest's media loading process ignores them due to the extension.
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## Lines / "Commands"
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Irrlicht only looks at the first characters needed to tell commands apart (imagine a prefix tree of commands).
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Superfluous parameters are ignored.
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Numbers are formatted as either:
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* Float: An optional minus sign (`-`), one or more decimal digits, followed by the decimal dot (`.`) then again one or more digits
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* Integer: An optional minus sign (`-`) followed by one or more decimal digits
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Indexing starts at one. Indices are formatted as integers. Negative indices relative to the end of a buffer are supported.
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* Comments: `# ...`; unsupported commands are silently ignored as well
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* Groups: `g <name>` or `usemtl <name>`
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* Subsequent faces belong to a new group / material, no matter the supplied names
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* Each group gets their own material (texture); indices are determined by order of appearance
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* Empty groups (groups without faces) are ignored
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* Vertices (all numbers): `v <x> <y> <z>`, global to the model
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* Texture Coordinates (all numbers): `vt <x> <y>`, global to the model
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* Normals (all numbers): `vn <x> <y> <z>`, global to the model
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* Faces (all vertex/texcoord/normal indices); always local to the current group:
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* `f <v1> <v2> <v3>`
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* `f <v1>/<t1> <v2>/<t2> ... <vn>/<tn>`
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* `f <v1>//<n1> <v2>/<n2> ... <vn>/<nn>`
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* `f <v1>/<t1>/<n1> <v2>/<t2>/<n2> ... <vn>/<tn>/<nn>`
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## Coordinate system orientation ("handedness")
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Vertex & normal X-coordinates are inverted ($x' = -x$);
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texture Y-coordinates are inverted as well ($y' = 1 - y$).
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## Example
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```obj
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# A simple 2³ cube centered at the origin; each face receives a separate texture / tile
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# no care was taken to ensure "proper" texture orientation
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v -1 -1 -1
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v -1 -1 1
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v -1 1 -1
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v -1 1 1
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v 1 -1 -1
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v 1 -1 1
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v 1 1 -1
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v 1 1 1
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vn -1 0 0
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vn 0 -1 0
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vn 0 0 -1
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vn 1 0 0
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vn 0 1 0
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vn 0 0 1
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vt 0 0
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vt 1 0
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vt 0 1
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vt 1 1
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g negative_x
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f 1/1/1 3/3/1 2/2/1 4/4/1
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g negative_y
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f 1/1/2 5/3/2 2/2/2 6/4/2
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g negative_z
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f 1/1/3 5/3/3 3/2/3 7/4/3
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g positive_x
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f 5/1/4 7/3/4 2/2/4 8/4/4
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g positive_y
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f 3/1/5 7/3/5 4/2/5 8/4/5
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g positive_z
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f 2/1/6 6/3/6 4/2/6 8/4/6
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```
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190
minetest/obj.lua
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minetest/obj.lua
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local assert, tonumber, type, setmetatable, ipairs, unpack
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= assert, tonumber, type, setmetatable, ipairs, unpack
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local math_floor, table_insert, table_concat
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= math.floor, table.insert, table.concat
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local obj = {}
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local metatable = {__index = obj}
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local function read_floats(next_word, n)
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if n == 0 then return end
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local num = next_word()
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assert(num:find"^%-?%d+$" or num:find"^%-?%d+%.%d+$")
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return tonumber(num), read_floats(next_word, n - 1)
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end
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local function read_index(list, index)
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if not index then return end
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index = tonumber(index)
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if index < 0 then
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index = index + #list + 1
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end
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assert(list[index])
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return index
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end
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local function read_indices(self, next_word)
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local word = next_word()
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if not word then return end
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-- TODO optimize this (ideally using a vararg-ish split by `/`)
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local vertex, texcoord, normal
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vertex = word:match"^%-?%d+$"
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if not vertex then
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vertex, texcoord = word:match"^(%-?%d+)/(%-?%d+)$"
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if not vertex then
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vertex, normal = word:match"^(%-?%d+)//(%-?%d+)$"
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if not vertex then
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vertex, texcoord, normal = word:match"^(%-?%d+)/(%-?%d+)/(%-?%d+)$"
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end
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end
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end
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return {
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vertex = read_index(self.vertices, vertex),
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texcoord = read_index(self.texcoords, texcoord),
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normal = read_index(self.normals, normal)
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}, read_indices(self, next_word)
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end
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function obj.read_lines(
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... -- line iterator such as `modlib.text.lines"str"` or `io.lines"filename"`
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)
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local self = {
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vertices = {},
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texcoords = {},
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normals = {},
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groups = {}
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}
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local groups = {}
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local active_group = {name = "default"}
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groups[1] = active_group
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groups.default = active_group
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for line in ... do
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if line:byte() ~= ("#"):byte() then
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local next_word = line:gmatch"%S+"
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local command = next_word()
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if command == "v" or command == "vn" then
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local x, y, z = read_floats(next_word, 3)
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x = -x
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table_insert(self[command == "v" and "vertices" or "normals"], {x, y, z})
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elseif command == "vt" then
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local x, y = read_floats(next_word, 2)
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y = 1 - y
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table_insert(self.texcoords, {x, y})
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elseif command == "f" then
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table_insert(active_group, {read_indices(self, next_word)})
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elseif command == "g" or command == "usemtl" then
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-- TODO consider distinguishing between materials & groups
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local name = next_word() or "default"
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if groups[name] then
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active_group = groups[name]
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else
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active_group = {name = name}
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table_insert(groups, active_group)
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groups[name] = active_group
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end
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assert(not next_word(), "only a single group/material name is supported")
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end
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end
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end
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-- Keep only nonempty groups
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for _, group in ipairs(groups) do
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if group[1] ~= nil then
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table_insert(self.groups, group)
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end
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end
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return setmetatable(self, metatable) -- obj object
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end
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-- Does not close a file handle if passed
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--> obj object
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function obj.read_file(file_or_name)
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if type(file_or_name) == "string" then
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return obj.read_lines(io.lines(file_or_name))
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end
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local handle = file_or_name
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-- `handle.read, handle` can be used as a line iterator
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return obj.read_lines(assert(handle.read), handle)
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end
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--> obj object
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function obj.read_string(str)
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-- Empty lines can be ignored
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return obj.read_lines(str:gmatch"[^\r\n]+")
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end
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local function write_float(float)
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if math_floor(float) == float then
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return ("%d"):format(float)
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end
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return ("%f"):format(float):match"^(.-)0*$" -- strip trailing zeros
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end
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local function write_index(index)
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if index.texcoord then
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if index.normal then
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return("%d/%d/%d"):format(index.vertex, index.texcoord, index.normal)
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end
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return ("%d/%d"):format(index.vertex, index.texcoord)
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end if index.normal then
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return ("%d//%d"):format(index.vertex, index.normal)
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end
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return ("%d"):format(index.vertex)
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end
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-- Callback/"caller"-style iterator; use `iterator.for_generator` to turn this into a callee-style iterator
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function obj:write_lines(
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write_line -- function(line: string) to write a line
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)
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local function write_v3f(type, v3f)
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local x, y, z = unpack(v3f)
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x = -x
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write_line(("%s %s %s %s"):format(type, write_float(x), write_float(y), write_float(z)))
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end
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for _, vertex in ipairs(self.vertices) do
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write_v3f("v", vertex)
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end
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for _, normal in ipairs(self.normals) do
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write_v3f("vn", normal)
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end
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for _, texcoord in ipairs(self.texcoords) do
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local x, y = texcoord[1], texcoord[2]
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y = 1 - y
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write_line(("vt %s %s"):format(write_float(x), write_float(y)))
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end
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for _, group in ipairs(self.groups) do
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write_line("g " .. group.name) -- this will convert `usemtl` into `g` but that shouldn't matter
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for _, face in ipairs(group) do
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local command = {"f"}
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for i, index in ipairs(face) do
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command[i + 1] = write_index(index)
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end
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write_line(table_concat(command, " "))
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end
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end
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end
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-- Write `self` to a file
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-- Does not close or flush a file handle if passed
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function obj:write_file(file_or_name)
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if type(file_or_name) == "string" then
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file_or_name = io.open(file_or_name)
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end
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self:write_lines(function(line)
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file_or_name:write(line)
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file_or_name:write"\n"
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end)
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end
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-- Write `self` to a string
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function obj:write_string()
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local rope = {}
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self:write_lines(function(line)
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table_insert(rope, line)
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end)
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table_insert(rope, "") -- trailing newline for good measure
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return table_concat(rope, "\n") -- string representation of `self`
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end
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return obj
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