nono's 3D OBJ viewer for ComputerCraft!

if not fs.exists("3d") then fs.makeDir("3d") end

local objList = {
    {n="Teapot",u="https://raw.githubusercontent.com/alecjacobson/common-3d-test-models/master/data/teapot.obj"},
    {n="Suzanne",u="https://raw.githubusercontent.com/alecjacobson/common-3d-test-models/master/data/suzanne.obj"},
    {n="Beetle",u="https://raw.githubusercontent.com/alecjacobson/common-3d-test-models/master/data/beetle.obj"}
}

local function centerText(text)
    local w, _ = term.getSize()
    local x = math.floor((w - #text) / 2)
    term.setCursorPos(x, term.getCursorPos())
    print(text)
end

local function showMenu()
    term.clear()
    centerText("=== nono's 3D OBJ viewer for ComputerCraft ===")
    print("\nSelect an .obj file:")
    for i,obj in ipairs(objList) do
        print(i..". "..obj.n)
    end
    print("Enter choice (1-"..#objList..") or 0 to exit:")
    local c = tonumber(read())
    if c == 0 then return nil end
    return c and c >= 1 and c <= #objList and objList[c] or nil
end

local function downloadObj(obj)
    centerText("Downloading "..obj.n.."...")
    local f = "3d/"..obj.n..".obj"
    if fs.exists(f) then
        print("File already exists. Use cached version? (y/n)")
        if read():lower() == "y" then return true end
    end
    local success = shell.run("wget", obj.u, f)
    return success
end

local function parseObj(f)
    local v,faces,vn = {},{},{}
    for l in io.lines(f) do
        local p = {}
        for w in l:gmatch("%S+") do table.insert(p,w) end
        if p[1]=="v" then
            table.insert(v,{x=tonumber(p[2]),y=tonumber(p[3]),z=tonumber(p[4])})
        elseif p[1]=="vn" then
            table.insert(vn,{x=tonumber(p[2]),y=tonumber(p[3]),z=tonumber(p[4])})
        elseif p[1]=="f" then
            local face = {}
            for i=2,#p do
                local indices = {}
                for index in p[i]:gmatch("(%d+)/?") do
                    table.insert(indices, tonumber(index))
                end
                table.insert(face, indices)
            end
            table.insert(faces,face)
        end
    end
    if #faces == 0 then
        for l in io.lines(f) do
            local p = {}
            for w in l:gmatch("%S+") do table.insert(p,w) end
            if p[1]=="f" then
                local face = {}
                for i=2,#p do
                    table.insert(face, {tonumber(p[i])})
                end
                table.insert(faces,face)
            end
        end
    end
    return v,faces,vn
end

local function rotateX(p,a) return {x=p.x,y=p.y*math.cos(a)-p.z*math.sin(a),z=p.y*math.sin(a)+p.z*math.cos(a)} end
local function rotateY(p,a) return {x=p.x*math.cos(a)+p.z*math.sin(a),y=p.y,z=-p.x*math.sin(a)+p.z*math.cos(a)} end
local function rotateZ(p,a) return {x=p.x*math.cos(a)-p.y*math.sin(a),y=p.x*math.sin(a)+p.y*math.cos(a),z=p.z} end
local function translate(p,tx,ty,tz) return {x=p.x+tx, y=p.y+ty, z=p.z+tz} end

local function project(p,w,h,zoom,offsetX,offsetY)
    local f = math.min(w, h) * zoom
    local x = (p.x*f)/(p.z+10)+w/2+offsetX
    local y = (p.y*f)/(p.z+10)+h/2+offsetY
    return math.floor(x),math.floor(y)
end

local function getAsciiChar(intensity)
    local chars = " .'`^\",:;Il!i><~+_-?][}{1)(|/tfjrxnuvczXYUJCLQ0OZmwqpdbkhao*#MW&8%B@$"
    return chars:sub(math.floor(intensity * #chars) + 1, math.floor(intensity * #chars) + 1)
end

local function showControlGuide()
    term.clear()
    centerText("=== Control Guide ===")
    print("\nWASD: Rotate model")
    print("IJKL: Move model")
    print("UO: Move model forward/backward")
    print("Arrow Up/Down: Zoom in/out")
    print("X: Exit viewer")
    print("\nPress any key to continue...")
    os.pullEvent("key")
end

local function render(v,faces,vn)
    local w,h = term.getSize()
    local ax,ay,az,zoom = 0,0,0,0.5
    local tx,ty,tz = 0,0,0
    local running = true
    local buffer = {}
    local lastTime = os.clock()
    local frameCount = 0
    local fps = 0

    for y=1,h do
        buffer[y] = {}
        for x=1,w do
            buffer[y][x] = {char=" ", depth=math.huge}
        end
    end

    while running do
        local startTime = os.clock()

        for y=1,h do
            for x=1,w do
                buffer[y][x] = {char=" ", depth=math.huge}
            end
        end

        local p2d = {}
        for i,vert in ipairs(v) do
            local rotated = rotateZ(rotateY(rotateX(vert,ax),ay),az)
            local translated = translate(rotated,tx,ty,tz)
            p2d[i] = {project(translated,w,h,zoom,0,0)}
        end

        for _,face in ipairs(faces) do
            local v1,v2,v3 = v[face[1][1]], v[face[2][1]], v[face[3][1]]
            local normal = {
                x=(v2.y-v1.y)*(v3.z-v1.z)-(v2.z-v1.z)*(v3.y-v1.y),
                y=(v2.z-v1.z)*(v3.x-v1.x)-(v2.x-v1.x)*(v3.z-v1.z),
                z=(v2.x-v1.x)*(v3.y-v1.y)-(v2.y-v1.y)*(v3.x-v1.x)
            }

            local mag = math.sqrt(normal.x^2 + normal.y^2 + normal.z^2)
            if mag > 0 then
                normal.x, normal.y, normal.z = normal.x/mag, normal.y/mag, normal.z/mag
            else
                normal.x, normal.y, normal.z = 0, 0, 1
            end

            local intensity = math.max(0.1, normal.x*0.5 + normal.y*0.5 + normal.z*0.5)

            for i=1,#face do
                local x1,y1 = p2d[face[i][1]][1],p2d[face[i][1]][2]
                local x2,y2 = p2d[face[i%#face+1][1]][1],p2d[face[i%#face+1][1]][2]
                local depth = (v[face[i][1]].z + v[face[i%#face+1][1]].z) / 2

                local dx,dy = math.abs(x2-x1), math.abs(y2-y1)
                local sx,sy = x1 < x2 and 1 or -1, y1 < y2 and 1 or -1
                local err = dx-dy

                while true do
                    if x1 > 0 and x1 <= w and y1 > 0 and y1 <= h then
                        if depth < buffer[y1][x1].depth then
                            buffer[y1][x1] = {char=getAsciiChar(intensity), depth=depth}
                        end
                    end

                    if x1 == x2 and y1 == y2 then break end
                    local e2 = 2*err
                    if e2 > -dy then err = err - dy; x1 = x1 + sx end
                    if e2 < dx then err = err + dx; y1 = y1 + sy end
                end
            end
        end

        term.clear()
        for y=1,h do
            for x=1,w do
                term.setCursorPos(x,y)
                term.write(buffer[y][x].char)
            end
        end

        frameCount = frameCount + 1
        if os.clock() - lastTime >= 1 then
            fps = frameCount
            frameCount = 0
            lastTime = os.clock()
        end

        term.setCursorPos(1,1)
        term.write("FPS: " .. fps .. " | Zoom: "..string.format("%.2f", zoom).." | Pos: "..string.format("%.2f", tx)..", "..string.format("%.2f", ty)..", "..string.format("%.2f", tz))
        term.setCursorPos(1,2)
        term.write("Rot: "..string.format("%.2f", ax)..", "..string.format("%.2f", ay)..", "..string.format("%.2f", az))
        term.setCursorPos(1,3)
        term.write("WASD: rotate, IJKL: move, UO: forward/back, Arrows: zoom, X: exit")

        local event, key = os.pullEvent("key")
        if key == keys.w then ax = ax + 0.1
        elseif key == keys.s then ax = ax - 0.1
        elseif key == keys.a then ay = ay - 0.1
        elseif key == keys.d then ay = ay + 0.1
        elseif key == keys.q then az = az + 0.1
        elseif key == keys.e then az = az - 0.1
        elseif key == keys.i then ty = ty + 0.1
        elseif key == keys.k then ty = ty - 0.1
        elseif key == keys.j then tx = tx - 0.1
        elseif key == keys.l then tx = tx + 0.1
        elseif key == keys.u then tz = tz + 0.1
        elseif key == keys.o then tz = tz - 0.1
        elseif key == keys.up then zoom = zoom * 1.1
        elseif key == keys.down then zoom = zoom / 1.1
        elseif key == keys.x then running = false
        end

        local endTime = os.clock()
        local frameTime = endTime - startTime
        if frameTime < 0.05 then
            os.sleep(0.05 - frameTime)
        end
    end
end

while true do
    showControlGuide()
    local obj = showMenu()
    if obj then
        if downloadObj(obj) then
            local path = "3d/"..obj.n..".obj"
            local v,f,vn = parseObj(path)
            if v and f then
                print("Rendering " .. obj.n .. " model...")
                os.sleep(1)
                render(v,f,vn)
            else
                print("Failed to parse .obj file.")
            end
        end
    else
        print("Exiting...")
        break
    end
end