Utah Teapot on OpenComputers

local component, computer, event, math = require("component"), require("computer"), require("event"), require("math")
local gpu, internet = component.gpu, require("internet")
local sin, cos, floor, min, max, huge, sqrt, rad, abs = math.sin, math.cos, math.floor, math.min, math.max, math.huge, math.sqrt, math.rad, math.abs
local byte, format = string.byte, string.format

local W, H = gpu.getResolution()
local buffer, vertices, faces = {}, {}, {}
local sinT, cosT = {}, {}

for i = 0, 360 do local r = rad(i) sinT[i], cosT[i] = sin(r), cos(r) end

local function downloadOBJ()
    local c = ""
    for chunk in internet.request("https://raw.githubusercontent.com/kevinroast/phoria.js/master/teapot.obj") do c = c .. chunk end
    return c
end

local function parseOBJ(c)
    for l in c:gmatch("[^\r\n]+") do
        local p = l:sub(1, 2)
        if p == "v " then
            local x, y, z = l:match("v%s+([%d.-]+)%s+([%d.-]+)%s+([%d.-]+)")
            vertices[#vertices + 1] = {tonumber(x), tonumber(y), tonumber(z)}
        elseif p == "f " then
            local v1, v2, v3 = l:match("f%s+(%d+)/?%S*%s+(%d+)/?%S*%s+(%d+)/?%S*")
            if v1 then faces[#faces + 1] = {tonumber(v1), tonumber(v2), tonumber(v3)} end
        end
    end
end

local function calcBoundingBox()
    local minX, minY, minZ, maxX, maxY, maxZ = huge, huge, huge, -huge, -huge, -huge
    for _, v in ipairs(vertices) do
        minX, minY, minZ = min(minX, v[1]), min(minY, v[2]), min(minZ, v[3])
        maxX, maxY, maxZ = max(maxX, v[1]), max(maxY, v[2]), max(maxZ, v[3])
    end
    return minX, minY, minZ, maxX, maxY, maxZ
end

local function rotateY(x, y, z, a)
    local c, s = cosT[a], sinT[a]
    return x * c + z * s, y, -x * s + z * c
end

local function project(x, y, z, s, oX, oY)
    return floor(x * s + oX), floor(-y * s + oY)
end

local function drawLine(x1, y1, x2, y2, char)
    local dx, dy = abs(x2 - x1), abs(y2 - y1)
    local sx, sy = x1 < x2 and 1 or -1, y1 < y2 and 1 or -1
    local err, e2 = dx - dy, 0

    while true do
        buffer[y1 * W + x1] = char
        if x1 == x2 and y1 == y2 then break end
        e2 = 2 * err
        if e2 > -dy then err, x1 = err - dy, x1 + sx end
        if e2 < dx then err, y1 = err + dx, y1 + sy end
    end
end

local function clearBuffer() for i = 1, W * H do buffer[i] = ' ' end end

local function renderBuffer()
    local i, s = 1, {}
    for y = 1, H do
        for x = 1, W do s[x] = buffer[i] or ' ' i = i + 1 end
        gpu.set(1, y, table.concat(s))
    end
end

local function normalize(x, y, z)
    local l = sqrt(x*x + y*y + z*z)
    return x/l, y/l, z/l
end

local function dot(x1, y1, z1, x2, y2, z2) return x1*x2 + y1*y2 + z1*z2 end

local function detectGPUTier()
    local maxRes, mem = gpu.maxResolution(), gpu.totalMemory()
    if maxRes >= 160 and mem >= 786432 then return 3
    elseif maxRes >= 80 and mem >= 196608 then return 2
    else return 1 end
end

local function main()
    gpu.setBackground(0x000000)
    gpu.setForeground(0xFFFFFF)
    gpu.fill(1, 1, W, H, " ")
    print("Downloading teapot.obj...")
    local objContent = downloadOBJ()
    print("Parsing OBJ file...")
    parseOBJ(objContent)

    local gpuTier = detectGPUTier()
    local faceSkip, rotationSpeed = 4 - gpuTier, gpuTier

    local minX, minY, minZ, maxX, maxY, maxZ = calcBoundingBox()
    local cX, cY, cZ = (minX + maxX) / 2, (minY + maxY) / 2, (minZ + maxZ) / 2

    for i, v in ipairs(vertices) do v[1], v[2], v[3] = v[1] - cX, v[2] - cY, v[3] - cZ end

    local modelSize = max(maxX - minX, maxY - minY, maxZ - minZ)
    local scale = min(W, H) * 0.8 / modelSize
    local oX, oY = W / 2, H / 2

    local angle, lastRender = 0, 0
    local frameCount = 0
    local lastFPSUpdate = computer.uptime()
    local fps = 0

    print("Rendering... Press 'x' to exit, '+' to zoom in, '-' to zoom out")
    os.sleep(1)

    local shadeChars = {[0]='·', ':', '.', '+', '%', '&', '#', '@'}

    local totalMemory = computer.totalMemory()

    while true do
        local currentTime = computer.uptime()

        frameCount = frameCount + 1
        if currentTime - lastFPSUpdate >= 1 then
            fps = frameCount / (currentTime - lastFPSUpdate)
            frameCount = 0
            lastFPSUpdate = currentTime
        end

        if currentTime - lastRender >= 0.05 then
            clearBuffer()

            local la = (angle * 2) % 360
            local lx, ly, lz = sin(rad(la)) * 2, 1, cos(rad(la)) * 2
            lx, ly, lz = normalize(lx, ly, lz)

            for i, face in ipairs(faces) do
                if i % faceSkip == 0 then
                    local v1, v2, v3 = vertices[face[1]], vertices[face[2]], vertices[face[3]]

                    local ux, uy, uz = v2[1] - v1[1], v2[2] - v1[2], v2[3] - v1[3]
                    local vx, vy, vz = v3[1] - v1[1], v3[2] - v1[2], v3[3] - v1[3]
                    local nx, ny, nz = uy*vz - uz*vy, uz*vx - ux*vz, ux*vy - uy*vx
                    nx, ny, nz = normalize(nx, ny, nz)

                    local shade = dot(nx, ny, nz, lx, ly, lz)

                    local rx, ry, rz = 2 * shade * nx - lx, 2 * shade * ny - ly, 2 * shade * nz - lz

                    local spec = max(0, dot(rx, ry, rz, 0, 0, 1)) ^ 8

                    local char = shadeChars[floor(max(shade, spec) * 7)]

                    for j = 1, 3 do
                        local v1, v2 = vertices[face[j]], vertices[face[j % 3 + 1]]

                        local x1, y1, z1 = rotateY(v1[1], v1[2], v1[3], angle)
                        local x2, y2, z2 = rotateY(v2[1], v2[2], v2[3], angle)

                        local px1, py1 = project(x1, y1, z1, scale, oX, oY)
                        local px2, py2 = project(x2, y2, z2, scale, oX, oY)

                        drawLine(px1, py1, px2, py2, char)
                    end
                end
            end

            renderBuffer()

            local memUsage = (1 - computer.freeMemory() / totalMemory) * 100

            local statsText = format("FPS: %.1f | MEM: %.1f%% | Zoom: %.2f", fps, memUsage, scale)

            gpu.setForeground(0xFFFF00)
            gpu.set(1, 1, statsText)
            gpu.setForeground(0xFFFFFF)

            angle = (angle + rotationSpeed) % 360
            lastRender = currentTime
        end

        local eventType, _, char = event.pull(0.01)
        if eventType == "key_down" then
            if char == byte('x') or char == byte('X') then
                break
            elseif char == byte('+') then
                scale = scale * 1.1
            elseif char == byte('-') then
                scale = scale * 0.9
            end
        end
    end

    gpu.fill(1, 1, W, H, " ")
    print("Exiting...")
end

main()