k4_V2 (pre-alpha)

k4 is a physics-based, minimalist multiplayer 3D game framework. It's core is relatively small, providing: Highly wide-compatibility graphics: minimum OpenGL 1.5 An audio subsystem (k3Mix) A menu subsystem (k3Menu) A physics subsystem (Open Dynamics Engine) A tiny entity subsystem Builtin first-person and third-person controls Multiplayer with an authoritative server model, optional peer-to-peer connectivity or IPv6 Client-side prediction & server reconciliation A scripting interface, currently implemented with Lua 5.3 Besides the above, everything is expected to be realized at the scripting level. Windows and Linux are natively supported. OpenGL ES support is planned, but low-priority. There are no plans for Mac support. The largest limitation as of now is the lack of rendering layers, which prevents certain effects such as portals or split-screen rendering. k4 accepts command-line parameters at launch in the form key=value, which include script: the script to automatically launch, string res%: game resolution, percentage of the window size core: override the decision to use the Core OpenGL profile, 0 or 1 vsync: number of refreshes to wait for mipmap: allows one to disable mipmapping for everything tc: allows one to disable texture compression

Demo

Scripting

Upon launch, k4 will automatically load a script within the assets directory. By default this is init, in assets/init.lua.

The smallest possible script is the following:

return {
	load = function()
	
	end
}

But this ignores the fact that scripts must set up controls, the player entity, menus, and more. As is, the scene will appear as a blank screen.

Scripts may load resources such as models, sounds, textures, and so on. Doing so is recommended only within the load function, otherwise loaded resources will stay loaded even after the engine switches scripts.

When the load function is called, entities and other items from the old scene are still left over. For most cases, it is recommended to immediately do game.cleanup() to reset the subsystems. The load function may assign functions to callbacks such as game.render, game.render2d or game.update, or any of game.triggers[i].

Once load finishes, game.join will be called, if it exists, to signify the joining of the main player. This is important for multiplayer scripts.

The module everything revolves around is game, which interfaces with k4.

local healthbar = game.ref("tex", "healthbar.dif.png")
local bgm_source = game.ref("stream", "bgm.ogg")

game.batch(gun, {0.2, -0.2, -0.5}, game.camera)

game.firstperson(false)

-- Shoot laser
local ray = game.ray(60)
local hit_pos = ray.pos
print("Hit ray at ", hit_pos[1], hit_pos[2], hit_pos[3])
print("Hit entity ", ray.hit)

-- Half resolution
game.set_texture_reduction(1)

game.cleanup()

game.load("lvl2")

local phys = game.ref("physics", "scene")
game.setphys(phys)

local sb = game.ref("tex", "sky1.cub.png")
game.skybox(sb)

-- Half resolution
game.set_reduction(0.5)

Controls

k4 stores a mapping of "controls" to inputs within the game.controls table. Six controls are reserved by k4 for standard 3D movement: forward, back, left, right, grab and jump. The script must assign these so that the game becomes playable. The simplest method, though worst for accessibility, is to hardcode the options like so:

game.controls["forward"] = "W"
game.controls["back"] = "S"
game.controls["left"] = "A"
game.controls["right"] = "D"
game.controls["jump"] = " "
game.controls["grab"] = 0

String values correspond to scancodes, therefore the currently active keyboard layout does not affect gameplay. Integers 0 through 2 refer to mouse buttons.

Custom controls may be bound. If a control is either pressed or released, the game.ctrl handler will be called.

function game.ctrl(control_name, action)
	if control_name == "shit" then
		if action == 0 then
			-- Pressed (release your shit)
		elseif action == 2 then
			-- Released (hold in your shit)
		end
	end
end

The only input that may not be bound to is the Escape key. If said key is pressed, k4 will instead call the game.escape callback function.

Entities

k4 intentionally provides a minimal entity system.

game.addentity accepts an entity descriptor and adds an entity to the scene, of which there may be at most 65535. There are four entity components exposed to the script side: physics, render, movement, boned.

Once an entity is created, any of its components may be retreived by the script for inspection or modification with game.getcomponent:

local p = game.getcomponent(entity_id, "physics")
local player_position = p.pos
local player_quaternion = p.rot

-- Double player speed
p.speed = p.speed * 2

If a specific ID is required, the entity descriptor may optionally accept an id key with an integer below 65535. If this ID is already taken, this is erroneous.

A component object is a view to memory; it must not be saved across game ticks, or else you risk corrupting memory. Always call game.getcomponent again instead of storing it somewhere.

Physics component

The physics component grants an entity a shape in the physical scene. The descriptor of the physics component is mostly self-explanatory. The shape is determined by the existence of the mutually-exclusive box, sphere, capsule or trimesh keys.

Fields:

Render component

The render component exposes the entity to the renderer.

Fields:

Boned component

The boned component allows the model in the render component to be animated using bone animation. Without this component, the model will stay in its bind pose.

Movement component

Without the movement component, the entity cannot move. The only valid keys within the movement component is dir, a vec3 which defines the direction in which the entity should move, and jump, which specifies whether the entity wishes to jump. If a player has been assigned to this entity, its movement component will be continually modified to correspond to the player's inputs.

-- Move entity 0 to the spawn point
game.getcomponent(0, "physics").pos = {0, 5, 0}

game.kill(0)

game.addentity{
	render = {mdl = "modelname"},
	physics = {
		dynamics = "dynamic",
		capsule = {radius = 0.24, length = 1.3},
		pos = {0, 5, 0},
		rot = {0, 0, 0, 1},
		trigger = 0,
		speed = 6.5
	},
	movement = {
		dir = {0, 0, 0}
	}
}

Triggers

Any physical entity in k4 may have a corresponding integer trigger field. If the entity comes in contact with any other physical object, a corresponding callback function will be called. These callback may be assigned to the game.triggers table.

-- Place a large trigger box at the center of the scene
game.addentity{
	physics = {
		dynamics = "static",
		box = {w = 10, h = 10, l = 10},
		pos = {0, 0, 0},
		trigger = 1
	}
}

game.triggers[1] = function(id, e1, e2, event)
	print(e1 .. " and " .. e2 .. " have collided")
end

In the callback, id is the ID of the trigger (1 in the above example). e1 and e2 are the entity IDs which have collided. If the object in collision is actually not an entity, but the static physics mesh, then its entity ID will be nil. Similarly to controls, event being 0 marks the start of collision, 2 marks the end, and 1 is for the ticks in between.

k4 supports at most 65535 triggers in the scene. Triggers are 1-indexed, and the ID 0 means a lack thereof.

Killing entities or spawning them within a trigger is not allowed.

Audio

The audio subsystem is designed as a graph of audio processing nodes, which allows you to perform audio effects on or extract data from sounds.

3D sounds are currently unavailable, but are envisioned as being filters on top of mono sounds.

local bgm_source = game.ref("stream", "bgm.ogg")
local bgm = game.mix.sound(bgm_source)

game.mix.play(bgm)

game.mix.stop(bgm)

local p = game.mix.power(bgm)
print(p.rms)

local q = game.mix.queue()

q:clear()

game.mix.stopsmooth(bgm)

q:add(bgm)

Clipboard

The clipboard interface is quite self-explanatory. Drag & dropping files is a planned feature.

-- Get text from the clipboard
print(game.clipboard.text)

-- Override text in the clipboard
game.clipboard.text = "pnis Ayy lmao"

Menus

k3Menu is an interface for stylizable retained UIs. Everything in k3Menu is an object that may contain other objects. This way k3Menu forms a tree. The top-level object the user will interact with is a screen.

local screen = game.ui.screen()

local lbl = game.ui.label(fnt, 12, "Hello, World")

-- Activate screen
game.menu = screen

local screen = game.ui.screen()

local btn = game.ui.textbutton(fnt, 12, "Press Harder")

btn:set_bounds(0, 0, 500, 200)

btn:on("click", function()
	print(":)")
end)

local my_scrollbox = game.ui.scrollbox()
my_scrollbox.content:add_child(lbl)

game.screen:arrange()

local obj = game.ui.obj()

local lbl = game.ui.label(fnt, 12, "Cake")

local input = game.ui.textinput(fnt, 12, "Eat", "Cake")

screen:add_child(btn)
screen:add_child(lbl)
screen:add_child(input)

local img = game.ui.image("menu_icon.dif.png")

my_scrollbox.content:set_layout_linear()

game.screen:measure()

All objects have the x, y, w and h fields, which are self-explanatory. Because manually setting these is often impractical, k3Menu offers automatic layouts inspired by WPF.

With layouts there exist two additional events: measure and arrange. In measure, an item and all of its children determine their desired size. Afterwards, the arrange event takes in these desired sizes and finds a balanced size and placement for all. To trigger a relayout, first measure should be called, then arrange.

A top-down list layout can be set with k3menuitem:set_linear_layout().

It is fine to have custom measure and arrange callbacks if k3's builtin layouts and styles aren't enough. For example, an inventory system which downright ignores the measure event because it doesn't care:

panel:on("arrange", function()
	local ratio = 1408 / 1328
	
	local h = math.min(menu.h, 120 * game.dpmm)
	local w = ratio * h
	local x = menu.w / 2 - w / 2
	local y = menu.h / 2 - h / 2
	
	panel:set_bounds(x, y, w, h)
	
	local slots_w = math.floor(w * 0.9)
	local slots_h = math.floor(h * 0.9)
	local slot_w = slots_w / 9
	local slot_h = slot_w
	local slot_padding = slot_w * 0.05
	for i, slot in ipairs(slots) do
		local slot_ix = (i - 1) % 9
		local slot_iy = (i - 1) // 9
		
		slots[i]:set_bounds(
			x + (w - slots_w) / 2 + slot_ix * slot_w + slot_padding,
			y + slots_h - ((h - slots_h) / 2 + slot_iy * slot_h + slot_padding),
			slot_w - slot_padding * 2,
			slot_h - slot_padding * 2
		)
	end
end)

game.dpmm is the number of dots per millimeter (akin to DPI), which is useful for setting limits on object sizes.

You may set values for predefined "properties" on objects, accessible by the prop_* set of fields. For example, to change the horizontal alignment of a label, one may do:

lbl.prop_horizontal_alignment = "left"

List of properties:

List of events:

Animations

An "animator" may be created with game.animator(model) and passed to game.batch when rendering. Each animator is created for a specific model; it is invalid to use an animator from one model on another.

Once an animator is created, either an animation or a tree of animations may be set on the animator with k3animator:set, then played with k3animator:play.

Basic animation example:

local animator = game.animator(mdl)
animator:set({type = "base", id = 1, loop = false})
animator:play()

Blend animation example:

local animator = game.animator(mdl)
animator:set({
	type = "blend",
	{
		anim = {type = "base", id = 1, loop = true},
		w = 1,
		speed = 1,
		offset = 0,
	},
	{
		anim = {type = "base", id = 2, loop = true},
		w = 1,
		speed = 1,
		offset = 0,
	}
}):play()

If w is a scalar, it defines the weight of each animation. Alternatively w may be a table, assigning a weight per each bone (see k3mdl:query_weights). This is useful for example, for combining walking and fighting animations.

Additive animation example:

gun_anim:set({
	type = "add",
	
	{type = "base", id = 1, loop = true},
	{type = "base", id = 2, loop = true},
	
	scale = 1.2,
}):play()

Animation blending is not animation addition. The former unlike the latter is commutative.

To apply an animator on an entity use the "boned" component:

game.getcomponent(entity_id, "boned").animator:set({type = "base", id = 1, loop = false}):play()

Planar water

k3 has a planar water implementation that simulates waves using finite differences on the CPU. A planar water model can be created with game.water, which accepts a material for rendering, and can be passed to game.batch.

Water is purely for rendering and any physical behavior must be done separately.

local lava_obj = game.water(10, 10, 25, game.ref("mat", "lava"))

lava_obj:disturb(0, 0, 1, colliding_player_speed / 10)

lava_obj:simulate(0.03, 2, 1, 20)

lava_obj:update()

Particle systems

A particle system is created with game.particle_system. A system uses exactly one material for its particles. Like water, a particle system may be passed to game.batch.

local parts = game.particle_system(1500, game.ref("mat", "particle_smoke"))

parts:update()

Fields:

Example usage:

local part = game.particle_system(1500, game.ref("mat", "particle_smoke"))
part.origin = {p.pos[1], p.pos[2] - 1.0, p.pos[3]}
part.enabled = true
part.rate = 500
part.cone_direction = {0, 1, 0}
part.cone_angle = math.pi / 3
part.gravity = {0, 0.35, 0}
part.lifetime = 2.5
part.remaining = 0.75
part.color_start = {1, 1, 1, 1}
part.color_end = {1, 1, 1, 0}

Networking

A k4 scene can easily transition from singleplayer to multiplayer by calling either game.net.host or game.net.join.

For hosting, k4 will attempt to automatically open a port through either the PCP or NAT-PMP protocols (unimplemented). The server generates a "peercode" with game.net.gen_peercode, which returns a string containing a few IP addresses.

However, automatic port forwarding is likely to fail due to poor adoption of both PCP and NAT-PMP. Should this occur, players will be forced to manually establish connections via what is known as hole punching. To do this, the client should also generate his/her peercode with game.net.gen_peercode. The player responsible for the client should then pass this peercode to the host player. The host side should then "punch a hole" through its NAT to the client using game.net.punch. Once said function is called, it may become possible for the client to connect.

If the server turns out to be behind a symmetric NAT, then game.net.gen_peercode will also fail, in which case either a port must be manually opened, or a different player must host the game.

Server-side:

game.net.host()

-- If punching is required, get some_clients_peercode (like with the clipboard), then do
game.net.punch(some_clients_peercode)

Client-side:

game.net.join(server_peercode)

Once a connection is established, the game.join handler will be called by k4 with a k4peer object representing the connection to the newly-joined client. Messages may be sent to a client by calling k4peer:send. The server may broadcast messages to all players (excluding itself) with game.send. For clients, game.send sends only to the server. Clients cannot send messages to other clients.

Messages may be received by assigning a handler to game.receive.

Messages can be any serializable Lua types (no lightuserdata, userdata, thread and function types).

function game.join(cli)
	cli:send("Ping")
end

function game.receive(cli, msg)
	if msg == "Ping" then
		cli:send("Pong")
	end
end

While k4 itself provides seamless transitions to multiplayer, this still requires effort from the script writer.

Firstly, k4 employs client-side prediction, which means the client will keep the simulation running without waiting for the server to send the authorative state. As this is a prediction, it can be wrong, in which case the client will be forced to rollback the state to what the server sent. This means that, for example, a player can enter a trigger and suddenly exit it because of a misprediction. Meanwhile on the server, the player could have never entered the trigger at all.

Secondly, k4 employs server reconciliation, which means the client will try prediction again after receiving the authorative state from the server, using the recent player inputs. Using the same example, this means a player could enter a trigger, exit it, and enter it again, while on the server the player could've entered it only once.

Thirdly, client-side prediction is done only for k4's own entity system, which ignores any properties that could be defined by scripts, e.g. player health. This means if a script's logic depends on things that could be mispredicted, divergence from the server state is inevitable.

Accounting for all of this, to write multiplayer-compliant scripts, one must take into account the following rule: all major gameplay events must be explicitly confirmed by the server through messages instead of being predicted by the client. Clients should allow an entity's health to go below 0 without killing it, servers should periodically send true entity healths and announce kills in case clients missed it. A script can know if it is authorative with the boolean game.authority.

game.net.gen_peercode(function(peercode, err)
	-- ... get peercode to other player ...
end)

game.net.join(server_peercode)

game.send("Yo")

game.net.host()

game.net.punch(client_peercode)

cli:send("Yo")

Creating resources for k4

All assets in the resource system are loaded from a prioritized list of sources, one of which is the assets directory. All graphical resources (models, materials and textures) are within the subdirectory mdl, audio files are within aud and raw triangle meshes are within phys. It is planned to allow archive files to be sources in a future version of k4.

Audio

Audio files must be Ogg containers with the Vorbis codec, and they must be of 44.1kHz sample rate. Audio is streamed from the disk, therefore length will not impact RAM usage.

Similarly to Sonic Robo Blast 2, k4 supports the LOOPPOINT metadata field, which specifies the sample index where a loop should begin.

Models

k4's graphics engine, k3, has a standard file format for models. Models currently support position, UV, color and bone attributes, but it is planned to allow generic attributes.

An export script is available for Blender 2.79, and another script exists for converting from the GLTF2 file format. Beware, because k3 is more or less advanced in different areas compared to either GLTF2 or Blender, most properties are completely ignored by the scripts. For example, only the names of materials are transferred, so k4 may find it within it's own resources. All textures attached to the material are ignored, and you must specify them manually in the material files.

Textures

Texture filenames must be suffixed with one of .dif.png for diffuse textures, .nrm.png for normal textures, .dsp.png for displacement textures, .emt.png for emissive textures, .rgh.png for roughness textures and .alp.png for alpha textures. These are necessary so k3 may properly interpret the colors and render them.

Cubemap textures are needed for, for example, skyboxes. Although a k4 script will load cubemap textures with a .cub.png suffix (game.ref("tex", "sky1.cub.png")), you actually need six files in the filesystem, one for each side of the cube: .px.png for the +X side, .py.png — +Y, .pz.png — +Z, .nx.png — -X, .ny.png — -Y, .nz.png — -Z.

Fonts

Fonts are simply TrueType files.

Materials

Graphics resources are the most complex because of k3's hardware compatibility. Materials must specify properties for each graphics backend.

k4 uses the Lua interface to allow specifying materials, therefore material files are simply Lua scripts. The material is composed of a prioritized list of rendering techniques. k3 will attempt to initialize the first technique, and will use the rest as fallbacks. If all techniques fail, k3 will use the always supported primitive form of rendering.

In each technique we have a list of rendering passes, the maximum of which is currently hardcoded to 1. The render pass may specify certain rendering options such as additive rendering.

Here is an example material script that uses either a GL3.3 shader (with the GLCORE macro defined), or a GL2.1 shader, or a shaderless mode with a single texture, in that order. If all three modes fail, k3 will use a gray color as set in primitive.

Because Mesa is stupid, core-profile rendering is strictly equivalent to an OpenGL version equal to or above 3.3, even though this isn't required by the OpenGL standard.

return {
	skinning = false,
	primitive = {
		diffuse = {0.5, 0.5, 0.5, 1}, shininess = 16,
		specular = {0, 0, 0, 1}, emission = {0, 0, 0, 0},
	},
	-- First technique
	{
		-- First pass
		{
			-- Request GLSL
			glsl = {
				vs = {330, "regular.vs.glsl"},
				fs = {330, "regular.fs.glsl"},
				defs = {GLCORE = ""}, u = {u_dif = 0},
			},
			-- Set texture units
			units = {
				{tex = "checkers.dif.png"},
			}
		}
	},
	-- Second technique
	{
		-- First pass
		{
			-- Request GLSL
			glsl = {
				vs = {120, "regular.vs.glsl"},
				fs = {120, "regular.fs.glsl"},
				defs = {}, u = {u_dif = 0},
			},
			-- Set texture units
			units = {
				{tex = "checkers.dif.png"},
			}
		}
	},
	-- Third technique
	{
		-- First pass
		{
			-- No GLSL
			-- Set texture units
			units = {
				{tex = "checkers.dif.png"}
			}
		}
	},
}

Getting started

For the majority of cases, the vanilla distribution of k4 is enough to begin scripting. It is built to run on at least the Pentium 4. It contains boilerplate resources, sample 3D models to play with and a starting script with a simple 3D scene. Note that k4 remains in an alpha state, and breaking changes will be common.

Download Vanilla Distribution

But it is even encouraged to modify k4 itself, if you need the performance. For example, the voxel demo shows bad hiccups when placing or removing blocks, because the Lua script is the bottleneck in regenerating chunk models. This shows the need for a voxel extension to k4 (and k3).

Source Code

Support

Yeah

Planned features:

1. Decals
2. Animation blending & layering
3. 3D sounds
4. Customizable rendering pipeline
5. Resource archives
6. WebAssembly scripting?
7. OpenGL ES support
8. Joystick support, including flick stick movement