A beginner opens the editor, sculpts a hill, places rocks and draws a road, and by evening the world has fallen apart. The problem is not clumsy hands. It is the order of work. A world is built in passes, from large to small and over the whole map at once, because a late change to something large costs the most. The lesson then shows how world tools recompute a world after a change, which kinds of tools exist, and where to start your first world. It opens the course “Your own open world.”

Why this matters

Here is a typical first evening with a world editor. You create the ground and sculpt a hill with a brush. You place a dozen rocks on the slope, draw a road up to the hill and plant a forest around it. Then the hill looks badly placed, and you move it to the left. On one side the rocks and trees now hang in the air; on the other they have sunk into the ground. The road now runs up a steep slope. You dig a lake nearby, and trees stand in the water. You spend an hour fixing it by hand, and the world still doesn’t come together.

It can go the other way, too. One corner is polished to a shine, with moss on the rocks, flowers by the fence and a gate. But the walk from the village to the lake takes several minutes across an empty green plain. The very first walk shows that the lake is too far away, so it has to move, pretty corner and all.

Neither failure comes from lack of skill. The small things were placed before the large ones, and the small things stand on the large ones. Move a large thing, and the small ones break.

Picture a garden plot. Nobody plants roses until the excavator has dug the pond: the excavator would drive straight over the roses. First comes the plan of the plot on paper, then the earthworks, then the paths and the spot for the house. Next come the lawn and the trees, and the bench, the lamps and the flowers come last. A big game world is built in the same order, and the lesson keeps returning to this plot.

The world is a stack of layers

Everything in a world stands on something. A tree stands on the ground. A road lies on the ground and goes around the water. A village sits by the road and the water. Grass grows where there is no road or water. Light falls on everything at once, and shadows depend on every hill and house. Together they form a stack: each layer rests on the ones below it.

That gives a simple rule. When you change a layer, you have to check and fix everything above it. Move a hill, and you re-place the forest on it, fix the road across it, repaint the slopes and check the shadows. If nothing sits above the layer, the change is free. So the cost of a change is everything already placed on top of it.

The plot works the same way. Moving the pond while it is only drawn on the plan takes an eraser. Moving it when the lawn, paths and roses are around it means digging again, leveling, resowing and replanting. It is the same pond, but the work is different, because of what stands on it.

So the later you change something large, the more it costs. While the world is still gray, moving a hill takes a minute. When the hill carries a forest, rocks, a trail and a viewpoint, it takes a day of work, and you risk missing something. The biggest thing in any world is its scale. Road widths, house sizes, distances and travel time all depend on it. Changing the scale at the end means redoing almost everything. The next lesson takes it up: “World scale.”

Six passes from large to small

Since small things stand on large ones, you build the world from the bottom of the stack up. Each layer is one pass: you cover the whole map and do only that layer. Each pass ends with a check: “Is it ready for the next layer?” “Ready” doesn’t mean “perfect.” It means “good enough that the next layer won’t force you to tear it down.”

Pass 1. Concept and scale

Everything starts on paper. Decide what the world is about and what it feels like: quiet forests and lakes, a village by the water, a road over a pass. Decide how people move through it: on foot, by car, by boat. Decide how big it is, counted not in kilometers but in minutes of travel (world size in minutes). Mark the regions that have their own character and the landmarks that can be seen from afar (open-world design). Mark where people live and where the roads lead (world logic).

On the plot, this is a plan on graph paper: where the pond is, where the house is, where the apple trees are. A change on the plan costs only an eraser.

Done when there is a map with a scale bar: regions, main landmarks, water, villages and main roads. And you can say how many minutes it takes to get from the village to the lake on foot and by car.

Pass 2. Shape of the ground and water

You block in the large forms: mountains, valleys, plains, lakes, riverbeds. Work roughly, the way an excavator does, with no footpaths and no pebbles. In games the ground is almost always a grid of heights (lesson on landscape). In this pass you sculpt it with large brushes or import a ready-made grid from a terrain generator. Water lies in low places, and rivers flow only downhill (lesson on water). Valleys and passes settle right away where roads can run: a car can’t climb a slope that is too steep.

Done when the silhouettes of mountains and hills read from the player’s eye height, water lies in the low places, rivers flow downhill, and there is room for roads in the valleys and passes.

Pass 3. Roads and places as gray boxes

Roads and trails go down as lines. Villages, hamlets, bridges, viewpoints and cave entrances go in as gray boxes of the right size: a house is a box the size of a house, with no windows or roof. This is a world blockout (also called a graybox): a draft made of simple shapes that you can already walk through (lesson on blockout).

On the plot, you mark out the house and paths with stakes and string, then walk between them. Is it a comfortable walk from the gate to the porch? Does the path run into the shed?

Next you walk and drive through it with a stopwatch (gray world and the stopwatch). How long does it take to walk from the village to the lake? Can the car make the climb? Is the landmark mountain visible from the road? Is this stretch boring?

Done when the whole world can be walked and driven, travel times match the concept, the car takes every climb, and the landmarks are visible from where they were meant to be.

Pass 4. Ground cover: soil, forest, grass

The ground gets a material: grass on the plain, rock on steep slopes, sand by the water, packed earth on the trails (ground material). Forests, clearings, forest edges and meadows appear (biomes, scattering by rules), and so does grass (grass and foliage). There are now hundreds of thousands of objects, so ground cover is almost always laid by rules, not by hand. More on rules below.

On the plot, these are the lawn, trees and bushes.

Done when from far away the world reads as large patches: dark forest, light meadows, gray cliffs. The forest doesn’t creep onto roads or into the water. And the game doesn’t slow down in the thickest forest on the target computer.

Pass 5. Details, interiors, light and air

This pass adds rocks by the trail, fences, carts, signs and jetties by the water; house interiors (interior and exterior); light, fog, sky and time of day (air and distance); and sound. This is what makes the world feel alive up close. Details are the most numerous and the smallest things in the world, so they come last.

A simple sun and sky are in the world from day one, because without them you can’t make out the shapes. In this pass you don’t switch the light on, you tune it: the mood, the time of day, the fog that hides the distance.

On the plot, these are the bench, the lamps and the flower beds.

Done when the main places look finished up close, light and fog lead the eye to the landmarks, and nothing floats or sinks.

Pass 6. Streaming in chunks and budget

A big world doesn’t fit into memory whole. The engine cuts it into squares and loads only the ones around the player, and distant places are replaced by simplified copies. A whole city block or a patch of distant forest is merged into one simple model. This is HLOD, hierarchical level of detail (lesson on LOD). The lesson “Open world: how it loads in pieces” has the details. The budget belongs here too: how much frame time and memory you can spend (frame budget, open-world budget).

In the list this pass is the last one, but it starts on day one. You create the world with streaming from the start: converting a finished world to streaming takes a long time. And you measure the budget on every pass, especially on ground cover, where the frame rate usually drops. What remains for the last pass is polish: loading distances, simplified copies, measurements along the whole route.

Done when the whole route can be walked and driven without hitches on the target computer, and you can’t see chunks loading ahead.

A pass covers the whole map

The second rule: do each pass over the whole map instead of polishing one corner to a shine. The excavator digs the whole plot, not one corner at a time with a flower bed in each. Painters do the same: they work on the whole picture at once, from general to specific, and don’t finish one eye while the canvas has no face yet.

There are four reasons:

• Big mistakes show only across the whole world. Is the village too far from the lake? Does the road lead the right way? Is the forest boring? A walk across the whole map answers all of this. Build corner by corner, and you can walk the route only at the very end, when fixing a mistake throws away beauty that is already done.
• The player sees the world whole. From a hill the player sees the whole valley. A polished corner next to an empty plain looks worse than an even world that is only halfway done.
• One pass, one hand. A layer made in one go over the whole map comes out even: the same rules, the same forest density, the same style. Corners made at different times don’t match: by the last corner you draw better than you did on the first.
• If time runs out, the world is still whole. With passes you end up with a complete world at medium readiness. With corners, half the world is beautiful and half is empty.

With passes, the mistake turns up at hour 32. Two layers are done across the whole map, ground with water and roads with places, so the route can already be walked. The world is still gray, and only those two layers have to be redone on four tiles: 8 hours, and the world is done by hour 88. Corner by corner, the route becomes walkable only at hour 77. By then the three tiles by the lake are finished, 17 hours go to waste, and the world is done only by hour 97. This model is a toy: in real life the passes differ in cost, and there is more than one mistake. But the conclusion holds: the sooner the whole world can be walked, the cheaper big mistakes are.

Tools that let you redo things

The order of passes makes late changes rare, but it doesn’t remove them. A walk shows that the lake is too far. The first players complain that the valley is cramped. So world tools are built to make changes to the base cheap. They store a recipe instead of a result: what to do and by what rule. Change the base, and the tool recomputes the result. This is non-destructive editing: the base is never damaged, and you can remove or redo any edit.

A real plot doesn’t work like this: after you move the pond, you replant the trees by hand. But imagine a gardener who is given not “plant a birch right here” but an instruction: “birches along all the paths every five meters, no closer than two meters to the water.” You move a path, and the gardener replants the birches on his own. The editor has such a gardener, and he works in seconds. Here the analogy with the plot stops working: a human gardener would need a week.

Non-destructive tools for a world come in three kinds.

Edit layers for the ground

You don’t sculpt the ground straight into a single surface; you sculpt it in layers. The base is terrain from a generator. On top of it sit a “hill” layer, a “lake bed” layer and an “embankment for the road” layer. Each layer stores only its own change. Turn a layer off and its edit disappears, while the others stay. These are edit layers.

On the plan of the plot, tracing paper plays this role: you lay one sheet over the plan for each kind of change. Take a sheet away, and the plan is as it was. The lesson on landscape explains how the engine stores and combines layers.

Splines: a line through a few points

To mark out a curved garden path, you lay a garden hose on the lawn and adjust it in three or four places, and it bends smoothly by itself. A spline is the same kind of line in the editor: a smooth curve through a few control points. You can build a road, a river, a fence, wires or a wall along it.

Move one point, and everything along the line rebuilds itself: the road bends, the ground under it levels out, the fence posts move into place again. Five points instead of five hundred pieces of road. The lesson “Roads and trails” has more on roads.

Scattering by rules

You don’t plant a forest tree by tree. You write rules. Some rules say where trees grow: not in water, not on a road, not on a slope steeper than a set limit, and denser in low places. Others say how many: the density comes from a density map, a gray picture where lighter means denser, often drawn with noise (a random pattern of smooth blotches). Still others say which trees: each one gets a random rotation, size and tint. One number, the seed, controls all the randomness: the same seed gives the same forest. The computer scatters thousands of objects by these rules and recomputes them after every change to the ground, water or roads. This is scattering by rules; in Unreal it is PCG (Procedural Content Generation).

A person writes the rules, and all the taste lives in them: forest edges, clearings, lone trees in a meadow, rocks at the foot of cliffs. The lesson “Scattering by rules” shows how to build such rules. The flow “Plant a forest by rules in PCG” shows how to build them in Unreal.

Big worlds are built this way, too. In Far Cry 5 the terrain kept changing throughout development, and Houdini tools recomputed the world from scratch every night: forests, cliffs, rivers, fences, power lines. Artists cleared the sites and laid roads with splines, and rules filled in everything around them.

In the demo below, all three tools work together on a small world. Move the hill and the lake, and compare what breaks when you place everything by hand with what recomputes itself when you place it by rules.

In “By hand” mode every object remembers where it was put. Move the hill, and the trees on the old slope hang in the air, while on the new slope they sink into the ground. The ground cover stays painted for the old shape: a rock on flat ground, sand where there is no shore anymore. On the second pass the same edit breaks nothing, because nothing stands on the ground yet.

In “By rules” mode the forest, rocks, fence, ground cover and road height recompute by themselves. Look at the distant trees: they don’t move, and only what is near the edit changes. But some things stay your job. Drag the lake onto the road, and the rules won’t move the road. Where it goes is your decision, and moving a couple of points fixes it. The houses stay where you put them, too: there are few of them, and they are decisions, not rules.

By hand: at the end and for accents

Hand placement is the most expensive to change, because each object remembers its place and height. Some tools help. Unreal adjusts the height of grass and trees that you paint onto a landscape with a brush when you sculpt the ground. But nothing moves them out of a new lake or off a new road.

So you place only a few things by hand, and at the end. A lone oak on a hill, a cart by the gate, rocks at a cave entrance, a boat by the jetty. These are accents: a dozen objects that tell the story of a place. You also fix by hand the places where the rules came out badly. In Unreal 5.8, PCG keeps such manual edits on top of the result: move or remove a few trees, and the edit survives the recompute.

A rule of thumb: thousands by rules, dozens by hand, and by hand at the end.

A map of world tools

Every engine names its tools differently, but the kinds are the same. There are seven. Splines and rules were covered above, so they get one line each here; every kind has its own lesson with the details.

• Ground from a heightmap. The ground is a grid of evenly spaced points, each with its own height. The heights are stored as a gray picture, a heightmap: the lighter the pixel, the higher the point. Brushes raise, lower and smooth the ground, while terrain generators produce it in one go, already carved by erosion (the marks left by water and rockslides). A heightmap can’t do overhangs or caves; those are made as separate cliff models. For the details, see “Landscape.”
• Bodies of water. A lake, a river and the sea are objects with a shape and a level. A lake has an outline and a water height; a river has a spline from source to mouth, with a width and a current. They carve their own bed into the ground, using their own edit layer. For the details, see “Water in the world.”
• Splines. Roads, trails, rivers, fences and wires along a line. For the details, see “Roads and trails” and scattering along roads and shores.
• Scattering by rules. Forests, grass, rocks, debris by the roads. For the details, see “Scattering by rules.”
• Hand brush. A brush plants copies of a model over a surface: density, random size and rotation (grass placement). It is good for small places and accents, but it knows nothing about new roads and water.
• Ready-made blocks. You assemble a house with an interior and furniture, a yard with a well, or a dock once and save it as one whole. This is a prefab; in Unreal it is a Level Instance. You place copies around the world, and when you fix the original, all the copies change. It is like a ready-made gazebo from a kit: you put it up whole, not board by board.
• Streaming in chunks. The world is laid out on a grid of cells, and only the cells around the player are loaded. In the editor, only the cells around the place where you work load. Each object lives in its own file, so several people can edit one world at once. In Unreal this is World Partition. For the details, see “Open world: how it loads in pieces.”

How this works in Unreal

Unreal Engine 5.8 has a tool for every pass. Together they make an order of work.

Where to start

Build your first world in the same order as a big one, only smaller.

1. A map on paper. A lake, a village, a hill with a landmark, a road between them, a forest, and a scale bar for estimating minutes of travel (map with a ruler and a grid). The map can be crude: what matters is where things are and how far apart.
2. A gray world. Rough terrain in a few big brush strokes or from a generator. Water is a plane at the right height, roads are lines, houses are boxes of the right size. No materials, no forest: only sizes, distances and what is visible from where (“Blockout: a world of boxes”).
3. A walk and a drive with a stopwatch. Walk from the village to the lake, drive the whole road, note the time. Write down where it is boring, where the car can’t make the climb, and where the landmark is out of sight. Fix the gray world and walk it again. Two or three such rounds are normal.
4. Only then ground cover and details. First the ground material and the forest by rules, then the accents by hand. Check the frame rate right after the forest, not at the end.

Where it breaks

• Rocks and trees hang in the air or have sunk into the ground. You placed them by hand before the ground settled, and then changed the ground. Scatter by rules, and place by hand at the end. If it has already happened, the Select Invalid button in Unreal’s Foliage Mode selects the copies that have come loose from the ground.
• After a walk the houses feel like sheds, the road like a highway, and the lake is ten minutes away. You checked the scale late. Set it on paper, check it with a stopwatch in the gray world, and take the sizes of houses and roads from the player’s body (build the world around the body).
• The forest stands in the lake, grass grows on the road. The rules have no exceptions for water and roads, or you planted the forest with a brush before the water and roads existed. Add “not in water, not on a road” to the rules and recompute.
• The road climbs a cliff, and the car can’t drive it. You drew the road before the terrain settled, or changed the terrain afterward. Make the road a spline, and check the slopes by driving through the gray world.
• The world is the same everywhere. Everything was made with the same rules, with no regions and no accents. Give each region its own rules and add a few hand-placed accents (open-world design).
• The game slowed down once the forest was planted. You didn’t measure the budget on the ground cover pass. Measure after every pass (frame budget).
• The world takes a long time to open in the editor, and two people can’t edit it at once. You created the world without streaming in chunks, all in one file. Start from the Open World template.

Connections: the same idea in other fields

• DrawingWork “from general to specific”: first large patches of light and shadow across the whole sheet, then forms, then details. The silhouette and the large patches read sooner than the small things. Value and silhouette
• Game designA game is also tested on a gray draft made of boxes, before any art: big mistakes are cheaper to find while nothing is on top. How to run a playtest
• MaterialsA master material and its instances also store a recipe instead of a result: fix the master, and all the instances update. Instances
• TexturesA graph in Substance Designer is non-destructive, like edit layers for the ground: change the height at the start of the graph, and the normal, AO and color recompute. Everything from height
• SoundSounds follow the same logic as the forest: in Horizon Zero Dawn one system scatters plants, sounds and animals around the player. Ambience
• GraphicsFrom far away only the large masses of a world remain: simplified copies replace city blocks and forests. So check ground cover from far away, too. HLOD

Sources and further reading

• Étienne Carrier, “Procedural World Generation of Far Cry 5,” GDC 2018How the world was recomputed every night while the terrain kept changing: water, cliffs, forests, fences and wires by rules. A summary of the talk: Christian Mills’s notes
• Epic Games, “Unreal Engine 5.7 is here”PCG is production-ready; PCG Editor Mode brings splines, a brush and volumes for rules. unrealengine.com
• Epic Games, “Unreal Engine 5.8 Release Notes”Manual edits on top of PCG output, and the experimental Mesh Terrain. Unreal documentation

Key points

• A world falls apart not from lack of skill but from the order of work: small things stand on large ones, and a change to something large breaks everything that has already been placed on top.
• You build a world in six passes: concept and scale, ground and water, roads and places as gray boxes, ground cover, details with light and air, streaming and budget. Each pass ends with a check: “is it ready for the next one?”
• A pass covers the whole map: big mistakes show only on the whole world, and finding them is cheaper while little stands on top.
• World tools store a recipe, not a result: edit layers, splines and rules recompute when the base changes.
• Place thousands of objects by rules and dozens by hand, at the end. Where the village is and where the road leads are decisions, and they stay with you.
• Streaming in chunks and the budget come last in the list but not in time: switch streaming on from day one, and measure the budget on every pass.
• Start your first world with a map on paper, then build a gray world of boxes and rough terrain, then walk and drive it with a stopwatch, and only then add details.