From greybox to dressed level: replacing placeholder props with AI

The greybox is a contract

A greybox (or blockout) is a level built from simple shapes to test gameplay before art exists. Each grey cube is a promise: "something this size, here, doing this job". Cover from enemy fire. A climbable ledge. A landmark the player navigates by. When you replace placeholders with real props, the risk is breaking those promises: a crate that's taller than the cover cube, a statue that blocks a sightline.

Done carefully, art replaces the cubes without changing the game.

Step 1: Annotate the placeholders

Before replacing anything, give each placeholder a name and a purpose:

• cover_low_01 — low cover, must stay under 1 m.
• landmark_tower — visible from the spawn.
• clutter_shelf_03 — decoration only, no collision needed.

Many teams use colour coding in the greybox for this (cover in one colour, climbable in another). Write down the dimensions of anything gameplay depends on.

Step 2: Turn each placeholder into a brief

For each prop, write a one-line brief: what it is, roughly how big, and any constraint. "Stack of sandbags, 0.9 m high, 2 m wide, chest-high cover."

This brief becomes your prompt or the description next to your sketch. A sketch is often the better input here, because you can draw the exact proportions the placeholder needs. Sketch to 3D for game assets covers that workflow.

Step 3: Generate, then fit to size

Generated models don't arrive at real-world scale, so you'll fit each one to its placeholder:

1. Generate the prop and export FBX or GLB.
2. Import it next to the placeholder.
3. Scale it uniformly until the critical dimension matches (height for cover, footprint for a floor prop). Avoid non-uniform scaling unless the proportions are wrong anyway; it distorts textures.
4. Apply the scale so it's real geometry, ideally in Blender before export. Wrong size on import explains units in each engine.

Step 4: Match the pivot

Placeholders usually have their pivot at the bottom centre. Set the new prop's pivot to the same place so it drops into the same position and rotation. Fix a pivot point shows how in Blender and in-engine.

Then swap: place the new prop at the placeholder's transform, and hide (don't delete) the placeholder until you've tested.

Step 5: Re-test gameplay

A real prop is not a cube. Check:

• Collision. Does the player still fit through gaps? Can they still mantle the ledge? Use simple collision shapes that match the old placeholder where gameplay depends on it. Adding collision.
• Cover height and silhouette. Does the prop hide a crouched player as the cube did?
• Sightlines. Detailed silhouettes (spikes, branches) can block or reveal more than intended.
• Navigation mesh. Rebake and check AI paths.
• Readability. Can the player still tell cover from decoration at a glance?

Step 6: Dress the rest

Once the gameplay-critical props are swapped and tested, add clutter that the greybox didn't specify: small props, decals, variety. This is where generating variations (three barrels instead of one) pays off. AI 3D props for indie games covers which props generate well.

Tips

• Swap in passes, by area or by prop type, and playtest between passes.
• Keep placeholder and prop on separate layers so you can toggle back.
• Don't let art change the layout unless the designer agrees. If a beautiful statue needs more space, that's a design conversation.

Got a greybox full of cubes? Generate the first replacements in Vi3W and swap them in one area at a time.