From AI concept to Unreal Engine: adding cloth and bone physics to a 3D character

What actually makes a 3D character feel alive? It’s not just high-res textures or a clean sculpt. The real magic comes from motion – subtle secondary movements in hair, clothing, accessories, and anything that should wobble, sway, or bounce when the character moves.

With modern AI tools, you can now go from a 2D concept to a fully rigged 3D character much faster. The missing piece is often physics: jiggly bones and cloth simulation. This guide walks through a full workflow – from AI-assisted concepting and 3D generation to Blender assembly and Unreal Engine 5 physics setup – using a character called Violet as an example.

Planning your character for physics

Before touching any tools, it helps to plan your character around how you want it to move. In the example, Violet has a hat with decorative pieces and a dress that should react to motion. These are perfect candidates for physics.

The key principle: split the character into logical parts. This makes it easier to generate, rig, texture, and later apply physics.

Typical splits include:

• Head
• Hair
• Body/torso
• Dress or skirt
• Hat and accessories (bows, bells, ribbons, etc.)

Keeping the body and dress separate is especially important if you plan to use cloth simulation on the dress. The same logic applies to hats, capes, scarves, or anything that should move independently.

Using AI to generate character parts

You can use modern image and 3D AI tools to speed up concepting and modeling. The workflow in this guide uses:

• AI image models (such as GPT Image 2 or similar) to generate concept art and clean views of each part.
• AI 3D tools (like Triple P1 or other 3D AIs) to turn those images into meshes.

The process usually looks like this:

1. Generate clean concept images for each part (head, hair, dress, hat, etc.).
2. Use prompts that clearly describe the shape, style, and angle you want.
3. Send each part into your AI 3D modeler to generate a mesh and base texture.

Triple P1, for example, lets you:

• Set a polygon budget per object (e.g., 3–3.5k polys for a hat, 5–7k for hair, 10–15k for a full body).
• Generate meshes quickly (often in a few seconds).
• Auto-generate base textures and even PBR maps if you’re going for realism.

Other tools can do something similar, but may require more manual steps like high-poly sculpting, retopology, and baking. The trade-off is simple: more automation and better topology usually cost money; more manual work can be done for free but takes longer. If you’re interested in this part of the pipeline, also check out how AI can turn a single image into a production-ready 3D model.

Cleaning and assembling the character in Blender

Once you’ve generated all the parts, bring them into Blender for assembly and cleanup.

Typical steps:

1. Import all meshes into a single Blender scene.
2. Align and position each part around a main reference body (e.g., the torso or full body mesh).
3. Use Sculpt Mode and basic mesh editing to tweak shapes so they fit together cleanly (e.g., adjust the hat to sit properly on the head, tuck legs into the dress, etc.).
4. Delete hidden geometry that will never be visible (for example, parts of the body fully covered by the dress). This reduces complexity and avoids physics conflicts later.
5. For symmetrical parts like hands or feet, consider mirroring one side to save texture space and keep things consistent.

This is also a good time to separate or merge objects based on how you plan to animate them. Anything that will get cloth simulation should remain its own mesh (e.g., the dress). Accessories that might get jiggly bones (like ribbons or bells) should be separate too.

Rigging the character with an auto-rigger

With the mesh assembled, the next step is to create a skeleton. An auto-rigger like AccuRig can save a lot of time.

The basic auto-rig workflow:

1. Export the assembled character from Blender as FBX. When exporting, use Path Mode: Copy and enable "Embed Textures" / Pack so textures travel with the file.
2. Import the FBX into your auto-rigging tool.
3. Place markers on key body joints (hips, knees, shoulders, elbows, wrists, fingers, etc.).
4. Let the tool generate the skeleton and skin weights.
5. Test basic movements to ensure limbs bend correctly. If your character has stylized proportions, you may need to tweak offsets.

Once you’re happy, export the rigged character using an Unreal Engine–compatible skeleton preset if available. Then bring that rigged mesh back into Blender for physics-specific adjustments.

Preparing for jiggly bones vs cloth simulation

There are two main ways to add secondary motion in Unreal Engine:

• Physics (jiggly) bones: extra bones driven by physics instead of keyframe animation. Great for hair strands, tails, ribbons, bells, and similar appendages.
• Cloth simulation: physics applied directly to a mesh surface, ideal for dresses, skirts, capes, and loose fabric.

For cloth simulation to work well, you generally don’t want standard bones to control that area. Otherwise, animation and physics will fight each other.

Removing bone influence from cloth areas

In Blender:

1. Select the armature and the mesh (e.g., the dress).
2. Switch to Weight Paint mode.
3. Use Ctrl+Shift+click on bones to see which areas they influence (red = strong influence, blue = none).
4. For bones that currently affect the dress but shouldn’t (like thigh bones), use the Gradient tool with weight set to 0 to paint out their influence from the lower part of the dress.
5. Aim for a smooth gradient so the top of the dress still follows the body, but the bottom is free for cloth simulation.

Pink areas in weight paint indicate no bone is assigned. That’s fine for sections that will be fully driven by cloth simulation.

Adding new jiggly bones in Blender

For parts like Violet’s hat decorations, you’ll want extra bones that can later be turned into physics bones in Unreal.

To add them:

1. Select the armature and go into Edit Mode.
2. Find the parent bone (e.g., the head bone). Even if it’s small or hard to see, you can locate it via the bone list.
3. With the parent bone selected, press E to extrude a new bone. Position and rotate it so it follows the shape of the accessory (for example, a chain of 3 bones along a hat ribbon).
4. Repeat to create a chain of bones that roughly follows the mesh you want to jiggle.
5. Give your new bones clear names (e.g., Hat_Main_L, Hat_1_L, Hat_2_L, etc.), so you can easily find them in Unreal later.

Painting weights for the new bones

New bones won’t do anything until they have weights.

1. Select the armature and the accessory mesh (e.g., the hat).
2. Enter Weight Paint mode.
3. Use Ctrl+Shift+click to select each new bone and paint its influence using the Gradient or Brush tools.
4. At each joint, paint a smooth gradient so the mesh bends naturally instead of creasing sharply.
5. Check for conflicts: if the entire hat is still heavily weighted to the original head bone, remove that influence from the jiggly section. You can do this via the Vertex Groups panel by selecting the head group and painting weight 0 over the parts you want controlled by the new bones.

Test in Pose Mode by rotating the new bones. The accessory should bend and follow them smoothly, without being pulled by other bones.

Exporting to Unreal Engine 5

When the rig and weights look good, export from Blender again:

1. Use FBX export.
2. Pack or copy textures so they’re embedded.
3. In the Armature section, disable "Add Leaf Bones" to avoid extra unwanted bones.
4. If you don’t need animations in this export, you can disable them.

In Unreal Engine 5:

1. Create or open a third-person template project.
2. Drag and drop your FBX into the Content Browser.
3. Import with default skeletal mesh settings.

You can then retarget the existing third-person animations (e.g., from the default mannequin) to your new character by using Unreal’s retargeting tools. This gives you walking, running, and idle animations without having to create them from scratch. For a deeper dive on animation retargeting and local character animation, see how to animate any 3D character with NVIDIA Komodo locally.

Setting up physics assets and colliders

To make physics work, Unreal uses a Physics Asset that wraps your skeleton in simple shapes (capsules, spheres, boxes). These shapes act as colliders for both physics bones and cloth.

Open your character’s Physics Asset and you’ll see:

• A skeleton tree on the left.
• Colliders (usually capsules) attached to major bones.
• Optional constraints (joint limits) between colliders.

First, clean up the default colliders:

1. Resize and rotate them so they roughly match the character’s body (torso, arms, legs).
2. Replace shapes where needed (e.g., use capsules for limbs, spheres for joints) to better fit the anatomy.
3. Pay special attention to leg colliders, since they’ll interact with the dress cloth later.

Adding colliders for jiggly bones

Your new jiggly bones from Blender won’t have colliders by default.

1. In the skeleton tree, enable "Show All Bones" so you can see every bone, including the new ones.
2. Select the jiggly bones (e.g., Hat_1_L, Hat_2_L, etc.).
3. Right-click and choose Add Shape → Capsule (or Sphere, depending on the shape).
4. Manually position and scale these colliders so they follow the chain of bones along the accessory.

This step can be a bit tedious, but it’s crucial: cloth and other physics will collide with these shapes, not the detailed mesh.

Adding constraints (joints) for physics bones

Colliders alone aren’t enough. You also need constraints that define how each bone can move relative to its parent.

1. Select a collider that needs a joint (e.g., Hat_2_L).
2. Right-click and add a Constraint to its parent bone (e.g., Hat_1_L).
3. Position the constraint at the joint location (where the bones meet).
4. In the constraint settings, adjust the angular limits or leave them fairly open if you want loose, jiggly motion.

Conceptually:

• The green cone represents swing (how far the bone can swing around).
• The red cone represents twist (how much it can rotate around its own axis).

For organic, flexible parts like ribbons or hair, you can often keep the constraints fairly permissive and rely on damping and mass settings to keep things under control.

Configuring physics bones in Unreal

Once colliders and constraints are in place, you can tune how the jiggly bones behave.

1. In the Physics Asset, select the colliders for your jiggly chain.
2. Set Mass values: heavier near the root (e.g., 3 kg), lighter towards the tip (e.g., 2 kg, then 1 kg). This creates a natural-feeling hierarchy.
3. Adjust Linear Damping (e.g., around 20) and Angular Damping (e.g., around 100) to control how quickly the bones settle back to their rest pose.
4. For constraints, enable Angular Drive with Twist and Swing so the engine knows how to drive the motion back to the default pose. Lower strength means more floppy motion; higher strength means stiffer behavior.

Next, decide which bones are driven by physics and which are purely animation-driven:

1. In the Physics Asset, select all bones except your jiggly chain.
2. Set their Physics Type to Kinematic so they only follow animations.
3. Leave the jiggly bones as simulated so they respond to physics.

Enabling physics in the animation blueprint

To actually see the physics in-game, you need to enable it in the Animation Blueprint:

1. Open your character’s Anim Blueprint.
2. In the Anim Graph, insert a Rigid Body node.
3. Connect your main animation state output into the Rigid Body node, then into the final pose.

Compile and play. When your character moves, the jiggly bones should now react with secondary motion based on your physics settings.

Adding cloth simulation to the dress

With jiggly bones working, it’s time to make the dress behave like real fabric.

Creating cloth data

1. Open the Skeletal Mesh in Unreal and go to the Clothing tools (enable the Clothing panel if it’s hidden).
2. In the mesh section, find the material element that corresponds to the dress (e.g., Element 1). Use isolate/solo to confirm it’s the right part.
3. Right-click that section and choose Create Clothing Data from Selection. Give it a name (e.g., Dress_Cloth).

Painting cloth influence

Now you define which parts of the dress are simulated and how strongly.

1. Activate Cloth Paint mode.
2. Choose the Brush tool and set a moderate radius (e.g., 7).
3. Start with a low Paint Value (around 15) and Strength around 0.5.

The idea is similar to weight painting:

• Higher values = more affected by cloth simulation (looser, more movement).
• Lower values = more anchored to the body (less movement).

A practical approach for a dress:

1. Paint the bottom edge of the dress with a higher value (e.g., 15) to make it very free and flowy.
2. Move up and paint a middle band with a lower value (e.g., 7) to create a smooth transition.
3. Near the waist or hips, paint a thin band with an even lower value (e.g., 3) so the top of the dress stays more attached to the body.

This gradient ensures the dress swings naturally from the bottom while staying stable at the top.

Applying and testing the cloth

1. Exit paint mode and right-click the dress section again.
2. Choose Apply Clothing Data and select the asset you just created (e.g., Dress_Cloth / LOD0_1 etc.).
3. The cloth simulation should start immediately in the viewport when you move or simulate the character.

This is where your earlier collider work pays off. The dress will collide with the body’s capsules and spheres instead of clipping through the legs.

If you see the cloth intersecting with the legs or behaving strangely, revisit the Physics Asset and refine the leg colliders (size, shape, and position). The closer they match the body’s silhouette, the better the cloth will behave.

Bringing it all together

By combining AI-assisted modeling with traditional 3D tools and Unreal Engine physics, you can go from a 2D concept to a lively, game-ready character much faster than before:

1. Use AI image and 3D tools to generate clean, optimized parts for your character.
2. Assemble and clean everything in Blender, planning ahead for which parts will get physics.
3. Auto-rig the character, then add custom jiggly bones and weights where needed.
4. Export to Unreal Engine, set up Physics Assets, colliders, and constraints.
5. Configure jiggly bones via mass, damping, and constraints, and enable them in the Anim Blueprint.
6. Add cloth simulation to dresses or other garments by painting influence and relying on well-placed colliders.

Once you’ve done this workflow a couple of times, it becomes a powerful template you can reuse for different characters, outfits, and styles – all starting from AI-generated concepts and models.