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02_Rigging_3D_models.md

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replicAnt

"generating annotated images of animals in complex environments with Unreal Engine"

by Fabian Plum, René Bulla, Hendrik Beck, Natalie Imirzian, and David Labonte (2023)

Rigging 3D models (subjects)

Requirements:

Behind every great synthetic dataset stands a great 3D model. We will assume that you already have a retopologised 3D model you wish to use.

rendering of a retopologised low-polygonal 3D model (with a wireframe overlay)

Important Blender shortcuts to make your life (so much) easier:

Now, again: the most important shortcuts to make interacting with blender easier.

Relating to the viewport:

  • Middle Mouse Button: (Hold and move your mouse to) rotate the viewport.

  • Middle Mouse Button + SHIFT: (Hold and move your mouse to) translate the viewport.

  • num-pad keys: Moves the viewport to a pre-defined position.

The following shortcuts work in any blender panel, so they are useful to remember:

  • G: Move – move a selected object around. Click the left mouse button to confirm the new position.

  • S: Scale – scale a selected object up or down. Here, this will help you to adjust the size of the tracking marker.

  • R: Rotate – rotate a selected object. Adding the rotation of a marker can help retain its identity across frames.

You can also add constraints to the above commands to specify their effect. After pressing any of the keys above you can additionally use:

  • X / Y / Z: Restrict the axis in which you are performing the action.

  • CTRL: Jump between whole number steps

  • SHIFT: Perform the change in finer increments. Helps a lot when making small changes.

You always confirm your change by clicking the LEFT Mouse Button.

  • And, of course, CTRL + Z is your friend to revert your last steps (we all make mistakes). By default, you can go back 32 steps, but you can increase that number under Edit/Preferences…/System menu, if you want/need to.

For a handy "cheat-sheet", have a look at Blender 3.0 Shortcuts

For a "not-so handy HotKeys In-depth Reference", refer to the official HotKeys In-depth Reference

Rigging workflow

To make use of our provided animation blueprints, follow our standard rigging convention.

In theory, any model and rig combination is possible. Make sure, your model is posed with all legs/equivalent in ground contact in a somewhat natural pose. This is important, as we are going to use IK solvers to have the appendages interact with elements in the generated environment.

In this guide we will only cover rigging using our pre-configured armature. More advanced users may wish to create their own custom armatures and blue-prints.

Bringing the pre-configured armature into your project

Open the Blender project file containing your retopologised 3D model and in click on File > Append. Navigate to the example_data directory and select the base_rig.blend file.

Navigate to the Objects folder, select the Armature and import it.

The Armature should now be loaded into your scene at the world origin. Next, select the Armature and switch into Edit Mode to begin moving the set of virtual bones into their desired location on the model.

Now (in Edit Mode), select the entire Armature (with A) and move it upwards to roughly align with the centre of your model.

Next, we activate Snap To > Volume so that the joints we move automatically snap to the centre of the surrounding mesh. Make sure to also select Snap With > Center, as the default is Closest.

Furthermore, make sure "X mirror" is turned off while editing. Otherwise, any adjustment made will affect the respective joints (left and right) on both sides of the Armature.

Now, one by one, select each joint and move it to their desired location. Refer to our reference figures below:

NOTE:

Take your time with the placement of joints. The more accurately you assign them in your model, the more realistic the resulting poses will be. You can always go back and refine their position, but you may have to repeat the weight painting process detailed below afterward.

(Joint naming convention details)

(top-down view of the assigned Armature)

(side view of the assigned Armature)

(Close-up of the end configuration of each leg. As the generator reports back only the location of the base of each bone, the tail end of every chain of bones may lie outside the model)

After you have completed the joint placement process, we can now parent the 3D model (mesh) to the Armature.

First, select the 3D model, then, while holding SHIFT, select the Armature.

Now, press CTRL + P and select Armature Deform > With Automatic Weights.

Finally, select your 3D model and switch into Weight Painting mode. On the right side open the Vertex Group panel and, one by one, check that every virtual bone only affects the desired area. If it also affects adjacent areas of the mesh, use the Add and Subtract brushes to paint in the areas correctly.

Be very diligent with this final step, as otherwise your model will not deform correctly inside the generator, and everyone will be disappointed.

When you are done, select the Armature and switch into Pose Mode to ensure all joints behave as expected and only affect the desired areas of the mesh!

Next up:

03 Bringing 3D_models into Unreal

In case you encounter any problems, consult our troubleshooting guide, or consider raising an issue on the replicAnt GitHub page.

License

© Fabian Plum, Rene Bulla, David Labonte 2023 MIT License