This is a bit of a weird one but let me describe to you what I mean cause im not 100% familiar with model terms or sdk terms or blender or whatever. I want to know what setting or origin point or bone or whatever determines where the model is based off the ground. When I am in VRC without full body, my (digitigrade) legs end up touching the “ground” but as I move up and down just from shifting in my chair, my legs stretch and compress but my feet stay on the “ground” itself. What is determining that so my feet know to not just… plummet into the dirt if I crouch down irl? or end up floating a ton of the ground if I suddenly stand up from sitting mode?
When exporting from blender, the origin of your armature will determine the bottom of your avatar. Make sure it is placed at blender’s world origin.
VRChat tries to place your feet at the origin of your avatar, and VRChat also tries to place the origin of your avatar on the world floor by using colliders. When you crouch in your VR headset, VRChat does several things to keep your avatar planted on the ground.
First, VRChat places your avatar bottom on the floor using physics and collisions, moving your entire avatar. It then blends in a crouching animation based on the height of your headset. This lowers the spine and raises the legs. The animation also lowers the root bone (the waist) so that the avatar’s feet are roughly placed at the bottom. Finally, it runs an algorithm that computes a pose for the upper and lower legs to place your feet more firmly on the ground.
Here is a very crappy drawing that demonstrates this process. The grey thing is your headset:
VRChat struggles with digitigrade legs because VRChat, like many other games, uses only the hips and knees to place feet on the ground. Digitigrade animals make complex use of their ankles, in addition to their hips and knees, to place their toes on the ground. So humans (and VRChat) have two points of rotation in their legs, but digitigrade animals have three.
You can see this process partially fail by standing on the edge of a collider (e.g. on stairs). One of your feet will be placed on the ground, but the other one will hover in the air.