Animations (Syncing and getting started)

[cuppajoeman]

Hi there,

In my game engine I already have code for loading in animations from .fbx files through assimp, I have this function called set_bone_transforms:

https://github.com/cpp-toolbox/rigged_model_loading/blob/b11401e02585ba9738133a2fa4eab974210856ec/rigged_model_loading.cpp#L509

it returns a vector of glm::mat4 which represent the transformations for each bone (they are the full transforms after the skeletal hierarchy is parsed), then in my shader each vertex has a collection of indices into the bone animation transform matrix, resulting in a weighted transformation.

Note: in my shader each vertex can only be influenced by at most 4 bones and only has support for 100 bones in total right now

Now I want to have a low poly player model that I can use for hitscan against an animated player in jolt that is synchronized to the animated player model that we see in-game

Research:

I found SkeletalAnimation which seems promising and looks like I need to call the Sample function there which takes in a SkeletonPose which I think is initialized by the SetSkeleton function which takes in a Skeleton, which seems to contain the hierarchy of bones (armature in blender?)

Questions:

• Are Joints the same thing as bones?
• Can anyone point me to some sample code for constructing a skeleton from a source file?
• Since each vertex is only influenced by 4 bones how can I make the same occur within the jolt animation
• Is there any other things I need to take into account?

[jrouwe]

Are Joints the same thing as bones?

Yes

Can anyone point me to some sample code for constructing a skeleton from a source file?

JoltPhysics/Samples/Utils/RagdollLoader.cpp

Lines 124 to 137 in c93401e

	// Create skeleton
	Ref<Skeleton> skeleton = new Skeleton;
	uint lower_body = skeleton->AddJoint("LowerBody");
	uint mid_body = skeleton->AddJoint("MidBody", lower_body);
	uint upper_body = skeleton->AddJoint("UpperBody", mid_body);
	/*uint head =*/ skeleton->AddJoint("Head", upper_body);
	uint upper_arm_l = skeleton->AddJoint("UpperArmL", upper_body);
	uint upper_arm_r = skeleton->AddJoint("UpperArmR", upper_body);
	/*uint lower_arm_l =*/ skeleton->AddJoint("LowerArmL", upper_arm_l);
	/*uint lower_arm_r =*/ skeleton->AddJoint("LowerArmR", upper_arm_r);
	uint upper_leg_l = skeleton->AddJoint("UpperLegL", lower_body);
	uint upper_leg_r = skeleton->AddJoint("UpperLegR", lower_body);
	/*uint lower_leg_l =*/ skeleton->AddJoint("LowerLegL", upper_leg_l);
	/*uint lower_leg_r =*/ skeleton->AddJoint("LowerLegR", upper_leg_r);

Since each vertex is only influenced by 4 bones how can I make the same occur within the jolt animation

You can't. You connect rigid bodies to joints. The vertices of a rigid body are by definition fixed so you don't have to skin them. Skin weights are usually stored with the render mesh rather than the skeleton or the animation.

Is there any other things I need to take into account?

The API doesn't require using SkeletonAnimation or Skeleton, all interfaces also offer passing an array of Mat44. You can keep the hierarchical information in your own classes if you want. I expect very few people using these two classes as each engine will have it's own version and there are much better ways to store and compress animations than what I have implemented.

See e.g.:

JoltPhysics/Jolt/Physics/Ragdoll/Ragdoll.h

Lines 163 to 164 in c93401e

	/// Lower level version of SetPose that directly takes the world space joint matrices
	void SetPose(RVec3Arg inRootOffset, const Mat44 *inJointMatrices, bool inLockBodies = true);