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Improve arm IK code #112

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Improve arm IK code #112

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cf7df15
Remove redundant trig functions
FirEmerald Jan 31, 2026
28f717d
propagate back arm IK inversion fix
FirEmerald Jan 31, 2026
2ba0b32
Fix comment describing poledir
FirEmerald Jan 31, 2026
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14 changes: 7 additions & 7 deletions src/hooking/skeleton.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -134,16 +134,16 @@ class Skeleton {
dist = glm::clamp(dist, epsilon, l1 + l2 - epsilon);

// law of cosines for angle at shoulder (alpha)
float cosAlpha = (l1 * l1 + dist * dist - l2 * l2) / (2 * l1 * dist);
float alpha = glm::acos(glm::clamp(cosAlpha, -1.0f, 1.0f));
float cosAlpha = std::clamp((l1 * l1 + dist * dist - l2 * l2) / (2 * l1 * dist), -1.0f, 1.0f);
float sinAlpha = -std::sqrt(1.0 - cosAlpha * cosAlpha);

// plane construction
glm::vec3 dirNorm = glm::normalize(dir);
glm::vec3 planeNormal = glm::normalize(glm::cross(dirNorm, poleVector));
glm::vec3 ortho = glm::normalize(glm::cross(planeNormal, dirNorm));

// arm 1 direction (world)
glm::vec3 arm1Dir = glm::normalize(dirNorm * cos(alpha) + ortho * sin(alpha));
glm::vec3 arm1Dir = glm::normalize(dirNorm * cosAlpha + ortho * sinAlpha);

// arm 2 direction (world)
glm::vec3 elbowPos = rootPos + arm1Dir * l1;
Expand All @@ -153,12 +153,12 @@ class Skeleton {
glm::vec3 x1 = arm1Dir * boneForwardSign;
glm::vec3 z1 = planeNormal;
glm::vec3 y1 = glm::cross(z1, x1);
glm::mat3 rot1World = glm::mat3(x1, -y1, -z1);
glm::mat3 rot1World = glm::mat3(x1, y1, z1);

glm::vec3 x2 = arm2Dir * boneForwardSign;
glm::vec3 z2 = planeNormal;
glm::vec3 y2 = glm::cross(z2, x2);
glm::mat3 rot2World = glm::mat3(x2, -y2, -z2);
glm::mat3 rot2World = glm::mat3(x2, y2, z2);

// convert to local space
glm::mat4 arm1Local = glm::inverse(parentWorld) * glm::mat4(rot1World);
Expand Down Expand Up @@ -464,8 +464,8 @@ void CemuHooks::hook_ModifyBoneMatrix(PPCInterpreter_t* hCPU) {
glm::vec3 targetPos = glm::vec3(targetModel[3]);

// pole vector (elbow direction)
// left: left-down-back, right: right-down-back
glm::vec3 poleDir = isLeft ? glm::vec3(1.0f, -1.0f, -0.5f) : glm::vec3(-1.0f, -1.0f, -0.5f);
// left: right-up-front, right: left-up-front
glm::vec3 poleDir = isLeft ? glm::vec3(-1.0f, 1.0f, 0.5f) : glm::vec3(1.0f, 1.0f, 0.5f);

// rotate pole vector by body rotation (Skl_Root)
if (Bone* rootBone = s_skeleton.GetBone("Skl_Root")) {
Expand Down