Use fast inverse-transpose to compute the width factor

Assets/ScriptableRenderLoop/HDRenderLoop/Material/Lit/Lit.hlsl

     P2 = mul(P2, transpose(basis));
 
     // Compute the binormal.
-    float3 B = normalize(cross(P2 - P1, P1));
+    float3 B = normalize(cross(P1, P2));
 
     float ltcValue;
 

Assets/ScriptableRenderLoop/ShaderLibrary/AreaLighting.hlsl

     float tLDDL2 = l2rcpD / (d * d + l2 * l2);
     float intWt  = LineFwt(tLDDL2, l2) - LineFwt(tLDDL1, l1);
     float intP0  = LineFpo(tLDDL2, l2rcpD, rcp(d)) - LineFpo(tLDDL1, l1rcpD, rcp(d));
-    return intP0 * normal.z + intWt * tangent.z;
+    // Guard against numerical precision issues.
+    return max(intP0 * normal.z + intWt * tangent.z, 0.0);
+}
+
+// Computes 1.0 / length(mul(ortho, transpose(inverse(invM)))).
+float ComputeLineWidthFactor(float3x3 invM, float3 ortho)
+{
+    // transpose(inverse(M)) = (1.0 / determinant(M)) * cofactor(M).
+    // Take into account that m12 = m21 = m23 = m32 = 0 and m33 = 1.
+    float    det = invM._11 * invM._22 - invM._22 * invM._31 * invM._13;
+    float3x3 cof = {invM._22, 0.0, -invM._22 * invM._31,
+                    0.0, invM._11 - invM._13 * invM._31, 0.0,
+                    -invM._13 * invM._22, 0.0, invM._11 * invM._22};
+
+    // 1.0 / length(mul(V, (1.0 / s * M))) = abs(s) / length(mul(V, M)).
+    return abs(det) / length(mul(ortho, cof));
-    // Inverse-transform the endpoints and the binormal.
+    // Inverse-transform the endpoints.
-    B  = mul(B,  invM);
+
+    float width = ComputeLineWidthFactor(invM, B);
 
     if (P2.z <= 0.0)
     {
     // Integrate the clamped cosine over the line segment.
     float irradiance = LineIrradiance(l1, l2, P0, T);
 
-    // Compute the width factor. We take the absolute value because the points may be swapped.
-    // TODO: consider using an inverse-transpose of 'invM' - potentially faster.
-    float width = abs(dot(B, normalize(cross(T, P1))));
-
-    // Guard against numerical precision issues.
-    return max(INV_PI * width * irradiance, 0.0);
+    return INV_PI * width * irradiance;
 }
 
 #endif // UNITY_AREA_LIGHTING_INCLUDED