I mainly want to understand the reconstruction after converting cone beam to parallel beam in the backprojectors_VD.cu file. In the coneParallelBackprojectorKernel_vox function, I understand that backprojection weight does not need to be calculated in parallel beam backprojection, while it is required in cone beam geometry. However, I am not sure whether this backprojection weight is needed in the Cone Beam Parallel Backprojector. In the code:
// Calculate backprojection weight-related parameters
const float v_denom = sqrtf(R * R - s * s) - (cos_phi * x + sin_phi * y);
const float v_denom_inv = 1.0f / v_denom;
const float backprojectionWeight = doWeight ? R : R*R * v_denom_inv;

In the subsequent code, it is multiplied by sqrtf(1.0f + v_arg*v_arg):

tex3D<float>(g, s_arg, v_phi_x_first + k_offset * v_phi_x_step_A, L) * backprojectionWeight * sqrtf(1.0f + v_arg*v_arg);

Could you recommend any papers or articles where I can refer to this backprojection weight?
Additionally, regarding const float asin_tau_over_R = asin(tau / R);, is this used for cone beam artifact correction?