compute_alpha
s32 alpha_y0 = compute_alpha(from->sigma.y0);
s32 alpha_y1 = compute_alpha(from->sigma.y1);
s32 alpha_u0 = compute_alpha(from->sigma.u0);
s32 alpha_u1 = compute_alpha(from->sigma.u1);
s32 alpha_v0 = compute_alpha(from->sigma.v0);
s32 alpha_v1 = compute_alpha(from->sigma.v1);