Eta param order could be wrong when calculating half vector

[bing007]

Hi, I couldn't wrap my head around about the order of eta arguments being passed to computeNormalizedRefractionHalfVector in BsdfTransmissionCookTorrance.cc

// Compute microfacet / half vector, pointing towards the surface side
// with the lowest ior
Vec3f m;
if (!computeNormalizedRefractionHalfVector(mEtaI,
                                           slice.getWo(),
                                           etaT,
                                           wi,
                                           m)) {
    return sBlack;
}

but the method is expecting the order etaT, wo, etaI, wi, m as shown below as well as in the paper:

// H is always pointing towards medium with lowest IOR, but only if wo and wi
// are pointing on opposite side of the surface (wrt. N).
finline bool
computeNormalizedRefractionHalfVector(const float iorWo, const scene_rdl2::math::Vec3f &wo,
        const float iorWi, const scene_rdl2::math::Vec3f &wi, scene_rdl2::math::Vec3f &H)
{
    static const float halfVectorLengthMinSqr = 0.001f * 0.001f;

    H = -(iorWo * wo + iorWi * wi);

    float lengthHSqr = lengthSqr(H);
    if (lengthHSqr > halfVectorLengthMinSqr) {
        H *= scene_rdl2::math::rsqrt(lengthHSqr);
        return true;
    } else {
        return false;
    }
}

Thanks,

[bing007]

If we search the usage of these ETAs, there appear to be other usages with similar issues.

eval does not produce any result as it always returns at this check:

    // Exclude cases where microfacet m is pointing to the opposite side of
    // the surface (wrt. N). This corresponds to the X+(m.n) term of the normal
    // distribution function D(m); see eq. (25), (30) or (33)
    const float cosThetaM = dot(m, mFrame.getN());
    if (cosThetaM <= sEpsilon) return sBlack;