getUVWderivs and getUVWbases.

[Karba]

I don't understand what this functions do exactly.
Please help me.

[vlado]

getUVWderivs() is used to determine the texture filtering extents for the ShadeContext:: DUVW() method, in the following way:

Point3 RayContext::DUVW(int channel) { // return UVW derivs for point
	if (duvwCache==channel) return duvw;
	duvwCache=channel;
	if (!rayresult.sd) return duvw=Point3(0,0,0);
	VR::Vector derivs[2];
	((VR::ShadeData*) rayresult.sd)->getUVWderivs(*this, channel, derivs);
	duvw=computeDiags(derivs[0], derivs[1]);
	if (!_finite(duvw.x+duvw.y+duvw.z)) {
		duvw=Point3(0.0f, 0.0f, 0.0f);
	}
	return duvw;
}

The actual implementation of getUVWderivs() for a triangle mesh is as follows:

void DefaultShadeData::getUVWderivs(const VR::VRayContext &vri, int channel, VR::Vector derivs[2]) {
	if (static_cast<const SequenceData&>(vri.vray->getSequenceData()).inMtlEditor) channel=1;
	RayContext &rc=*((RayContext*) &vri);

	VR::Vector tv[3];
	if (!getTexCoords(tv, channel, rc)) {
		derivs[0].makeZero();
		derivs[1].makeZero();
		return;
	}

	VR::Vector te0=tv[1]-tv[0];
	VR::Vector te1=tv[2]-tv[0];

	VR::Vector offs=VR::Vector(rc.rayresult.wpoint)-rc.rayresult.faceBase;
	VR::Vector &e0=rc.rayresult.faceEdge0;
	VR::Vector &e1=rc.rayresult.faceEdge1;
	VR::Vector nrm=e0^e1;

	double D=e0*(e1^nrm);
	if (nrm.lengthSqr()>(D*D)*1e18f) {
		derivs[0].makeZero();
		derivs[1].makeZero();
		return;
	}

	nrm/=D;
	// double inverse=1.0f/D;

	VR::Vector su=(e1^nrm);
	VR::Vector sv=(nrm^e0);

	double dudx=rc.rayresult.dPdx*su;
	double dvdx=rc.rayresult.dPdx*sv;

	double dudy=rc.rayresult.dPdy*su;
	double dvdy=rc.rayresult.dPdy*sv;

	derivs[0]=(te0*dudx+te1*dvdx);
	derivs[1]=(te0*dudy+te1*dvdy);
}

Best regards,
Vlado

[vlado]

getUVWbases() is used for bump mapping and returns the orientation of the U, V and W texture vectors, in world space. It is used for the ShadeContext:: DPdUVW() method like this:

void RayContext::DPdUVW(Point3 dP[3], int channel) { // Bump vectors for UVW (camera space)
	if (!rayresult.sd) return;
	VR::Vector bases[3];
	if (getVRayFlag(VR::VOUT_SMOOTH_UVW_DERIVS)) {
		((VR::ShadeData*) rayresult.sd)->getSmoothUVWbases(*this, channel, bases);
	} else {
		((VR::ShadeData*) rayresult.sd)->getUVWbases(*this, channel, bases);
	}

	bases[0].makeNormalized0();
	bases[1].makeNormalized0();
	bases[2].makeNormalized0();

	if (!_finite(bases[0].x) || !_finite(bases[0].y) || !_finite(bases[0].z)) vray->getSequenceData().progress->DumpMsg(MID_RENDER, PWRN_START, "Invalid bump basis (XYZ %g, %g, %g).\n", bases[0].x, bases[0].y, bases[0].z);

	dP[0]=_VectorFrom(toPoint3(bases[0]), REF_WORLD);
	dP[1]=_VectorFrom(toPoint3(bases[1]), REF_WORLD);
	dP[2]=_VectorFrom(toPoint3(bases[2]), REF_WORLD);
}

For a triangle mesh, the implementation is as follows:

void DefaultShadeData::getUVWbases(const VR::VRayContext &vri, int channel, VR::Vector bases[3]) {
	if (static_cast<const SequenceData&>(vri.vray->getSequenceData()).inMtlEditor) channel=1;
	RayContext &rc=*((RayContext*) &vri);

	VR::Vector tv[3];
	if (!getTexCoords(tv, channel, rc)) {
		bases[0].makeZero();
		bases[1].makeZero();
		bases[2].makeZero();
		return;
	}

	float du0=tv[1].x-tv[0].x;
	float du1=tv[2].x-tv[0].x;

	float dv0=tv[1].y-tv[0].y;
	float dv1=tv[2].y-tv[0].y;

	VR::Vector &e0=rc.rayresult.faceEdge0;
	VR::Vector &e1=rc.rayresult.faceEdge1;

	double D=du0*dv1-du1*dv0;
	double DD=D*D;

	VR::Vector b0=(e0*dv1-e1*dv0);
	VR::Vector b1=(e1*du0-e0*du1);

	if (b0.lengthSqr()<DD*1e18f) bases[0]=b0/D;
	else {
		if (e0.lengthSqr()<(du0*du0)*1e18f) bases[0]=e0/du0;
		else if (e1.lengthSqr()<(du1*du1)*1e18f) bases[0]=e1/du1;
		else bases[0].makeZero();
	}

	if (b1.lengthSqr()<DD*1e18f) bases[1]=b1/D;
	else {
		if (e0.lengthSqr()<(dv0*dv0)*1e18f) bases[1]=e0/dv0;
		else if (e1.lengthSqr()<(dv1*dv1)*1e18f) bases[1]=e1/dv1;
		else bases[1].makeZero();
	}
	bases[2]=bases[0]^bases[1];
}

Best regards,
Vlado

[Karba]

Thank you very much!