Function sh_add

StereoKitC/spherical_harmonics.cpp:56–86

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void sh_add(spherical_harmonics_t &to, vec3 light_dir, vec3 light_color) {
	light_dir = { -light_dir.x, -light_dir.y, light_dir.z };

	// From DirectXMath's XMSHEvalDirectionalLight. See:
	// https://github.com/microsoft/DirectXMath/blob/master/SHMath/DirectXSH.cpp#L4476

	// CosWtInt
	const float fCW0 = 0.25f;
	const float fCW1 = 0.5f;
	const float fRet = MATH_PI / (fCW0 + fCW1);
	light_color = light_color * fRet;

	// The rest is a mix of XMSHEvalDirectionalLight, XMSHEvalDirection, and
	// sh_eval_basis_2
	const float z2 = light_dir.z*light_dir.z;
	const float s1 = light_dir.y;
	const float c1 = light_dir.x;
	const float s2 = light_dir.x*s1 + light_dir.y*c1;
	const float c2 = light_dir.x*c1 - light_dir.y*s1;

	to.coefficients[0] += light_color * 0.282094791773878140f;
	to.coefficients[2] += light_color * 0.488602511902919920f*light_dir.z;
	to.coefficients[6] += light_color * (0.946174695757560080f*z2 + -0.315391565252520050f);
	to.coefficients[1] += light_color * -0.488602511902919920f*s1;
	to.coefficients[3] += light_color * -0.488602511902919920f*c1;
	const float p_2_1 = -1.092548430592079200f*light_dir.z;
	to.coefficients[5] += light_color * p_2_1*s1;
	to.coefficients[7] += light_color * p_2_1*c1;
	to.coefficients[4] += light_color * 0.546274215296039590f*s2;
	to.coefficients[8] += light_color * 0.546274215296039590f*c2;
}

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