blender/intern/cycles/kernel/osl/nodes/node_fresnel.h

/*
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 */
 
float fresnel_dielectric(vector Incoming, normal Normal, float eta)
{
	/* compute fresnel reflectance without explicitly computing
	 * the refracted direction */
	float c = fabs(dot(Incoming, Normal));
	float g = eta * eta - 1 + c * c;
	float result;

	if (g > 0) {
		g = sqrt(g);
		float A = (g - c) / (g + c);
		float B = (c * (g + c) - 1) / (c * (g - c) + 1);
		result = 0.5 * A * A * (1 + B * B);
	}
	else
		result = 1.0;  /* TIR (no refracted component) */

	return result;
}