py_sha_fresnel

Demonstrates how to implement a shader that computes a fresnel effect.

DETAILS

The shader evaluates the Fresnel response and shows how a ShaderData plugin reports its output and exception color.

To run this example, place the file in the Cinema 4D plugins folder and restart Cinema 4D.

Topics:

• Computing a Fresnel shader response
• Returning shader output and exception colors

Files

py_sha_fresnel.pyp

"""Demonstrates how to implement a shader that computes a fresnel effect.

The shader evaluates the Fresnel response and shows how a ShaderData plugin reports its output and exception color.

To run this example, place the file in the Cinema 4D plugins folder and restart Cinema 4D.

Topics:
    - Computing a Fresnel shader response
    - Returning shader output and exception colors
"""
__copyright__ = "Copyright 2026, MAXON Computer"
__author__ = "Maxime Adam"
__date__ = "25/08/2026"
__license__ = "Apache-2.0 license"
__version__ = "13.0.0"

import math
import c4d

# Be sure to use a unique ID obtained from developers.maxon.net
PLUGIN_ID = 1027089


class PyFresnel(c4d.plugins.ShaderData):

    def __init__(self):
        # If a Python exception occurs during the calculation of a pixel colorize
        # this one in red for debugging purposes
        self.SetExceptionColor(c4d.Vector(1, 0, 0))

    def FaceForward(self, N, I):
        return abs((-I) * N) * N

    def Fresnel(self, I, N, etasqrt):
        cos_theta = I * N

        fuvA = etasqrt - (1.0 - (cos_theta * cos_theta))
        fuvB = abs(fuvA)
        fu2 = (fuvA + fuvB) / 2.0
        fv2 = (-fuvA + fuvB) / 2.0
        fv2sqrt = 0.0 if fv2 == 0.0 else math.sqrt(abs(fv2))
        fu2sqrt = 0.0 if fu2 == 0.0 else math.sqrt(abs(fu2))

        fperp_temp = ((cos_theta + fu2sqrt) * (cos_theta + fu2sqrt)) + fv2
        if fperp_temp == 0.0:
            return 1.0

        fperp2 = (((cos_theta - fu2sqrt) * (cos_theta - fu2sqrt)) + fv2) / fperp_temp

        fpara_temp = ((etasqrt * cos_theta + fu2sqrt) *
                      (etasqrt * cos_theta + fu2sqrt)) + fv2sqrt * fv2sqrt
        if fpara_temp == 0.0:
            return 1.0

        fpara2 = (((etasqrt * cos_theta - fu2sqrt) * (etasqrt * cos_theta - fu2sqrt)) + -
                  fv2sqrt * -fv2sqrt) / fpara_temp

        return 0.5 * (fperp2 + fpara2)

    def Output(self, sh, cd):
        """Called by Cinema 4D for each point of the visible surface of a shaded object to return the color.

        Important: No OS calls are allowed during this function. Doing so could cause a crash, since it can be called in a multi-processor context.

        Args:
            sh (c4d.BaseShader): The shader node connected with this instance.
            cd (c4d.modules.render.ChannelData): Channel data to use and/or modify.

        Returns:
            c4d.Vector: The color of the shaded from 0 = black to 1 = white
        """
        # If shader is computed in 3d space
        if cd.vd:
            # default IOR value
            ior = 1.6
            n = self.FaceForward(~cd.vd.bumpn, ~cd.vd.ray.v)
            fresnel = self.Fresnel(-(~cd.vd.ray.v), ~n, ior * ior)

            return c4d.Vector(fresnel)

        # If shader is computed in 2d space, return black
        else:
            return c4d.Vector(0.0)


if __name__ == '__main__':
    # String resource, see c4d_symbols.h, have to be redefined in python
    IDS_PY_FRESNEL = 10000
    c4d.plugins.RegisterShaderPlugin(
        PLUGIN_ID, c4d.plugins.GeLoadString(IDS_PY_FRESNEL), 0, PyFresnel, "", 0)

Example on GitHub

res/c4d_symbols.h

enum
{
	// End of symbol definition
	IDS_PY_FRESNEL=10000,
  _DUMMY_ELEMENT_
};

Example on GitHub

res/strings_us/c4d_strings.str

STRINGTABLE
{
    IDS_PY_FRESNEL  "Py SDK - Fresnel";
}

Example on GitHub