Basic Illumination Model in C#

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Introduction

In my previous article Simple Ray Tracing in C#, we saw how to display spheres using a simple ray tracing algorithm. Now, we will start from the last point and will work with basic illumination models.

By definition, we have three types of light:

• Ambient
• Diffuse
• Specular

Ambient

It is considered as the light distributed by the environment, which contributes to the global illumination independent of the light position, objects, or observer.

Diffuse

It is the light the contribution of which depends on its incidence angle. Diffuse light hit can be reflected in all directions.

Specular

Specular lights represent the bright spots in objects; the more reflective it is the smaller the bright spot.

Background

To proceed with an illumination algorithm, we need to get the ambient, diffuse, and specular constants for the material we want to model; for example, here we use brass constants which are:

Exponent 27.8974

The ray tracing algorithm used is the same as you can see on Simple Ray Tracing in C# with the improvements of a basic illumination model.

The Equations

A Light Source is defined as an R³ (x,y,z) point with a (v_x,v_y,v_z) direction vector.

An Observer is defined as an R³ (x,y,z) point with a (v_x,v_y,v_z) direction vector.

Theta is defined as the angle between a light ray and a normal vector at the intersection point P on the surface.

Phi is defined as the angle between the reflected light ray at the intersection point P on the surface and the viewer ray to the same point P.

The Sphere Equation

• r² = (x-cx)²+(y-cy)²+(z-cz)²

Illumination on a given pixel:

• I_Ambient = I * K_Ambient
• I_Diffuse = I * K_Diffuse * cos(theta)
• I_Specular = I * K_Specular * cos(phi)ⁿ
• I = I_Ambient + I_Diffuse + I_Specular

Reflection calculation:

• i' = i - (2 * n * dot(i, n))

where

• i = incidence light ray
• n = normal at intersection
• i' = reflected ray

The Code

...
if (spherehit != null)
     {
     double intersx  = px + t * vx, intersy = py + t * vy,
                       intersz = pz + t * vz;
     double vNormalX = intersx - spherehit.cx,
                       vNormalY=intersy - spherehit.cy,
                       vNormalZ=intersz - spherehit.cz;
     double cost     = tAlgebra.GetCosAngleV1V2(lvx, lvy, lvz, vNormalX,
                       vNormalY, vNormalZ);
     if (cost < 0) cost = 0;

     double vReflX = 0, vReflY = 0, vReflZ = 0;
     double vEye2IntersX = px - intersx, vEye2IntersY = py - intersy,
                           vEye2IntersZ = pz - intersz;

     tAlgebra.Reflect(lvx,lvy,lvz, vNormalX,vNormalY,vNormalZ,ref vReflX,
                      ref vReflY, ref vReflZ);
     double cosf = tAlgebra.GetCosAngleV1V2(vReflX, vReflY, vReflZ,
                   vEye2IntersX, vEye2IntersY, vEye2IntersZ);
     if (cosf < 0) cosf = 0;

     double result1 = cost * 255.0;
     double result2 = Math.Pow(cosf, spherehit.shininess) * 255.0;
     double rgbR = (spherehit.ambientR * 255.0)+(spherehit.diffuseR *
                    result1) + (spherehit.specularR * result2);
     double rgbG = (spherehit.ambientG * 255.0) +(spherehit.diffuseG *
                    result1) + spherehit.specularG * result2);
     double rgbB = (spherehit.ambientB * 255.0) +(spherehit.diffuseB *
                    result1) + (spherehit.specularB * result2);
     rgbR = Math.Min(rgbR, 255);
     rgbG = Math.Min(rgbG, 255);
     rgbB = Math.Min(rgbB, 255);
     color = Color.FromArgb((int)rgbR, (int)rgbG, (int)rgbB);
     ...
     }
...