Microsoft XNA Game Studio Creator’s Guide- P14:The release of the XNA platform and specifically the ability for anyone to write Xbox 360 console games was truly a major progression in the game-programming world. Before XNA, it was simply too complicated and costly for a student, software hobbyist, or independent game developer to gain access to a decent development kit for a major console platform. | 68 MICROSOFT X N A GAME STUDIO CREATOR S GUIDE float3 unitDirection normalize lightDirection L - dot product of surface normal and light direction float cosine dot unitNormal unitDirection R 2 N - L float3 reflection normalize 2 cosine unitNormal - unitDirection n specular reflection. float specularLevel pow dot reflection unitDirection 2 specularColor color intensity specularLevel return specularColor The diffuse light is simply the dot product between the light direction vector and the object being lit. The dot product approaches 1 for full intensity with the directness of the light to the surface. When this calculation is done in the pixel shader the result is interpolated between pixels to produce a nice smooth-looking light. As you move the camera closer to the wall the light radiates brightly and fizzles outward from the point that is directly in front of the camera. Diffuse light is modeled by the following equation Diffuse Color Diffuse Intensity To make the light fade from the center even more dramatic the light intensity is scaled with a fallOff variable. fallOff is the inverted exponent of the scaled distance d between the light and pixel. exp d equals ed where e is approximately . Because cos a as the angle between the surface normal and light vector decreases cos a approaches 1 and diffuse light increases. Shining a light directly at a surface normal generates a brighter reflection than a light shone at an angle away from the normal vector. PointLightDiffuse calculates the color added by the diffuse light float4 PointLightDiffuse VStoPS IN unit direction of light vector L floats lightDirection normalize lightPosition - brightest angle between L and N 0 float diffuseLevel dot lightDirection get distance from light to pixel float distance distance lightPosition 69 CHAPTER 22 I compute a falloff for the lighting float scale float fallOff clamp exp distance .
