3/23/2005 © Dr. Zachary Wartell 1 Illumination Models and Surface- Rendering Methods

3/23/2005 © Dr. Zachary Wartell 2 light models and surface rendering light model – model for calculating the light intensity at a single surface point surface rendering – procedure for applying a lighting model to obtain pixel colors for each pixel on a projected surface. Options: –compute light model at every pixel ray-tracing, scan-line graphics hardware circa 2000 (but more computation→lower framerate) –compute light model at few surface locations and interpolate color of other pixels older scan-line graphics hardware, modern hardware must still balance realism vs framerate.

3/23/2005 © Dr. Zachary Wartell 3 Light in real world medium emission spectrum reflection spectrum phototopic curve (eye sensitivity)

3/23/2005 © Dr. Zachary Wartell 4 Light in graphics medium emission spectrum reflection spectrum Display RGB pixels space “outside” display typically not computationally modeled

3/23/2005 © Dr. Zachary Wartell 5 Light interactions Light interacts with a surface in some combination of: emission reflection –on surface : mirror, specular or diffuse –suspended particles: random scattering transmission –transparent, translucent, refraction absorption

3/23/2005 © Dr. Zachary Wartell 6 Distance Attenuation emission spectrum Display RGB pixel At farther distances same amount of energy is spread over a larger area (section of sphere). In particular: energy/area = 1 / distance 2

3/23/2005 © Dr. Zachary Wartell 7 Directional Light Sources Point Light Source V light θlθl V obj α If α < θ l then P is illuminated by light; otherwise it is not. V obj  V light = cos α, so test V obj  V light ≥ cos θ l P assumes vectors are normalized and 0 ≤ θ l ≤ 90 ◦

3/23/2005 © Dr. Zachary Wartell 8 Surface Lighting Effects: Diffuse Reflection diffuse reflection – reflection intensity equal in all outgoing directions. Reflected light color is due to combination of incoming spectrum and surface absorption spectrum

3/23/2005 © Dr. Zachary Wartell 9 Surface Lighting Effects: Specular Reflection Specular reflection – reflected light is concentration in a particular outgoing direction. It appears as a highlight or bright spot on surface. In physical world reflected light is primarily due to incoming light spectrum, but graphics models usually include surface reflection spectrum also.

3/23/2005 © Dr. Zachary Wartell 10 Surface Lighting Effects: Ambient Light ambient light – approximates all the combined light that a surface receives from all other non- emitting surfaces. Light is not direct from light source but reflected off an intermediate surface. This is a very coarse approximation to reality. Doesn’t model “color bleeding”.

3/23/2005 © Dr. Zachary Wartell 11 Diffuse reflection of Point Light Source CGwOG, Hearn&Baker

3/23/2005 © Dr. Zachary Wartell 12 Diffuse Reflection of Ambient and Point Light CGwOG, Hearn & Baker

3/23/2005 © Dr. Zachary Wartell 13 Specular Reflection incident angle = reflection angle = θ mirror reflection – all reflected light follows R specular reflection – reflected light distributed in “reflection” cone around R N L V R θ θ Φ mirror shiny specular dull specular

3/23/2005 © Dr. Zachary Wartell 14 Combined Illumination Model ●to keep I in [0,1] range: clamp final result, normalize individual terms, or scale based on maximum of all pixels