Chris Mayer & Nic Shulver Shading Definitions Light emanating from small surfaces is called a Point light sourcee.g. the sun (not really small though!) Light emanating from a large surface is call a Distributed light sourcee.g. fluorescent tube Multiple reflections of light from nearby objects combine to produce uniform illumination called Ambient lighte.g. walls

Chris Mayer & Nic Shulver Shading Definitions Diffuse reflection occurs when light hits a “dull” surface and the surface re-emits some of the light in all directions Specular reflection is the direct reflection of light by a “shiny” surface Specular reflection Diffuse reflection

Chris Mayer & Nic Shulver Shading Ambient Light Ambient light can be considered to be of uniform intensity in all directions It is “cloudy day” light, no specific origin so no shadows It is usually modelled as a constant within a scene The intensity of a surface exposed to ambient light depends on the reflectivity of the surface and the ambient light level

Chris Mayer & Nic Shulver Shading Diffuse reflection The intensity of a surface due to a point light source is calculated from Lambert’s cosine law, so objects appear brighter when light is almost “behind” your viewpoint It also depends on the distance of the object from the light source and the reflectivity of the surface

Chris Mayer & Nic Shulver Shading Specular reflection At certain points a shiny surface reflects all incident light. This produces a spot of reflected light that is the same colour as the incident light  RN L Dull surfaces have a wider reflection range  RN L Shiny surfaces have a narrow reflection range

Chris Mayer & Nic Shulver Shading Flat Each surface intensity is calculated using ambient and diffuse reflections The whole surface is coloured with this one flat colour Objects look faceted Mach banding occurs Very quick to calculate

Chris Mayer & Nic Shulver Shading Gouraud Assumes that uniform shading is not desired It interpolates the brightness over the surface Intensity at each vertex is calculated and then linearly interpolated across the surface Smoother appearance, in fact it can smooth hard edges too much Not good at incorporating specular reflection

Chris Mayer & Nic Shulver Shading Phong Phong Shading Extends Gouraud’s method but instead of interpolating intensities it interpolates the normals and calculates the intensity at every point on the surface It renders specular reflections properly It is very expensive in computer time From a lecture by B.Wyvill

Chris Mayer & Nic Shulver Shadows (Hard and Soft) umbra penumbra

Chris Mayer & Nic Shulver Umbra-Penumbra Umbra -penumbra dependence on the size of the light source Penumbra Umbra

Chris Mayer & Nic Shulver Shadows Shadows can be used to anchor objects used to emphasize the changing direction of the light source hard edged or soft edged contain both an umbra and penumbra Shadows are a quality add on No one particular shadow algorithm

Chris Mayer & Nic Shulver Shadows For static scenes, shadows are fixed and do not change as viewpoint changes If relative positions of objects and light sources change the shadows have to be recalculated There is a high computational overhead