Illumination and Shading
• How to shade surfaces based on the
position,orientation,characteristics of the
surfaces and the light sources illuminating
them
Illumination Models
• It express the factors
• Determine a surface’s color at a given point
• Called lighting models or shading models
Shading Model
• Determines when the illumination model is
applied
• What arguments it will receive
• Ex:some shading models invoke an illumination
model for every pixel in the image
• Others invoke an illumination model for only
some pixels , and shade the remaining pixels by
interpolations (it is a method of constructing new
data points within the range of a discrete set of
known data points).
Ambient light
• light that is already present in a scene, before
any additional lighting is added. It usually refers
to natural light, either outdoors or coming
through windows etc. It can also mean artificial
lights such as normal room lights.
• Fluorescent lighting in an office is an example of
ambient lighting.
• In renderings, an ambient light source is
sometimes used as a default light source to give a
baseline level of lighting to the model.
Shading Model
• It defines how light is scattered or reflected
from a surface.
• Light sources shine on the various surfaces of
the objects.
• The incident light interacts with the surface in
3 different ways.
1.Some is absorbed by the surface and
converted to heat
2.Some is reflected from the surface
3.Some is transmitted into the interior of the
object as in the case of a piece of glass
Shading Model
• If all of the incident light is absorbed ,the
object appears black and is known as a black
body.
Example:Point light source
Example:Point light source
Example:Point light source
Example:Point light source
Diffuse Scattering
• Occurs when some of the incident light
penetrates the surface slightly
• And re-radiated uniformly in all directions.
• Scattered light interacts strongly with the
surface
• Its color is affected by the nature of the
material by which the surface is made
Example: Scattering
Specular Reflections
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Mirror like
Highly directional
Incident light does not penetrate the object
It is reflected directly from its outer surface
Reflected light color is same as the incident
light
• Make the material look like plastic
Example:Specular
Example:Specular
Flat Shading
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Simplest shading model for polygons.
Also called constant shading or faceted shading
Basic shading technique
The entire object has only one color
Color is computed at only one vertex, and entire
face is filled with that color.
• Brightness is drawn uniformly over the entire
face.
Flat shading
• Calculate single intensity value, use it to share
an entire polygon.
• Sample the value of the illumination equation
once for each polygon, hold the value across
the polygon to reconstruct the polygon’s
shade.
Flat shading
This approach is valid,if several assumptions are
true,
1.The light source is at infinity,so Ń.Ĺ is constant
across the polygon face.
2.The viewer is at infinity,so Ń.Ĺ is constant
across the polygon face.
3.The polygon represents the actual surface
being modeled,and is not an approximation to
a curved surface.
Example:Flat shading
Example:Flat shading
advantages
• Inexpensive to compute
• Appropriate for objects with flat surfaces
• Less pleasant for smooth surfaces.
Smooth(or)Interpolative shading
Smooth(or)Interpolative shading
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It calculates the color at each vertex
Produces much better image
It computes during scan conversion
More expensive to calculate
It attempts to emphasize edges between faces
Compute colors at more points on each face.
Showing the smooth underlying surface rather
than the individual polygons making up the
object.
Two types of smooth shading
1.Gourad shading
2.Phong shading