How can a computer draw an image that looks real?
- Reflection Models
- Glossy Materials
- Subsurface Scattering
Computer graphics is an ancient discipline with roots dating back to the 1960s. By modeling light, camera, materials and shapes, we can create virtual worlds. Rendering is interdisciplinary in nature, and combines ideas from physics, math and art.
Visible light is an electromagnetic, tranverse wave.
This means that the EM field wiggles sideways as the energy propagates forward. This wiggling in the EM field, can be seen as two fields wiggling.
But in general, you can think of light as a straight line. Light rays do not interefere with each other. light travels from source to the eye.
The simplest algorithm for modeling light is called ray tracing.
For each pixel, cast a ray from the light source, to the object underneath that pixel.
Ray tracing is an elegant technique that has its origins in lens making; Gauss traced rays through lenses by hand in the 19th century. Ray-tracing algorithms on computers follow the path of infinitesimal rays of light through the scene until they intersect a surface. This approach gives a simple method for finding the first visible object as seen from any particular position and direction and is the basis for many rendering algorithms.
In the above model, the light source is the blue concentric circle.
Materials, Microgeometry, microfacets
Try changing the roughness of the cube above to affect the glossiness of the reflected light rays below.
polarizes them. stretches the positive and negative charges
when an EM wave hits an atom,
In a gas,
The combination of dipole interactions is too complex to accurately simulate
we treat mediums as homogenous.
IoR - speed of light through medium and how much light
is absorbed by medium1
absorption determines the color
if it is colored, it absorbs some wave lengths more than others
scattering determines how cloudy the medium is
When modeling a surface to be rendered, we care only about a specific number of properties
Roughness or nanogeometry, or irregularities in the surface of the material that are smaller than the wave of light. This causes Diffraction.
The Huygens-Fresnel Principles
Plane wave -> spherical wave until it hits an optical, light bends around the corner
If the surface is optically smooth, diffraction results in reflectance.
ACTION : polish the material
the smaller the nano geometry, or the height of the bumps,
Micro geometry -
light has a very different direction
light spreads out in very wide cone
we don’t model the micro geometry explicitly
we view it statistically
the rougher the surface, the wider the cone
reflectance is the process by which a ray of light interacts with a surface.
when light bounces off a surface, it's properties change in 3 ways
a bdrf must be linear, reciprocal, conserve energy, isotropic??
Properties of light
- Spectra and Color
- Directional distribution the behavior of refracted light depends on whether it is a metal. Metal’s immediately absorb all refracted light because there is a sea of electrons on the surface of them.
nonmetals behave like cups of liquid the refracted light is scattered, absorbed or both.
if it’s transparent like glass, it will keep going all the way through. but usually it will be selectively be absorbed some percentage of the time. and the light that enters gets scattered at different angles if the pixel size size surface reflection term is specular refraction, absorption, scattering and more refraction, is diffuse quantify light as a number Radiance measures the intensity of light along a single ray spectral: rub bidirectional reflectance distribution function a function of l, v Lo(V) = integral of omega outgoing radiance = integral of incoming radiance TIMES times bdrf times cosine factor weighted average over incoming directions
Subsurface Scattering - TODO
The essential property of a lens is that it bends radiation, to converge upon a focus point.
- Field of view = Film size, focal length
- Depth of feld = Aperture, focal length
- Exposure = Film speed, aperture, shutter
- Motion blur = Shutter speed
A realistic Camera model for Computer Graphics
See the 2007 paper
Microfacet Models for Refraction through Rough Surfaces
See the 2001 review,
A Practical Model for Subsurface Light Transport↩