Download Lecture 9

ATEC 4371.001
Procedural
Animation
Introduction to Procedural
Methods in 3D Computer
Animation
Dr. Midori Kitagawa
In class
 Pay
attention
 Take notes
 Learn
 Be ready for a pop quiz
Week 10: Rendering
 Rendering
 Scan-line
 Ray-tracing
 Photon
mapping
 Physically based rendering
 Assignment 14 review
 Assignment 16
Rendering
 The
process of converting the abstract
representations of geometric entities into
the appropriate color values in an image.
Rendering algorithms
 Scan-line
 Ray-tracing
 Photon
mapping
 Physically based rendering
Scan-line algorithm
 Most
commonly used rendering algorithm,
especially in real-time.
 Fast
 E.g. Micro-polygon rendering in Houdini
Scan lines and pixels
 Any
digital image is composed of a 2D
grid of pixels.
 Scan line is each row of pixels.
Scan line
Scan-line algorithm
 looks
at each pixel,
one after the other,
scan line by scan
line, and calculates
the color that pixel
should be
rendered.
Scan-line algorithm
 The
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color of each pixel is computed using:
the color and other surface
characteristics of the surface visible from
the camera (i.e, the surface closest to the
camera),
the lights in the scene, and
the position of the camera.
Scan-line algorithm
 If
the surface closest to the camera is
transparent, the color of the pixel is
computed using the surface
characteristics of the next closest surface
as well.
 With the scan-line algorithm, light is never
refracted.
Ray-tracing
 traces
the origins of the imaginary light ray
that arrives the camera through each
pixel in the image plane.
Ray-tracing
 To
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achieve this, a ray is cast back into the
object space to determine whether the
ray was
absorbed or reflected by a surface,
refracted by a transmissive medium, or
originated directly from a light source.
Ray-tracing
 The
path of a ray may be
divided into two when part of
light is reflected by a surface
while another part travels
through the surface.
Ray-tracing
 good
at reflection, refraction and
shadows.
 more time-consuming than scan-line
rendering.
Ray-tracing
Scan-line
Direct vs. indirect
illumination
 Direct
illumination occurs when
a light source directly
illuminates objects in a scene.
 Indirect illumination occurs
when the light that is reflected
or transmitted by objects
illuminates other objects.
Global illumination
 Global
Illumination is the technique used
to simulate indirect illumination.
 Photon mapping is one of global
Illumination methods.
 Photon mapping is a two-pass rendering
algorithm that deals with both diffuse and
specular reflections.
Photon mapping: 1st pass
 In
the first pass, photons are shot from the
light into the scene.
 Photons are bounced around interacting
with all surfaces that they encounter.
Photon mapping: 1st pass

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Photons are stored in a photon map for later use.
The resolution of the photon map is independent
from the resolution of the geometry.
Only a few thousands to a million photons are
sparsely stored and the rest is statistically
estimated from the density of the stored photons.
After all the photons have been stored in the map,
an estimate of the illumination at each photon is
statistically computed.
Photon mapping: 2nd pass
 In
the second pass, the direct illumination
is computed like ray-tracing and the
indirect illumination is computed from
querying the stored photons in the photon
map.
Photon mapping
 In
short, photon mapping shoots photons
from the light and tracks their distribution
in the scene.
 It is fast but not as accurate as final
gather
Physically based rendering
(PBR)
 follows
the physical behavior of light and
surfaces as closely as possible.
 incorporates ray-tracing, photonmapping, and other methods.
Physically based rendering
(PBR)
 Fresnel
 Energy
conservation
PBR: Fresnel
 describes
the behavior of light reflected
by a surface having differing reflectivity
that occurs at different angles.
PBR: Fresnel
 brighter
reflections near the edges.
PBR: Energy conservation
 The
outgoing energy cannot be greater
than the incoming energy.
 Diffuse reflection + specular reflection <=
incoming light
in other rendering algorithms
 Diffuse
reflection and
specular reflection
are independently
controlled.
 Diffuse reflection +
specular reflection >
incoming light is
possible.
PBR: Energy conservation
 In
PBR, the energy conservation law is
enforced.