ATEC 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 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 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: 1 st 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: 1 st pass  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: 2 nd 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, photon- mapping, 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.