SLIDE 1
Computer Graphics (CS 563) Lecture 4: Advanced Computer Graphics Image‐Based Effects: Part 1 Prof Emmanuel Agu
Computer Science Dept. Worcester Polytechnic Institute (WPI)
SLIDE 2 Image‐Based Effects
Three main types of image‐based effects
- a. Image‐Based Rendering: Texturing to improve RT
performance so that algorithms run at 30 FPS.
Examples: billboards, sprites
- b. Image Processing Post‐process: Used to add effects to
rendered images
Examples: High Dynamic Range (HDR), tone mapping, motion blur
- c. Volumetric effects: atmospheric effects
SLIDE 3 Image‐Based Effects
Image‐based rendering:
A spectrum of IBR techniques proposed by J. Lengyel
More camera control Slower rendering Less camera control Faster rendering
SLIDE 4 Fixed View Effects
If viewpoint is fixed, can increase image quality Example: Few moving foreground elements in
complex scene
Render and store background rendering information
(z‐buffer, color buffer, etc)
SLIDE 5
Relighting
Starting from image, figure out light parameters used Re‐render image again with different lighting
SLIDE 6
Skyboxes
Distant objects positions (mountains, sun, sky) don’t
change much or suffer from parallax
Skybox: use a environment map to render distant
elements
SLIDE 7 Light Fields
Render object from many angles, store images in data
structure
At run time, interpolate to render intermediate positions
t s
v u
SLIDE 8 Sprites, Billboards, and Impostors
Sprites:
Pure 2D image
No warping, or projection (Example: mouse cursor)
Billboards:
Sprite applied to a polygon
Alpha channel usually employed
Uses texture mapping for acceleration
Impostors:
Billboards created on the fly.
Can represent complex models
Error metric associated w/ changed views
SLIDE 9 Layered Sprites
Scene as a series of Layers Each layer has depth associated Render Back‐to‐front (avoid Z buffer) Camera movement restricted: only perpendicular scene
SLIDE 10 Billboards
IBR: pre‐render geometry onto images/textures Map textures onto polygons, place in scene Rendering at runtime involves simple lookups, fast Orienting polygon based on View Direction Billboarding + Alpha + Animation = free forms
(smoke, fire, explosions, clouds etc.)
Real time cloud rendering, Mark J. Harris
SLIDE 11 Types of Billboards
Three vectors of interest: up vector, normal and
rotation vector (perp to up and normal)
Screen Aligned Billboard:
Image always parallel to screen with constant up vector Up vector = camera’s up vector
SLIDE 12
World Aligned Billboard
Rendered object usually has orientation in space Use object’s up vector to orient billboard For small sprites, just align with view plane Otherwise orient with viewer position
SLIDE 13 Axial Billboards
Cylindrical Symmetry ( trees, laser beams etc) Does not face straight‐on towards viewer Rotate around some world space axis, align to face user as
much as possible
Up vector fixed, view point direction is adjustable vector Tree example
Single billboard v/s solid surface tree
Up vector along tree trunk
SLIDE 14 Particle Systems
Set of separate small objects set into motion using an
algorithm
Method of animation – not rendering Simulating Fire, smoke, explosions, water flow, trees, galaxies Representation – Points, lines … Can be billboards too Idea: controls for creating, moving,
changing and deleting particles
http://en.wikipedia.org/wiki/Particle_system
SLIDE 15
Imposters
Created on fly Render a complex object into image texture Texture mapped onto Billboard Can reuse imposter for a few frames to boost
performance before update
SLIDE 16 Why Create Imposters?
Fast to draw Closely resemble the object Reuse for several viewpoints located close together Best for static and distant objects
Movement of object diminishes with distance from viewer
Overcome low LOD constraints, since a high quality
imposter can be created
SLIDE 17 Imposters
Impostors Made Easy – William Damon, Intel
No Impostors With Impostors
SLIDE 18 Billboard Clouds
Billboard Clouds, Decoret, Durand et al [SIGGRAPH‘03] Render complex mesh onto cloud of billboards Billboard inclined at different viewpoints Models with tens of thousands of trianges can be represented
convincingly with less than 100 textured billboards
SLIDE 19 Depth Sprite aka Nailboard
Give depth to image ! Each texel as RGBΔ ‐ Δ (transparency) is depth parameter Set Δ based on depth of actual geometry Superior to imposters because better visibility when penetrate
closeby objects
Accuracy varies with no. of bits to represent Δ
2 bits 4 bits 8 bits
http://zeus.gup.uni-linz.ac.at/~ gs/research/nailbord/
SLIDE 20 IBR: Pros and Cons
Pros
Simplifies computation of complex scenes Rendering cost independent of scene complexity
Cons
Static scene geometry Fixed lighting Fixed look‐from or look‐at point
SLIDE 21
References
Kutulakos K, CSC 2530H: Visual Modeling, course
slides
UIUC CS 319, Advanced Computer Graphics
Course slides
David Luebke, CS 446, U. of Virginia, slides Chapter 2 of RT Rendering Suman Nadella, CS 563 slides, Spring 2005
