Computer Graphics (CS 563) Lecture 4: Advanced Computer Graphics - PowerPoint PPT Presentation

Computer Graphics (CS 563) Lecture 4: Advanced Computer Graphics Image ‐ Based Effects: Part 1 Prof Emmanuel Agu Computer Science Dept. Worcester Polytechnic Institute (WPI)

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

Image ‐ Based Effects  Image ‐ based rendering:  A spectrum of IBR techniques proposed by J. Lengyel More camera control Less camera control Slower rendering Faster rendering

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)

Relighting  Starting from image, figure out light parameters used  Re ‐ render image again with different lighting

Skyboxes  Distant objects positions (mountains, sun, sky) don’t change much or suffer from parallax  Skybox: use a environment map to render distant elements

Light Fields  Render object from many angles, store images in data structure  At run time, interpolate to render intermediate positions t v s u

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 

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

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

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

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

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 

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

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

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

Imposters No Impostors Impostors Made Easy – William Damon, Intel With Impostors

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

Depth Sprite aka Nailboard  Give depth to image !  Each texel as RGB Δ ‐ 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/

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

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