(note: these notes were covered over 2+ days. day 1 we got through bump mapping, day 2 was all kinds of advanced mapping, leaving shadow volumes for day 3)
High Level Review: (see Lecture 7):
What are the goals of graphics for games?
- Fast (keep up the frame rate)
- Dynamic (respond to what’s going on)
- High Quality – works for the game
- works with the style (realism, cartoon, stylized, …)
- avoids visual artifacts
- provides enough cues
- Control / Authorability – we can get what we (or the art directory wants)
- without undue experimentation/work
Rendering Review
How does physics light a scene
Key concepts:
- surface interactions and micro-geometry, subserface
- local vs. non-local vs. global
- direct vs. global
- reflection vs. glossiness
Texture Mapping and Image-Based Stuff
Motivating Image-based methods
- pre-rendered
- sprites (depth ordering, alpha, …)
- shape-changing sprites
- texture maps
- billboards / skyboxes / special cases
- more general imposters
- layers
Review
- motivations
- texture coordinate generation
- filtering
Animation by fiddling with texture coordinates
Other texturing tricks
- volumetric textures
- world-space textures
- slide projector textures
- tool for lighting and shadows (later)
Other Maps
- Surface properties
- color
- material properties (shinyness, glossiness)
- local geometry (displacement map, normal/bump map, occlusion map)
Texturing Surfaces
Problem: still a flat surface
View Dependent Textures (very simple thing)
Displacement Maps (move geometry) – really messy to do
Bump Map / Normal Maps (move normals)
Parallax Mapping (constantly evolving)
- depending on view direction, look somewhere else
- walk along direction of ray (based on height)
Parrallax Occlusion Mapping (see gamedev.net)
walk along ray until you hit something
Relief Mapping (Manuel’s page) (paper)
- use image warps to pre-compute the ray casts
- where does the point go on the face of its bounding box
- extend this to shadowing
Light Maps (of various forms)
- pre-compute lighting on the surface (somehow)
- keep it around as a texture
- texture combining
Environment Maps
Basic version
- small object / big world assumption
- capture lighting box (envionment map)
- box vs. sphere vs. cylinder
- pure specular (reflections)
- filtering issues
Environment Map Lighting?
- Hack version (old car racing games)
- earth is brown
- sky is blue
- lighting colored by direction
Environment maps to capture light
- Captures all lights (direct and indirect)
- from a distance, permanent in the scene
- Issues
- dynamic range (direct light much brighter than non)
- so far only for specular
- Idea 1 – sampling beyond the specular ray
- issue – self-occlusions (ignore for now)
- rays for glossy
- rays for diffuse (lots)
- Idea 2 – small samples (but area)
- equivalence of samples and blur
- pre-filtering (to make equivalence of lots of samples)
- problem: how to pre-filter an environment map
- MIP-MAP per cube wall? (what about corners?)
- Spherical Harmonics
- Why?
- arbitrary complex lighting environments (not limited to number of lights)
- specular reflections
- glossiness
- dynamic lighting? (re-render the environment map)
Shadows
Some basics
- umbra / penumbra / lit area
- hard vs. soft shadows
- self shadowing
Hacky shadows can be useful! (ball over plane example)
Reasons why shadows are useful
Casting Shadows
Hierarchy of Hacks
- Drop shadows (splotches) on floor
- paint dark circles
- paint light areas (spot-light)
- how does this interact with existing colors
- prospective hacks (for arbitrary point lights)
Shadow Maps (NOT light maps)
- draw scene from the light’s point of view
- use as colors (everything is shadowed, even things casting shadows)
- use as items (so you can tell what is/isn’t shadowed)
- use as depth
- Gotchas
- other than spotlights (cube environment maps?)
- depth bias
- item buffering doesn’t really work (very small polygons, moving)
- get first and second objects (two depths)
- aliasing (need high resolution to get sharp edges)
Shadow Volumes
- basic idea (extrude each triangle)
- point-in-frustum checks to see if in shadow
- relatively efficient ways to use z-buffer and stencil buffers to perform checks
- where you see the polygons, turn off lighting
- 3 passes: draw z+dim, draw shadow volumes (stencil), draw bright (where not stenciled)
- several ways to do mask pass
- simple = (1) draw scene (dark) (2) draw back, things occluded get stenciled as on (since they are behind volume) (3) draw front (places where occluded is in front of voume) (4) un-shadow what’s not marked
- Gotchas
- need to combine/simplify geometry
- expensive to do lots of fills 3 times
Towards more global illumination
Ambient Occlusion
- soft shadows for self-shadowing
- adds darkness where you get less light
- independent of the direct lighting
Basic Ideas
Bent Normals
Implementation
- ray sampling
- multiple shadow drawing
- multiple direction rendering
- screen space methods
- hack – estimate how much forward facing occlusion
- naïve version does lots of texture reads
- clever random sampling tricks to minimize reads (16?)
Why is AO so cool
- easy and cheap
- pre-computed for objects
- SSAO for game scenes
- gets major depth effects
- dark in pockets
- creases cause changes
- low-frequency soft shadowing effects
