CS 354Shadows (cont’d) & Scene Graphs

Mark KilgardUniversity of TexasMarch 22, 2012

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Today’s material

In-class quiz On shadow algorithms lecture

Lecture topic Finish Shadow Algorithms Scene Graphs

Course status Return mid-terms & feedback

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My Office Hours

Tuesday, before class Painter (PAI) 5.35 8:45 a.m. to 9:15

Thursday, after class ACE 6.302 11:00 a.m. to 12

Randy’s office hours Monday & Wednesday 11 a.m. to 12:00 Painter (PAI) 5.33

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Last time, this time

Last lecture, we Discussed shadow generation algorithms

Shadow mapping Planar projected shadows Shadow volumes

This lecture Shadow generation algorithms Scene graph

Data structures to organize the rendering of 3D scenes

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Daily Quiz

1. Multiple choice: Shadow acne is a visual artifact that affects

a) shadow mapping

b) projected planar shadows

c) shadow volumes

d) stencil buffer updates

2. Multiple choice: An approach to reduce shadow acne is

a) avoiding the far clip plane

b) casting fewer shadow rays

c) polygon offset

d) acne cream

3. True or false: The stencil buffer maintains a floating-point value corresponding to each color sample.

On a sheet of paper• Write your EID, name, and date• Write #1, #2, #3, #4 followed by its answer

0

2

4

6

8

10

12

14

16

40-49 50-59 60-69 70-79 80-89 90-100

Mid-term feedbackTotal questions: 43Maximum possible: 83

Top grade: 75 Median: 59.0Average: 58.0Standard deviation: 9.63

Percentage Grade Distribution

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VisualizingShadow Volumes

Occluder and light projects a shadow volume

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Shadow VolumeResult

Objects within the volume are shadowed

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Shadow VolumeAlgorithm

High-level view of the algorithm Given the scene and a light source position,

determine the shadow volume (harder than it sounds)

Render the scene in two passes Draw scene with the light enabled,

updating only fragments in unshadowed region Draw scene with the light disabled,

updated only fragments in shadowed region

But how to control update of regions?

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2D Cutaway of aShadow Volume

shadowingobject

shadowvolume(infinite extent)

partiallyshadowed object

lightsource

eyeposition surface inside

shadow volume(shadowed)

surface outsideshadow volume

(illuminated)

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For Shadow VolumesWith Intersecting Polygons

Use a stencil enter/leave counting approach Draw shadow volume twice using face culling

1st pass: render front faces and increment when depth test passes

2nd pass: render back faces and decrement when depth test passes

This two-pass way is more expensive than invert

And burns more fill rate drawing shadow volumes

Inverting is better if no polygon intersections

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Why Eye-to-Object Stencil Counting Approach Works

Shadowing object Lightsource

Eyeposition

zero

zero

+1

+1+2 +2

+3

Example in 2D

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Illuminated,Behind Shadow Volumes (Zpass)

Shadowing object Lightsource

Eyeposition

zero

zero

+1

+1+2 +2

+3

Unshadowedobject

+ ---+ +

Shadow Volume Count = +1+1+1-1-1-1 = 0

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Shadowed, Nested in Shadow Volumes (Zpass)

Shadowing object Lightsource

Eyeposition

zero

zero

+1

+1+2 +2

+3

Shadowedobject

+ -+ +

Shadow Volume Count = +1+1+1-1 = 2

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Illuminated, In Front of Shadow Volumes (Zpass)

Shadowing object Lightsource

Eyeposition

zero

zero

+1

+1+2 +2

+3

Shadowedobject

Shadow Volume Count = 0 (no depth tests pass)

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Shadow VolumeIssues

Practical considerations [Bergeron 86] If eye is in shadow volume, need to determine

this and flip enabled & disabled lighting passes Shadow volume only as good as its tessellation

Shadows tend to magnify limited tessellation of curved surfaces

Open models and non-planar polygons Must cap the shadow volume’s intersection with

the near clipping plane

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Problem Created byNear Clip Plane (Zpass)

zero

zero

+1+1

+2

+2

+3

Near clipplane

Far clipplane

Missed shadow volume intersection due to near clip plane clipping; leads to mistaken count

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Render scene to initialize depth buffer Depth values indicate the closest visible fragments

Use a stencil enter/leave counting approach Draw shadow volume twice using face culling

1st pass: render back faces and increment when depth test fails 2nd pass: render front faces and decrement when depth test fails

Don’t update depth or color Afterward, pixel’s stencil is non-zero if pixel in

shadow, and zero if illuminated

Alternative Approach: Zfail

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Illuminated,Behind Shadow Volumes (Zfail)

Shadowing object Lightsource

Eyeposition

zero

zero

+1

+1+2 +2

+3

Unshadowedobject

Shadow Volume Count = 0 (zero depth tests fail)

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Shadowed, Nested inShadow Volumes (Zfail)

Shadowing objectLightsource

Eyeposition

zero

zero

+1

+1+2 +2

+3

Shadow Volume Count = +1+1 = 2

+ +

Shadowedobject

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Illuminated, In Front of Shadow Volumes (Zfail)

Shadowing object Lightsource

Eyeposition

zero

zero

+1

+1+2 +2

+3

Shadowedobject

Shadow Volume Count = -1-1-1+1+1+1 = 0

- +- - + +

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Problem Created byFar Clip Plane (Zfail)

zero

Near clipplane

Far clipplane

Missed shadow volume intersection due to far clip plane clipping; leads to mistaken count

+1

+1+2+2

+3

zero

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Problem Solved byEliminating Far Clip

zero

+1

+1+2+2

+3Near clip

plane

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Avoiding Far Plane Clipping

Usual practice for perspective GL projection matrix Use glFrustum (or gluPerspective) Requires two values for near & far clip planes

Near plane’s distance from the eye Far plane’s distance from the eye

Assumes a finite far plane distance Alternative projection matrix

Still requires near plane’s distance from the eye But assume far plane is at infinity

What is the limit of the projection matrix whenthe far plane distance goes to infinity?

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Standard glFrustum Projection Matrix

Only third row depends on Far and Near

0100

200

02

0

002

NearFar

NearFar

NearFar

NearFarBottomTop

BottomTop

BottomTop

NearLeftRight

LeftRight

LeftRight

Near

P

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Limit of glFrustum Matrix asFar Plane is Moved to Infinity

First, second, and fourth rows are the same as in P But third row no longer depends on Far

Effectively, Far equals ∞

0100

2100

02

0

002

lim

NearBottomTop

BottomTop

BottomTop

NearLeftRight

LeftRight

LeftRight

Near

FarinfPP

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Verifying Pinf Will Not ClipInfinitely Far Away Vertices (1)

What is the most distant possible vertex in front of the eye? Ok to use homogeneous coordinates OpenGL convention looks down the negative Z axis So most distant vertex is (0,0,-D,0) where D>0

Transform (0,0,-D,0) to window space Is such a vertex clipped by Pinf?

No, it is not clipped, as explained on the next slide

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Verifying Pinf Will Not ClipInfinitely Far Away Vertices (2)

Transform eye-space (0,0,-D,0) to clip-space

0

0

0

0100

2100

02

0

002

DNear

BottomTop

BottomTop

BottomTop

NearLeftRight

LeftRight

LeftRight

Near

w

z

y

x

D

D

y

x

c

c

c

c

c

c

15.05.05.05.0

D

D

w

zz

c

cw

Then, assuming glDepthRange(0,1), transform clip-space position to window-space position

So ∞ in front of eye transforms to the maximum window-space Z value, but is still within the valid depth range (i.e., not clipped)

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Robust Shadow Volumes sans Near (or Far) Plane Capping

Use Zfail Stenciling Approach Must render geometry to close shadow volume extrusion on the

model and at infinity (explained later)

Use the Pinf Projection Matrix No worries about far plane clipping Losses some depth buffer precision (but not much)

Draw the infinite vertices of the shadow volume using homogeneous coordinates (w=0)

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Visualize Net Stencil Rendering

Non-zero stencil state marks shadowed pixels w.r.t. the light source

Fully shaded scene Final stencil state

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Stencil VolumeRendering Animation

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Shadow Volumes Add “Invisible” Geometric Complexity

Visible geometry

Shadow volume geometry

Scene with multiple light sources

Abducted game images courtesyJoe Riedel at Contraband Entertainment

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Examples of Possible Silhouette Edges for Quake2 Models

An object viewed from the same basic direction that the light is shining on the object has an identifiable light-view silhouette

An object’s light-view silhouette appears quite jumbled when viewed form a point-of-view that does not correspond well with the light’s point-of-view

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Multiple Lights andShadow Volumes

Requires still more rendering passes,but can work!

Shadows from differentlight sources overlapcorrectly

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Shadow Volumes fromArea Light Sources

Make soft shadows Shadow volumes work for point light sources Area light sources are common and make

soft shadows Model an area light source as a collection of

point light sources [Brotman & Badler 84] Use accumulation buffer or additive blending

to accumulate soft shadow Linear cost per shadow volume sample

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Area Light Sources Cast Soft Shadows

umbra

area lightsource

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Soft Shadow Example

Eight samples (more would be better)

Note the bandingartifacts

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CombinedShadow Approaches

Two Approaches at Oncejust logo shadow volume

combinedapproach

just planar projectedshadows

teapot lacksshadowon the floor

logo lacks shadowon the teapot

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Pre-computedShadow Textures

Lightmaps are static shadow textures Compute lightmaps off-line with radiosity solver

local lighting models evaluated on tight grid can work well too

Lightmaps with soft shadows can be built faster with convolution approach, accelerated using the Fast Fourier Transform [Soler & Silion 98]

Pre-computed shadow textures work well for building interior with lots of diffuse surfaces

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Light maps can encodeStatic Pre-computed Shadows

(modulate)

=

light maps onlylight maps only

decal onlydecal only

combined scenecombined scene* Id Software’s Quake 2

circa 1997

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RenderingSoft Shadow Textures

Fast soft shadows [Heckbert & Herf 96] Render a shadow texture

For each shadowed polygon, project all light-source-occluding polygons into the shadowed polygon’s plane

* Similar to projected planar shadows* Model area light sources as multiple jittered points* Use additive blending to accumulate all contributions

Copy resulting image to the shadow texture Draw polygon in final scene with shadow texture

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RenderedSoft Shadow Texture Example

Shadow Texture and Final Result

Shadow texture, accumulationof nine jittered occluderprojections

Final resulting scene, shadowtexture modulated withwooden floor

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Scene Graphs

Refers to the application data structures for representing a 3D scene Typically object-oriented, often C++

Inheritance is very natural

Game engine Scene graph + much more Tightly coupled to

Content creation tools Compiled/packed game data

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Game Engines

Scene graph is usually the core of a game engine’s data structure

Game engines do much more Audio Networking Physics + collision detection Game play User input Artificial intelligence / strategy Control panel Scripting language environment Resource management: textures, geometry, etc.

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Tension in Scene Graph Design

Expressiveness Efficiency

Lotsof controlof scene

Highestperformance

Balanced

Good engineering is to balancing design choicesbased on application needs

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Scene Graphs

API examples Open Inventor, IRIS Performer Open Scene Graph

Game engine examples Id Tech Epic C4 Engine Ogre Unity

File format examples Virtual Reality Modeling Language (VRML)

Didn’t really work, re dux is X3D Scalable Vector Graphics (SVG)

Specialized for 2D scenes COLLADA

Scene interchange format X3D

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Layering

Application

Scene graph

Graphics APIOpenGL or Direct3D

Graphics hardware

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Common Nodes

Transform Nest hierarchically

Geometry Object (shape) Geometric mesh (geoset) Material or surface shader

Camera (view) Light Source Group Property

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Example: Performer

LOD = Level-of-detailSCS = Static coordinate systemDCS = Dynamic coordinate system

Billboard = geometry set alwaysoriented to point towards viewer Example: textured tree cut-outs

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Transform types

Standard transforms Rotation, translation, scale, shear

Rotations Quaternion, trackball

Animation Splines Blends

Unity transforms

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Hierarchical Scenes

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Light Node types

Directional light Point light Spot light

Advanced lights Shadowed light

Depends on multi-pass traversal of scene graph

Blinking light

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Camera types

Orthographic camera Perspective camera

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Geometry

Shapes are represented Geometry

Usually meshes Or patches for tessellation

Surface appearance Material parameters Shaders Textures

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Traversal

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Acceleration Structures

Binary Space Partitioning (BSP) trees Bounding spheres Bounding hulls Portal systems Octrees Grids

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View Frustum Culling

Avoid drawing objects in scene graph conservatively outside the view frustum

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OcclusionCulling

With occlusionculling

Without occlusion

culling

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Portal Culling

Good for indoor environments

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Maintaining Constant Frame Rate

Visual simulation and games aim for a high and constant frame rate 60 frames per second for illusion of animation

Means 16.6 milliseconds per frame “Never drop a frame” (hard in practice)

Tied to the refresh interval of the display device Strategies to maintain frame rate

Level-of-detail Hard to do this while avoiding “popping” artifacts

Maximizing cull-able rendering Dynamic resize the pixel resolution Tune the scene to avoid “complex” views

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Manipulators

Handle-box manipulatorTrackball manipulator

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Scene Graph Labor

High-level division of scene graph labor Four pipeline stages

App (application) Code that manipulates/modifies the scene graph in response to

user input or other events Isect (intersection)

Geometric queries such as collision detection or picking Cull

Traverse the scene graph to find the nodes to be rendered Best example: eliminate objects out of view

Optimize the ordering of nodes Sort objects to minimize graphics hardware state changes

Draw Communicating drawing commands to the hardware Generally through graphics API (OpenGL or Direct3D)

Can map well to multi-processor CPU systems

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Scene Graph Profiling

Scene graph should help provide insight into performance

Process statistics What’s going on? Time stamps

Database statistics How complex is the scene in any frame?

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Example:Depth Complexity Visualization

How many pixels are being rendered? Pixels can be rasterized by multiple objects Depth complexity is the average number of times a

pixel or color sample is updated per frame

yellow and black indicate higher depth complexity

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Example:Heads-up Display of Statistics

Process statistics How long is

everything taking?

Database statistic What is being

rendered?

Overlaying on active scene often value Dynamic update

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Next Class

Project 2 today (!?) On texturing, shading, & lighting Study GLSL Two weeks to complete

Next lecture Compression How can images and other graphics data be stored

more compactly?