The “Dark” Side of Game Development

Let’s see some ID…

Julian SpillaneCEO / Technical Director, Frozen North

Productions, Inc.○ We make video games.

Currently working on a next-gen XBLA project○ January 2008 release○ PC release to follow soon after…

Break it down… XNA?

How does it stack upReception by the pros

Shadowing in Games Shadowing Algorithms

Maps, volumes, geometry projection…Why I prefer maps

Basic Shadow Map Theory & Implementation Variants

DPSM, PSM, TSM, LiSPSM, PSSM, VSM… Tips for Optimization, Soft Shadows Further Reading

XNA’s Not Acronymed What IS XNA?

A managed framework for developing games on both PC and Xbox 360○ Graphics device, input, and audio management

along with math helpers Who is using XNA?

HobbyistsIndependent developers… and yes, professionals

○ Torpex Games, Hidden Path Entertainment, GarageGames Inc.

XNA’s Not Acronymed (cont’d) Why XNA?

Unmanaged background○ C, C++, assembly○ XNA exposed me to a new language

XNA allows for RAD○ Full games can be put together in obscenely short

periods of time○ Allows developers to focus on content, gameplay,

balancing over technical issuesXNA does things more inuitively than DirectX

○ Render Targets○ Device Management

Preaching to the choir? ;)

XNA’s Not Acronymed (cont’d)

Full game built in 2 weeks using XNA with no prior knowledge of the API…

Shadowing in Games Why are shadows important?

Add a sense of presence and volume to a sceneImportant visual cuesMake for interesting gameplay mechanicsAdds to realism (caveats?)Players expect dynamic lighting

○ Shadows play a major role

How many different ways are there to shadow?In a word: lots

Shadowing Algorithms Geometry Based

Projected Shadows○ Geometry collapse

using “Shadow Matrix” Shadow Volumes

○ Regular volumes○ Z-fail / “Carmack’s

Reverse”○ GPU-generated

Both cases ~O(n) in terms of scene complexity

Image Based Shadow Map

○ Depth-based shadowing system

○ Many variants: PSM, TSM, LiSPSM, PSSM, CSM, VSM, DPSM, etc.

○ Relatively O(1) in terms of geometric scene complexity

Forward Shadow Mapping○ Proposed technique

Shadowing Algorithms (cont’d) Geometry Based

Pros○ Sharp, crisp shadows○ Easy to compute

If it’s visible on-screen, you already have the data to shadow it○ Easily extendable to the vertex shader

Speed benefits through pipeline-integration Cons

○ Shadow complexity is O(n), n = # edges Means speed α complexity-1

Fine balance between object count and edge resolution○ Requires image-space modification or ugly jitter for soft-

shadows○ No support for alpha-blended textures without extreme

trickery We’re talking leprechaun-level, here…

Shadowing Algorithms (cont’d)Shadow Volumes – Algorithm, Demonstration

Images courtesy of Østfold University College, Norway and NVIDIA Corporation

Shadowing Algorithms (cont’d)

Shadowing Algorithms (cont’d)

Images courtesy of me. Demonstration of vanilla shadowing using PCF and screen-space blur.

Shadowing Algorithms (cont’d) This presentation will focus on shadow

mapping, its theory, implementation and variantsOn modern hardware, pros far outweigh consSelf-shadowing and alpha blending!Scenes in games are growing in complexity

○ Vertex-bound techniques will cause even more problems as time progresses

Independent of model / vertex data architecturePersonal preference ;)

What Is Shadow Mapping? Rendering from the light’s POV

Using results to generate projective texture○ Generally filled with single-color depth

information

Two-Pass ProcessGenerating the shadow mapShadowing the scene

Shader ApproachCan be done using FF, but deprecated

Creating the Shadow Map Theory

Render from light’s point of view○ Any object that can be seen must be lit○ Store the depth value of every element

Render the scene as normal○ Take the position of every vertex

Transform into light-space coordinatesRetrieve the point as it is seen from the point of view of

the light (depth map)

○ Depth-compare with light-view mapIf depth value of surface is further away, it must be

behind an occluder and shadowed (z-fail test)

Creating the Shadow Map The Shadow Map

Can be represented in many formats○ Best Choice: R32F (single channel float)

Only if hardware supports…

○ Can use ARGB, just less precisionUnless interesting depth hashing with the channels…

Use XNA RenderTarget2D○ Writeable surface, can convert to texture○ Easy to use

Creating the Shadow Map

Creating the Shadow Map

Creating the Shadow Map Effect

Let’s now consider the effect○ Needs to push unlit vertices

Return depth information

○ Store depth independentlyPOSITION cannot be read directly

○ Very simple shaderDon’t make it any more complex than it needs to be

Creating the Shadow Map

Creating the Shadow Map

Creating the Shadow Map

Statue seen from the light’s point of view. Blue because of R32F texture. Notice gradation: depth values

Shadowing the Scene Projection

Now we have depth values stored○ Need to render a second pass from camera○ Project the shadow texture into the scene and depth

compareYet another matrix

○ Texture bias and projection matrix Used to map position from projection space into texture space

)/5.0(5.0

)/5.0(5.0

0.10.0

0.00.10.00.0

0.00.05.00.0

0.00.00.05.0

texHeighttY

texWidthtX

tYtX

P

Shadowing the Scene

Depth ComparisonTransform position using previous matrix

○ Compare z-value to value stored in shadow sampler

○ if stored depth < visible depth at that pointMust be occluded and therefore shadowed

Easy pixel shader comparison○ Single texture lookup with 1 inline conditional

branch (ps_2_0 only)

Shadowing the Scene

Shadowing the Scene

Shadowing the Scene

Images courtesy of Frozen North Productions, Inc.

Shadowing the Scene

Final StepsMerging with the scene

○ Simply multiply shadow term by diffuse colour / lighting / texture

So…improvements?○ Aliasing artifacts, projection artifacts, hard

edges, the list goes on○ How do we mitigate the problems?

Variations on the techniqueI will cover overviews with links to papers and demos

Shadowing the Scene

Images courtesy of Frozen North Productions, Inc.

Shadow Map Variants Dual-Paraboloid Shadow Maps

Basic shadow mapping is inherently spot / directional○ Need creative solution to handle omnidirecitonal (point) lights

Paraboloid Mapping○ “[I]mage obtained by an orthographic camera view a perfectly

reflecting paraboloid.”○ Single map can cover a full hemisphere

Implies only 2 rendering passes for omni light vs 6 passes for cube lookup○ Find a point P in R3 on the paraboloid that reflects a given

direction towards +/- Z○ Depth value calculated by using the distance from surface point

to center of paraboloid Extends from a 2D-3D mapping to a 3D-3D mapping

“Shadow Mapping for Hemispherical and Omnidirectional Light Sources” Brabec, et al., Computer Graphics Group, Max-Planck Institute, Saarbrucken Germany○ http://www.mpi-inf.mpg.de/~tannen/papers/cgi_02.pdf

http://www.gamedev.net/reference/articles/article2308.asp

Shadow Map Variants Perspective Shadow Maps

Aliasing due to depth resolution issues PSMs

○ Generated post-perspective transformation○ Light is transformed projectively to the unit cube○ Reduces perspective aliasing since the depth values are

considered after perspective proejction Caveats

○ Cases to handle for objects behind the viewer but cast shadows (lit from behind the viewer)

○ Comparatively convoluted implementation “Perspective Shadow Maps”, Stamminger, Marc and

Drettakis, George, REVES – INRIA Sophia Antipolis, France○ http://www-sop.inria.fr/reves/Basilic/2002/SD02/

PerspectiveShadowMaps.pdf

Shadow Map Variants Trapezoidal Shadow Maps

Similar to PSMs○ Approximates eye frustum as seen from the light

using a trapezoid to warp it onto a shadow map○ Designed to address perspective-induced aliasing

and quality discontinuity (flickers)○ Biases shadow map so quality increases closer to the

camera“Anti-aliasing and Continuity with

Trapezoidal Shadow Maps”, Martin, Tobias and Tan, Tiow-Seng, School of Computing, National University of Singapore○ http://www.comp.nus.edu.sg/~tants/tsm/tsm.pdf

Shadow Map Variants Light-Space Perspective Shadow Mapping

LiSPSM○ Modification to the PSM algorithm

○ A bit of a mouthful○ Designed to address artifacts inherent in the PSM algorithm

○ Perspective distortion, missed shadow casters, singularities in post-projective space

○ Perspective transform specified in light-space○ Allow for treating all lights as directional while still maintaining perspective

benefits○ Far more complicated than regular shadow maps

○ Benefits outweigh time cost of implementation○ General opinion mixed

○ “Light Space Perspective Shadow Maps”, Wimmer et. al, Vienna University of Technology, Austria, 2005○ http://www.cg.tuwien.ac.at/research/vr/lispsm/

shadows_egsr2004_revised.pdf

Shadow Map Variants Parallel-Split Shadow Maps

PSSM Splits the view frustum into parts using plane parallel to the

viewing plane Generates smaller shadow maps for split parts

Tighter bounds on each map takes better advantage of texture resolution Results in crisper shadows at lower cost

Widely implemented Lots of available example code Current shadow map fad

Very useful for large, outdoor environments Makes good use of orthographic cameras

“Parallel-Split Shadow Maps for Large-Scale Virtual Environments”, Zhang et. al., Chinese University of Hong Kong, 2006 http://appsrv.cse.cuhk.edu.hk/~fzhang/pssm_vrcia/

shadow_vrcia.pdf

Shadow Map Variants

Shadow Map Variants Variance Shadow Maps

VSM Developed by a fellow UW alum and IGDA Toronto member

Andrew Lauritzen, University of Waterloo student Presented at i3D 2006 then GDC 2007

Taking the industry by storm Designed to solve aliasing issues by using calculated variance to

determine an upper-bound on the fraction of an occluded, shaded fragment Store mean and mean2 of a range of depths Allows computation of variance

Relatively simple to compute on modern GPUs A bit more overhead, but well worth the cost

Allows for mipmapping and texture filtering Generally uncomputable for SSM because of single depth value

Caveats Light bleeding on filter areas with high variance

“Variance Shadow Maps”, Donnelly, William and Lauritzen, Andrew, CGL, University of Waterloo http://www.punkuser.net/vsm/vsm_paper.pdf

Shadow Map Variants

Optimizations

Things to consider Texture size Conditional statements

Depth comparison in shader

Texture filtering Combining of algorithms

PSSM + VSM particularly interesting for large outdoor scenes

Selecting the right algorithm for the scene Unified solution does not exist

Soft Shadows

Large Topic Could spend another two hours on various

techniques

Various Techniques Percentage Closer Filtering Penumbra Calculations Screen-Space Blur Combinations…

Will demo PCF with Screen-Space Blur

Further Reading “Casting Curved Shadows on Curved Surface”,

Williams, Lance, CGL, New York Institute of Technology, New York, 1978 The original introduction of shadow mapping (yes it’s that

old) "Shadow mapping." Wikipedia, The Free

Encyclopedia. 1 Sep 2007, 09:57 UTC. Wikimedia Foundation, Inc. 29 Sep 2007 <http://en.wikipedia.org/w/index.php?title=Shadow_mapping&oldid=154989894>.

“Shadow Mapping Tutorial”, Paul’s Projects, http://www.paulsprojects.net/tutorials/smt/smt.html

“Hardware Shadow Mapping”, NVIDIA Corporation http://developer.nvidia.com/attach/8456

Questions?

Thank You

Contact Information Email: jspillane@frozennorth.net Office: (519) 513-2409 Website: www.frozennorth.net