the dark side of game development. lets see some id… julian spillane ceo / technical director,...
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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)
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 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
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
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
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 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
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
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