The “Dark” Side of Game Development Shadowing in XNA

Let’s see some ID… Julian Spillane CEO / 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? Shadowing in Games Shadowing Algorithms How does it stack up Reception 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? Who is using 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? Hobbyists Independent 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 issues XNA 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 scene Important visual cues Make for interesting gameplay mechanics Adds 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 Image 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) Image Based Pros Shadow complexity α scene complexity!!! Means cube equally expensive as Marcus Fenix ™ (sorta…) Gives shadowed alpha for free Soft shadows feasible in real-time Self-shadowing is free Cons Inherently aliased / artifact’d Aliasing from texture resolution and light projection settings Artifacts from limitations of texture projection Both are mitigated through technique variations Memory intensive Memory consumption α shadow quality Dependent on texture size No unified approach Different techniques for different situations

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 variants On modern hardware, pros far outweigh cons Self-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 architecture Personal 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 Process Generating the shadow map Shadowing the scene Shader Approach Can 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 coordinates Retrieve the point as it is seen from the point of view of the light (depth map) Depth-compare with light-view map If depth value of surface is further away, it must be behind an occluder and shadowed (z-fail test)

Creating the Shadow Map Can be represented in many formats Best Choice: R32F (single channel float) Only if hardware supports… Can use ARGB, just less precision Unless interesting depth hashing with the channels… Use XNA RenderTarget2D Writeable surface, can convert to texture Easy to use

Creating the Shadow Map Matrices Need light’s view, projection matrices Choice in projection Ortho best for large, directional lights (i.e. sun) LightProj = Matrix.CreateOrthographic(w, h, n, f) w, h are width and height of view volume n, f are near and far planes of the view volume Recommend calculating these instead of arbitrary Perspective for local, spot lights LightProj = Matrix.CreatePerspectiveFOV(f, a, n, f) f: perspective FOV, a: perspective aspect ratio Implies we need light information Position, target, up vector LightView = Matrix.CreateLookAt(position, target, up)

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 independently POSITION cannot be read directly Very simple shader Don’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 compare Yet another matrix Texture bias and projection matrix Used to map position from projection space into texture space

Shadowing the Scene Depth Comparison Transform position using previous matrix Compare z-value to value stored in shadow sampler if stored depth < visible depth at that point Must 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 Steps Merging 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 technique I 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 Various Techniques 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