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Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

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  1. Regressions in RT Rendering: LittleBigPlanet Post Mortem Alex Evans & Anton Kirczenow Media Molecule Advances in Real-Time Rendering in 3D Graphics and Games Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  2. Constraints & Mash-ups Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  3. LBP’s Constraints • (Back in 2006) • Blank slate • no code, no tech, no existing franchise • New, Unknown & Strange platform • but a single platform! • User generated content was to be key • no pre-computation • ‘worst’ case is the typical case: untrained ‘artists’ aka users • Distinctive art style based on ‘miniature world’ • style binding content: allow LBP’s style to shine through any UGC • My technical prejudices • 1.5 programmers & 36 months Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  4. Prejudices to force decisions • We had little time to mess about, so the overriding goal was always: whatever is easiest to code & plays to our strengths • no middleware • (home-made turtles all the way down please) • one code path for all rendering of all objects • eg same path for solid, transparent, fx... • all lighting per-pixel • no per-vertex or (shudder) per-object ‘optimizations’ • playing to strength of team: I hate vertices, and I don’t have time to write those secondary paths anyway • minimum number of ‘knobs’ for the artists - but not too few • relatively non-technical art team, busy with other tasks Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  5. Prejudices into Features • lots of dynamic geometry & cloth • (we can’t pre-compute anything anyway...) • motion blur, DOF, flexible color correction • used in unusual context • unbounded numbers of dynamic lights • emphasis on many layers of texture to achieve surface look • refractive glass • correct DOF / motion-blur behind transparent objects • ability to arbitrarily cover the entire world in 'stickers' with no loss in frame-rate • fluid & cloth to drive all FX • we almost avoided writing a particle system. almost. Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  6. Unusual vertex pipe • Avoid Vertex Shader slowness on RSX by using SPU instead • Add some sexy features to take advantage of SPU: • Cloth • Soft bodies • Patch tesselation • Point deformers Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  7. Anton shows all Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  8. Vertex pipeline: iterate spring/collision constraints Verlet Integrate Collision with world Input verts Find cluster matrix Rigid Skinning Input bones X X compute N, T Stitch seams Morph targets Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  9. Vertex motion • SPU’s job is to create world-space ‘polygon soup’ per frame • P & N are triple buffered, allowing simple Verlet physics • Pnew = 2P-drag*(P-Pold)-Pold+A • drag is set by collision routine • Meshes automatically turned into springs across grids of quads, with artist weighting in vertex colors • http://www.gamasutra.com/features/20000327/lander_02.htm • Vertex lerp’ed back to skinned pos • using artist defined weights Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  10. Collision Detection • SPU does collision against: • extruded 2D convex shapes (world) • swept ellipsoids (character) • vertex transformed into old & new ellipsoid position • raycast sphere vs ellipse in stationary ‘ellipse space’ raycast Pold Pold old Pnew new transform into ellipse space Pnew Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  11. Clusters • Lovely mesh-less algorithm: least-squares fit a new rigid matrix to a deformed cloud of vertices • remove center of mass - that gives translational component • matrix sum of dyadic of (P-COM) x (Prest-COMrest) • multiply by inverse of pre-computed matrix in rest pose • orthogonalize resulting best-fit matrix to give affine transform, and optionally preserve volume by normalizing • blend with original rigid body matrices to make as squishy/rigid as you want • more detail: Meshless Deformations Based on Shape Matching - Muller et al, SIGGRAPH 05 Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  12. flexibility of vertex pipeline • we ended up using that pipeline not just for meshes, decorations, cloth • but also SFX like confetti, dust, embers,... • allowing us to almost entirely avoid an explicit ‘particle system’ • with one exception: jet-pack smoke was a simple point-sprite based backend on the integrated/collided vertices... • rendered 1/4 screen-res for fill-rate reasons • each point sprite has a 2D gradient across it - gives remarkable sense of ‘3d-ness’ • every pixel samples the sun’s shadowmap to give volumetric shadows in the smoke Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  13. On to pixels: graph based? Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  14. fixed everything aint so bad? • the graph editor was never used by artists • they were happy to composite LOTS of texture layers at different scales • and left me to hard-code a single ‘uber-shader’ that handled everything else. • that BRDF is entirely ‘cooked up’ based on live shader editing and my eyeballs. approximately: • 2 strong rim-light with color set using artist controlled hemisphere based on (V.N)^2 with bias & scale • the 2 rim lights are lerp’ed between based on sunlight shadow • diffuse a mixture of N.L and (N.L)^2 • ambient a simple 2 color hemisphere • specular simply (R.L)^22 Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  15. shadowing • one sun light & arbitrary numbers of ‘sprite lights’ • world has shallow depth, so we got away with very simple shadowmapping • Initially TSM eg http://www.comp.nus.edu.sg/~tants/tsm/tsm.pdf • Finally, just orthogonal single 768x768 shadowmap! • very tightly tuned frustum of shadow casters • soft shadows via variance shadow maps + AO contact shadows • just the vanilla form of VSM: after Donnelly et al • lots of light leaking • used 16FP render targets with two copies of X, X^2 in R/G and B/A • second copy scaled by 32. recombined in shader -> less boiling Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  16. ambient occlusion for contacts • the character’s feet were small & felt floaty... • so we added an AO ‘light’ that analytically computed the occlusion of the character modeled as 5 spheres (2 feet, 2 legs, 1 groin) • see Inigo Quilez’ awesome site for derivation • http://iquilezles.org/www/articles/sphereao/sphereao.htm AO= (N.D) (r2/D.D) Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  17. Spritelights take 1 • as presented at SIGGRAPH 06, I started by slicing the view frustum and rendering light volumes directly into those slices (as a volume texture) • A sort of ‘irradiance volume’ - but with no directional component. (‘0th order SH’) • Use grad of irradiance in direction of normal to approximate N.L type lighting. Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  18. Sprite Lights Take 1 Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  19. Sprite Lights Take 1 Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  20. Sprite lights take 2: Defer? • Lay down a Z/N pre-pass at 2X MSAA ...then light as if it were grey plastic (specular -> alpha) • ...and do sprite lights in the traditional ‘deferred’ way • BUT! no G-buffer, no material properties, no MRTs: just Z, N in Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  21. Sprite lights take 2: Defer? • then re-render scene ‘forwards’, sampling ‘L’ into BRDF Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  22. sprite lights take 2: god rays • now we’re doing sprite lights properly, we might as well add some features: god rays! Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  23. god rays implementation • completely 2d screenspace effect • each light volume is rendered to an off-screen surface, one channel per light (in batches of 4) • pixel shader integrates light scattering along eye ray to first solid object (z buffer distance) • 3 pass ‘smear shader’ then smears • dark regions with MIN blending • each pass doubles distance of smear • similar to masa’s ‘rthdribl’ lens-flare streak shaders • http://www.daionet.gr.jp/~masa/rthdribl/ Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  24. god rays implementation • completely 2d screenspace effect • each light volume is rendered to an off-screen surface, one channel per light (in batches of 4) • pixel shader integrates light scattering along eye ray to first solid object (z buffer distance) • 3 pass ‘smear shader’ then smears • dark regions with MIN blending • C = sample(u,v) • for (i=0;i<5;i++) • u=(u-cu)*k+cu; v=(v-cv)*k+cv; • C=min(C, sample(u,v) + i/5.f); • (where k<1 sets smear radius, cu,cv is the screenspace position of the lightsource) Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  25. 2 layer transparency • deferred shading with alpha. what? Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  26. 2 Layer transparency • Exploiting the fact that we have 2X MSAA hardware • and we can ‘cast’ the ‘L’ buffer to be 2560x720p non MSAA • deferred lighting done at full 2560x720p resolution • allows us two independent Z values per final pixel • PS3 you have to resolve your 2560x720p into 1280x720 by hand. turn this into a virtue: • trade transparency for MSAA in different regions of the screen • custom resolve shader is quite tricky: uses alpha channel to control blend between the two samples - compositing either as ‘A over B’ or ‘(A+B)/2’ Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  27. 2 layer transparency & DOF &... Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  28. 2 is not a big number • leads to odd effects when you get more than 2 layers: • users actively are exploiting this to great effect! Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  29. 2 layers: Refraction • Glass shader only WRITES to even pixels in X (‘alpha layer’) • it only READS from odd pixels in X (‘solid layer’), sampling from it’s own screenspace pos P perturbed by the glass surface normal • k is the ‘index of refraction’ • P.xy+=k*N.xy • P.x-=frac(P.x)+0.5; <--- MAGIC! • Cout=tex2D(screen,P) • code above relies on using UV’s measured in pixels, not 0-1 Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  30. careful MB/DOF calculation • Motion-blur & DOF were vital to the miniature look • willing to spend lots of GPU cycles on this • however it still runs at half res: • PASS 0: downsample 2x in Y, 4x in X • blend 2 layers correctly according to degree to which each sample needs blurring. Alpha channel = coarse z buffer Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  31. careful MB/DOF calculation • PASS 1 & 2: 7 tap ‘classic’ DOF with variable circle of confusion, checking alpha channel for z conflicts • samples distorted along direction of screenspace motion • motion blur comes ‘for free’ using same sample points as DOF stationary pixel fast moving pixel • Final MSAA Resolve: • reads 1/2 res blurred image, and composites it over the full-res screen; • however it does this BEFORE blending the two alpha layers, and mixes the blurred image into each layer only according to that layer’s need. • This conditional blend happens at full res, so no need for bilateral filter et al Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  32. first stab at motion blur • we have a screenspace motion vector (x,y) for each pixel • however, boundaries of objects don’t look nice: • Initially, tried to render geometry extruded along direction of motion. however, fast rotating objects broke this: Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  33. improving motion blur • how to improve ‘leading’ and ‘trailing’ edges? • insight: use 2 frames of info - this frame (1) and old frame (0) • blur C1 with V0, and C0 with V1 (twice as many taps) • use Z-in-alpha to help composite samples together Frame 0 Frame 1 these regions get frame 0 background blurred by frame 1’s velocity while these regions get fr.1 background, with fr.0 velocity Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  34. improving motion blur: results Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  35. fluids: how? • Stam stable fluids, 2D water-grid PDE, or SPH? • Anton will take them all, thanks very much! • Stam-style stable fluids can be seen all over the place for smoke, fire & dissolve fx • Water Grid used for ‘sea level’ • Rigid body coupling to grid, generates... • ... splash particles, combining SPH water-volume preserving forces with existing vertex pipeline (verlet, collisions etc) Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  36. water grid • shallow water simulation - 256x128 on SPU • fixed grid only contains visible play area, and is post-perspective distortion to give even tesselation • The method is as in Claes Johanson’s MSc thesis, "Real Time Water Rendering: Introducing the projected grid concept” but tweaked • to keep the grid evenly spaced in X when the camera is very near the surface and looking forward. • V advects quantities with bicubic filtering • linear not good enough - leads to ‘viscous’ look • method is similar to Chapter 11 of ‘Fluid Simulation for Computer Graphics by Robert Bridson 2008’ Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  37. Water rendering • The water surface normals are generated by bicubic filtering a texture of the height field, linear isn’t good enough: • Refraction is just cheesy resampling of screen buffer with perturbed UVs by the surface normal • additional z check to ensure refracted sample is actually below surface: Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  38. Water rendering Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  39. water grid / rigid body coupling • For rigid body coupling: • find rigid bodies intersecting the water surface, take their AABBs and compute the change of "submerged-ness" in the water. • This is diffused and added to the water depth to create wakes and ripples. • The method is as per [Thurey et. al."Real Time Breaking Waves for Shallow Water Simulations" 2007] , • but with Stam style stable diffusion so it doesn't blow up so much. • We also add some of the velocities of the immersed objects to the water grid Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  40. Grid -> SPH: Splashes! • We generate splashes wherever the water height changes rapidly • emit particles where these both hold: • change in water height > threshold • gradient(water height) dot water velocity > 0 • Splash particles (and bubbles) also add depth to the water grid when they collide with it to cause secondary ripples. • splash particles are in an SPH simulation • as per Muller et. al. "Particle based Fluid Simulation for Interactive Applications" Eurographics 2003 • with a novel collision detection scheme that is more SPU friendly • more details in the course notes! Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  41. splat based splash rendering • 2 pass rendering: • 1st pass sorts coarse quads front-to-back, with UNDER compositing of N and alpha • this ensures isolated particles look sharp, while still blending adjacent particles smoothly • each particle is elongated along its direction of motion • particles near the water surface stretch to simulate joining • 2nd pass outputs color • uses stencil buffer to avoid overdraw • uses similar shader as the water surface to provide consistency Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  42. Incoming light on regular grid small area compared to grid = bright large area compared to grid = dark underwater & caustics! • inspired by Pixar’s ‘Finding Nemo’ DVD extras & • Advanced Animation and Rendering Techniques by Alan Watt & Mark Watt, 1992, Ch 10.1 • intersect refracted grid of sun-rays with fixed ground-plane beneath the water surface Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  43. underwater & caustics! • inspired by Pixar’s ‘Finding Nemo’ DVD extras & • Advanced Animation and Rendering Techniques by Alan Watt & Mark Watt, 1992, Ch 10.1 • simply intersect refracted grid of sun-rays with fixed ground-plane beneath the water surface • on SPU, for each vertex, compute change in area of surrounding refracted quads vs un-refracted quads • output as a gouraud shaded mesh, rendered by GPU • blend as MAX rather than +, to keep dynamic range down Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  44. Refraction • a 1024x512 caustic map renders in only 0.6ms! Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  45. underwater • when rendering main scene, project caustic texture through world Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  46. stable fluids • 3 layers of 256x128 2D fluid, used for smoke & dissolve FX • borrowing tricks from Mark Harris’ GPU implementation (GPU Gems) • GPU re-samples screenspace z & v buffers to initialize boundary conditions • fluid only run on 3 world aligned slices, that scroll with the camera • GPU copies from the backbuffer into the fluid buffer to cause objects to ‘dissolve’ • SPU outputs list of point sprites on a grid to avoid wasting fill rate where there is no fluid Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  47. breathe.... • ...and be thankful that we didn’t go through all the other fun stuff, like virtual texturing for stickering, or procedural mesh generation on the SPU... Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  48. remind me what that was about? • the key here is that many disparate techniques can be blended to create something unique looking • it’s valid to treat dev-time as a primary constraint • for LBP, we were able to trade accuracy & generality for ‘good enough’ approximations... • the devil joy is in the details! • the SPU freed us from worrying about casting every problem as a VS/PS stream processing ‘DX9’ combination • which ironically unlocked a lot of fun pre-GPU literature • avoiding special case code paths kept the complexity down and the code coverage good... • ...making it cheap to re-visit major decisions, like ‘forward’ vs ‘deferred’, and re-implement features several times. Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  49. conclusion • in other words, • have fun mixing up and blending as many ideas as you think will help you achieve the visual look you’re after • (or something) Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)

  50. thankyou! Advances in Real-Time Rendering in 3D Graphics and Games New Orleans, LA (August 2009)