Real-Time Rendering of Cartoon Smoke and Clouds Morgan McGuire Andi Fein Paulo Ferreira

Gundam

Motivation Real-time cartoon rendering for: NPR Video Games Production Animation Preview Visualization Simulation and Training These often contain smoke, fluids, and other amorphous shapes…

Captain Marvel, June 1947 Outlines Cel Shading Self-Shadowing  Marvel Comics 1947

Heart of Empire Published by Dark Horse Comics 1999

SuperMan/Batman #16

Teenage Mutant Ninja Turtles Cartoon (2004)

Worms 4 (Game 2005) Pre-rendered flat billboards Excessive detail No self-shadowing

Algorithm

Simulation Particle system Forces: Size = smoke density — Buoyancy — Wind — Drag — Gravity — Vortices — Turbulence Size = smoke density ODE physics engine at 30 Hz — Compressible: 100,000 particles — Incompressible: 1,000 particles

Simulation Use Particles from the game “Avalanche”

Simulation Instead of smoke and clouds, cartoon effect applied on the avalanche of the game

Rendering steps Precomputed Nailboards Outlines Cel Shading Shadows

Steps Elements of Style Outlines Shading Self-Shadowing

Nailboards

Normal-mapped Nailboards Precompute extended billboard textures Several puff orientations for variety Each particle is assigned one variation Normal Depth Color Opacity

Preprocessing step: Nailboard Create two textures to store data such as: unit camera-space surface normal N, signed camera space depth d, diffuse color C, and coverage mask alpha.

Outlines Edges of puff are deep and black Black Nailboard Depth Shaded Color Viewer Actual Billboard Depth

Depth and Outline Pass Add d from the texture to the depth at the pixel and the depth buffer test automatically produces correct intersections.

Outlines Edges of puff are deep and black Viewer 1 Nailboard Image

Outlines Edges of puff are deep and black Depth test suppresses excessive internal contours No internal contour Viewer 2 Nailboards Image

Outlines Edges of puff are deep and black Depth test suppresses excessive internal contours Large depth discrepancy restores external contours External contour distinguishing clouds Viewer 3 Nailboards Image

Outlines Use the alpha-shapes, which are slightly larger than the normal shapes, to create the outline, a flange of dark pixels slightly behind the billboard. The outline of these flange pixels is opaque because alpha is one, colored black because the C is black, and farther from the camera than the particle itself because d is zero.

Cel Shading Ambient: Gradient as function of y Diffuse:Arbitrary function, q(N · L) Pack both into a 2D texture, like X-Toon y N · L Lambertian Selle04 Two-Tone Three-Tone q = Example

Explosion B & W Ambient q =

Explosion Color Ambient q =

Cel-Shading Pass Get N, C and alpha from the textures.

Cel-Shading Pass With ambient light color Ka, light color Kl, camera-space direction L to the light source, and diffuse color C from the texture map, the pixel at (x, y) is colored by a pixel shader using the following equation: y N · L

Shadow Volumes Compute the silhouette of the caster Extrude this to form a polyhedron bounding the volume of shadowed space Shadows are intersection between the volume and the scene

Billboard Shadows Caster is perpendicular to viewer and light Volume is a rectangular prism Four sides are parallel to the view vector Only 3 polygons to be rendered: one billboard and two shadow faces

Nailboard Shadows Apply the depth displacement to the shadow volume as well as the billboard Shadow volume visualization

Shadow Volume Visualization

Shadowing Only ~300 polygons in this scene, including shadows

Results

Train

Magic Lamp

Purple Cloud

Explosion

Varying Puff Shape

Varying Puff Shape

Varying Puff Shape

Video

Video

Mimicking Artists

Lichtenstein Algorithm Artist  Estate of Roy Lichtenstein 1963

Context

Published by Dark Horse Comics 1999 Comparison Algorithm Artist

50,000 particles 50,000 Particles on GeForce6800 50,000 particles on GeForce6800; bus limited (factor of four is possible from clever encodings)

Depth Differences Image space outlining method [Saito and Takahasi] Benefits: Rendering time independent of the scene Handles intersecting surfaces Drawbacks: Can stall graphics pipeline Assumes one style for the entire scene