Simulating Wax Crayons Dave Rudolf David Mould Eric Neufeld Depart. of Computer Science, U. of Saskatchewan

Overview Previous Work Goals Representation of Paper and Wax Interactions between Wax and Paper Rendering Results

Previous Work Adobe Photoshop TM Conté Crayon Filter

Previous Work Corel Painter TM Grainy Crayon Brush

Previous Work Viscous Paint Model (Baxter et al.)

Goals Improve realism Consider individual strokes Suitable paper texture Approximate mechanics of crayons Simulate optical properties of wax Efficient Rendering Not too insanely realistic

Representation of Paper Height-Mapped Texture Texture Synthesis Other sources (scans of Real Paper)

Convolution Mask Somewhat Expensive Can be Pre-computed Van Wilk’s Lunar Texture

Other Textures Make no assumptions about texture

Representation of a Crayon 2D Mask Height Area

Compression of Wax

Assume that wax compresses linearly Hooke’s Law: F = Y  L A / L 0 Can’t solve directly Crayon & Paper profiles are not predictable Must binary-search for force ( F )

Compression of Wax

Wax Deposition

Determined by: Friction Force of Crayon Direction of Stroke Whether there is existing wax F f =  F n =  N cos(N, F c )

Wax Smearing

Depends on texture and crayon force Flow Smearing Independent of crayon heading S f xy =   z Directional Smearing Depends on crayon heading S d xy =  cos( (x, y), V )

Deposition and Smearing Without Smearing With Smearing

Rendering Need a good colour model Wax scatters and absorbs light Additive/Subtractive models are insufficient Kubelka-Monk Colour Model Approximates of Translucent Pigments Considers transmittance, scattering, and interference of pigment layers.

Approximated Kubelka-Monk Real Crayon Generated Crayon

Results Not Quite Interactive Between 0.3 and 2.0 seconds per frame Updating model takes the most time Not Quite Realistic No wax flaking, carving, transporting

Results

Questions ?