NPR in Ray Tracing

Dan Maljovec

ReferenceChoudhury, A. N. and Parker, S. G. 2009. Ray tracing NPR-style feature lines. In

Proceedings of the 7th international Symposium on Non-Photorealistic Animation and Rendering (New Orleans, Louisiana, August 01 - 02, 2009). S. N. Spencer, Ed. NPAR '09. ACM, New York, NY, 5-14.

Ray Stencil

Compare:• Geometry ID (gs)

• Depth (ts)

• Normal (ns)

S

N = 1

Types of Feature Lines

Compare:• Geometry ID (gs)

• Depth (ts)

• Normal (ns)

Intersections & silhouettes

Silhouettes& self-occluding

contours

Creases

S

N = 1

Edge Strength Function

• You want the largest edge strength to be when half of your rays see something different

• Let M = # of stencil rays we are going to shoot

Different Geometry

Compare:• Geometry ID

For each stencil ray rIf(gs != gr)

m++

Radiance = lambertian * eM(m)

S

M = # of stencil rays

Edge Strength Function

N = 1

Depth Discontinuity

Compare:• Depth

For each stencil ray rIf(|ts – tr| > Td)

d++

Radiance = lambertian * eM(d)

Td = depth threshold

M = # of stencil rays

Edge Strength Function

N = 1

S

Normal Discontinuity

Compare:• Normal

For each stencil ray rIf(||ns – nr|| > TN)

Radiance = black

TN = normal threshold

S

M = # of stencil rays

Edge Strength Function

N = 1

Results

Results

Alternative Method

Do we need to shoot the ray stencil? – We already have that information from the neighboring rays

Alternative Method

Do we need to shoot the ray stencil? – We already have that information from the neighboring rays

Solution: – Shoot one ray per pixel– Pass normal, geometry, color, and depth values to shader

that computes edge strengths.

Alternative Method

Do we need to shoot the ray stencil? – We already have that information from the neighboring rays

Solution: – Shoot one ray per pixel– Pass normal, geometry, color, and depth values to shader

that computes edge strengths.

Ideally, you would want to pass the Optix buffers to a CUDA program and process in parallel– Eliminating the overhead of involving OpenGL

Alternative Method

Alternative Method

Performance

Pure Optix Method• Works well for N = 1

– (5-point stencil)

• Slows down for N > 1

• Dependent on # of rays that need stencil rays

CG Hybrid Method• Slight stutter in animation

– Copy overhead?

• Scales better when N > 1

• Independent of number of scene layout

In practice, N > 2 doesn’t look very good, witness Exhibit A:

N = 3

N = 7

Open Issue

Open Issue

No Feature LinesThrough glass