Non-Photorealistic Rendering FORMS

Model dependent Threshold dependent View dependent Outline form of the object Interior form of the object Boundary Crease (+ other discontinuities) Crease (+ other discontinuities) Silhouette X X

Object Space Involve computations in 3D and produce a list of silhouette edges or curves for a given viewpoint 1. Brute Force 2. Edge Buffer 3. Probabilistic 4. Gauss Map Arc Hierarchy 5. Normal Cone Hierarchy 6. Implicit Surfaces 7. NURBS Surfaces Image Space involve image processing techniques 1. Two Pass Methods 2. Environment Map 3. One Pass Method 4. Model Augmentation 5. Depth Discontinuity Methods Silhouette Algorithms

Object Space Involve computations in 3D and produce a list of silhouette edges or curves for a given viewpoint 1. Brute Force 2. Edge Buffer 3. Probabilistic 4. Gauss Map Arc Hierarchy 5. Normal Cone Hierarchy 6. Implicit Surfaces 7. NURBS Surfaces Image Space involve image processing techniques 1. Two Pass Methods 2. Environment Map 3. One Pass Method 4. Model Augmentation 5. Depth Discontinuity Methods Silhouette Algorithms

Image Space  1. Two Pass Methods Back facing polygons are rendered first with either their depth decreased or their field of view narrowed. Front facing polygons are then rendered on top Raskar and Cohen Image precision silhouette edges 1999 ACM Symposium on Interactive 3D Graphics

Object Space Involve computations in 3D and produce a list of silhouette edges or curves for a given viewpoint 1. Brute Force 2. Edge Buffer 3. Probabilistic 4. Gauss Map Arc Hierarchy 5. Normal Cone Hierarchy 6. Implicit Surfaces 7. NURBS Surfaces Image Space involve image processing techniques 1. Two Pass Methods 2. Environment Map 3. One Pass Method 4. Model Augmentation 5. Depth Discontinuity Methods Silhouette Algorithms

Image Space  2. Environment Map Silhouette lines are added to a shading environment map as a preprocess Gooch et al Interactive technical illustration 1999 ACM Symposium on Interactive 3D Graphics

For each face of the cube, render the world from the center of the object with the cube face as the image plane. For each face of the cube, render the world from the center of the object with the cube face as the image plane. Or, take 6 photos of a real environment with a camera in the object’s position Or, take 6 photos of a real environment with a camera in the object’s position Cube Maps

Cubic Map Examples Remember The Abyss and Terminator 2?

Object Space Involve computations in 3D and produce a list of silhouette edges or curves for a given viewpoint 1. Brute Force 2. Edge Buffer 3. Probabilistic 4. Gauss Map Arc Hierarchy 5. Normal Cone Hierarchy 6. Implicit Surfaces 7. NURBS Surfaces Image Space involve image processing techniques 1. Two Pass Methods 2. Environment Map 3. One Pass Method 4. Model Augmentation 5. Depth Discontinuity Methods Silhouette Algorithms

Image Space  3. One Pass Method During a preprocess phase two cube maps are created, one of surface normals and one of eye linear maps. At runtime per pixel dot products are computed yielding silhouettes. Everitt One-pass silhouette rendering with geforce and geforce2 NVIDIA Corporation White Paper.

Image Space  3. One Pass Method V dot N to index into a 1D texture map with a steep ramp from white (for front faces) to black (for back faces). V dot N to index into a 1D texture map with a steep ramp from white (for front faces) to black (for back faces). Dot product per pixel (as opposed to per vertex) by using registers to combine 2 automatically generated texture cube maps associated with the surface normal and eye vectors respectively. Dot product per pixel (as opposed to per vertex) by using registers to combine 2 automatically generated texture cube maps associated with the surface normal and eye vectors respectively.

Image Space  3. One Pass Method If this dot product is used to index a 1D texture map, some front facing areas tend to get darkened. If this dot product is used to index a 1D texture map, some front facing areas tend to get darkened. This is because near the silhouette, the surface normal is nearly perpendicular to the eye vector and the dot product is close to zero. This is because near the silhouette, the surface normal is nearly perpendicular to the eye vector and the dot product is close to zero. Just steepening the color ramp, to remove some of this shading, outputs silhouette outlines of varying strength and quality. Just steepening the color ramp, to remove some of this shading, outputs silhouette outlines of varying strength and quality.

Object Space Involve computations in 3D and produce a list of silhouette edges or curves for a given viewpoint 1. Brute Force 2. Edge Buffer 3. Probabilistic 4. Gauss Map Arc Hierarchy 5. Normal Cone Hierarchy 6. Implicit Surfaces 7. NURBS Surfaces Image Space involve image processing techniques 1. Two Pass Methods 2. Environment Map 3. One Pass Method 4. Model Augmentation 5. Depth Discontinuity Methods Silhouette Algorithms

Image Space  4. Model Augmentation A second polygonal model is created with degenerate quads, the quads have no height, positioned normal to the surface at each edge of the model. At runtime each edge is checked to see if it is a silhouette. If an edge is a silhouette the corresponding quad is made non-degenerate i.e. the quad is given height Raskar Hardware support for non-photorealistic rendering SIGGRAPH/Eurographics Workshop on Graphics Hardware 2001 Mitchell Real-time non-photorealistic rendering ATI Corporation White Paper

Can we design hardware to support such API functionality? Can we render special features in a scene specified by a sequence of individually defined polygons?

Silhouettes. Back-facing polygons in red before (left) and after enlargement (right).

The effective projection (green) in the image plane of the polygon enlargement (black). The thickness of the rendered feature is controlled by changing the width of the enlargement. The enlargement is proportional to Z (V.C) / (V.N)

Object Space Involve computations in 3D and produce a list of silhouette edges or curves for a given viewpoint 1. Brute Force 2. Edge Buffer 3. Probabilistic 4. Gauss Map Arc Hierarchy 5. Normal Cone Hierarchy 6. Implicit Surfaces 7. NURBS Surfaces Image Space involve image processing techniques 1. Two Pass Methods 2. Environment Map 3. One Pass Method 4. Model Augmentation 5. Depth Discontinuity Methods Silhouette Algorithms

Image Space  5. Depth Discontinuity Methods Pixel depth is compared on a per pixel basis if (depth difference between two pixels) > user defined tolerance then one of the pixels is colored black Curtis Loose and sketchy animation SIGGRAPH 1998 Course Notes Mitchell Real-time image-space outlining for non-photorealistic rendering SIGGRAPH 2002 sketch paper ATI Corporation White Paper.