SLIDE 1
Advanced Computer Graphics CS 563: Non‐Photorealistic Rendering Frederik Clinckemaillie
Computer Science Dept. Worcester Polytechnic Institute (WPI)
SLIDE 2 Uses of Non‐Photorealistic R d i Rendering
Different styles of Non Photorealistic Different styles of Non‐Photorealistic
Rendering(NPR) are used for different reasons
Displaying information Displaying information Creating emotion Imitate art style Imitate art style
SLIDE 3
SLIDE 4 T Sh di Toon Shading
Cartoon‐like rending effects Silhouette Edges Simple Shading:
Simple Shading:
Solid shading Two‐tone shading Specular/Diffuse three‐tone shading
SLIDE 5 C l Sh di Cel Shading
Solid Shading is trivial Solid Shading is trivial Two‐Tone Shading
l l h d ff h d d d ( * l) f
Calculate the diffuse shading dot product (n * l) for
each vertex
If n * l < 0 surface is facing away If n l < 0, surface is facing away
Use values on a 1‐D texture map Implemented on CPU or as Vertex Shader Implemented on CPU or as Vertex Shader Tone Mapping
SLIDE 6 Additional Work in Cel Shading
Card and Mitchell [2002] Card and Mitchell [2002]
Determined how to perform algorithm on GPU
effectively effectively
Barla et al. [2001]
Added view dependent effects by using 2 D maps Added view‐dependent effects by using 2‐D maps
Rusinkiewicz et al. [2006]
P d Alt t h di d l ith hi h t t
Proposed Alternate shading model with high‐contrast
by adjusting effective light position
SLIDE 7 C l Sh di ( t ) Cel Shading (cont.)
Specular/Diffuse three tone shading Specular/Diffuse three‐tone shading
Calculate the specular shading dot product (L * R) for
each vertex each vertex
Map to 1‐D texture map
SLIDE 8 Silh tt Ed R d i Silhouette Edge Rendering
Adds silhouettes to emphasize edges Adds silhouettes to emphasize edges Different categories of algorithms:
S f A l
Surface Angle Procedural Geometry
Image Processing
Image Processing Vector Edge Detection Hybrid Hybrid
SLIDE 9 T f Ed Types of Edges:
Boundary edge Boundary edge
One not shared by two polygons
C d
Crease edge:
Angle is greater than 60 degrees
l d
Material edge
One where polygons are different materials
Silhouette edge
Two neighboring triangles face different directions as
d compared to eye vector
SLIDE 10
S f A l Silh tti Surface Angle Silhouetting
Using the dot product of the viewpoint and the Using the dot product of the viewpoint and the
surface normal R lt d t i l
Results are mapped to circle map Results close to 0 are near
Silhouette edge
Does not work for with
Surfaces with hard edges
No vertex interpolation
p
SLIDE 11 P d l G t Silh tti Procedural Geometry Silhouetting
Render front faces normally render the edges of Render front faces normally, render the edges of
the backfaces behind front faces Z bi th d
Z‐bias method
Move backfaces closer so that edges become in front
Ed thi k i t t t
Edges thickness is not constant
SLIDE 12
P d l G t Silh tti Procedural Geometry Silhouetting
Backface triangles are fattened Backface triangles are fattened. Slope of triangle and distance
from viewer determine fattening
Corners are cut of to avoid
elongated corners
SLIDE 13
R lt Results
SLIDE 14
H l M th d Halo Method
Move the backfaces outwards by shifting their Move the backfaces outwards by shifting their
vertices along the shared vertex normals C t h l b hi d f t f
Creates a halo behind front faces
SLIDE 15
Li it ti Limitations
Fattening cannot work on curved surfaces Fattening cannot work on curved surfaces Shell techniques work on curved surfaces only if
th f t ti b di l d the surface representation can be displaced along surface normals Z bi k ith ll d f
Z‐bias works with all curved surfaces Little control over edge appearance Semitransparent surfaces are difficult to render Edges look poor without antialiasing
SLIDE 16
Silh tti b I P i Silhouetting by Image Processing
Entirely based on data stored in buffers Entirely based on data stored in buffers No modifications on the geometry of the scene Look for silhouette edges by searching for z‐
buffer discontinuities
SLIDE 17 Silh tt Ed R d i Silhouette Edge Rendering
Process: Process:
Use Vertex shaders to render the world space normals
and z‐depth to a texture and z depth to a texture
Normals are colors, z‐depths are alpha channels Implement a Sobel edge detection filter
p e e t a Sobe edge detect o te
Benefits
Handles all primitives Handles all primitives Meshes do not need to be connected or consistent
Flaws Flaws
Edges might not be found if discontinuities are small
SLIDE 18
SLIDE 19 Silh tt Ed D t ti Silhouette Edge Detection
Other methods have weaknesses: Other methods have weaknesses:
Often need two passes to render the silhouette No control on edge rendering style No control on edge rendering style
Detect silhouette edges and render them directly
Ob i fi l h li d d
Obtain more fine control on how lines are rendered Edges become independent of the model
A d i d fi d b
An edge is defined by:
(n0 *v > 0) != (n1 * v > 0)
SLIDE 20 Fi di Silh tt Ed Finding Silhouette Edges
Standard: Loop through list of edges Standard: Loop through list of edges Improving efficiency:
l d l l
Removing triangles inside planar polygons Storing dot products for faces
I i ilh il i
In static scenes, preserve silhouettes until view moves
a certain distance
SLIDE 21
H b id Silh tti Hybrid Silhouetting
Northrup and Markosian
p
Find a list of silhouette edges. Render all triangles and silhouette edges Render all triangles and silhouette edges,
assigning each a different ID number(color)
ID buffer is read back and visible silhouette edges ID buffer is read back and visible silhouette edges
are determined from it
Visible segments are then checked for overlaps Visible segments are then checked for overlaps
and linked together
Stylized strokes are added on path Stylized strokes are added on path
SLIDE 22
SLIDE 23 References
T Ak i M ll E i H i d N H
Tomas Akenine‐Moller, Eric Haines, and Naty Ho
- man. Real‐Time Rendering, Third edition. Wellesley, MA: A K Peters, Ltd.,
2008.
