03/30/05© 2005 university of wisconsin npr today “stylized rendering techniques for scalable...

03/30/05 © 2005 University of Wisconsin

NPR Today

• “Stylized Rendering Techniques for Scalable Real-Time 3D Animation”, Adam Lake, Carl Marshall, Mark Harris and Marc Blackstein, NPAR 2001

• "Computer-Generated Pen-and-Ink Illustration of Trees", Oliver Deussen and Thomas Strothotte, SIGGRAPH 2000

• "Sketchy Drawings", Marc Nienhaus and Jurgen Dollner, AfriGraph 2004


Stylized Rendering Techniques for Scalable Real-Time 3D Animation

Paper Overview

Leo Chao


Paper Information

• Published– NPAR 2001

• (Non-Photorealistic Animation and Rendering)– Authors

• Adam Lake• Carl Marshall• Mark Harris• Marc Blackstein

• Graphics Algorithms and 3D Technologies Group– Intel Architecture Labs

• UNC – Chapel Hill


Paper Information

• Prior Work– No novel techniques, adapts from prior works to

hardware

• Motion Lines– Maic Masuch. “Speedlines: Depicting Motion in Motionless Pictures.” ACM SIGGRAPH 99

Technical sketch.– Siu Chi Hsu and Irene H. H. Lee. "Drawing and Animation using Skeletal Strokes". In

Proceedings of ACM SIGGRAPH 94, pages 109-118. 1994.

• Silhouette Edges– Lee Markosian, Michael Kowalski, Samuel Trychi, Lubomir Bourdev, Daniel Goldstein, and

John Hughes. Real-Time Nonphotorealistic Rendering. In Proceedings of ACM SIGGRAPH 97, pages 113-122. 1997.

• Cartoon Shading– Philippe Decaudin. Rendu de scenes 3D imitant le style dessin anime. Rapport de Recherche

2919, Institut National de Recherche en Informatique et en Automatique. 1996.


Contributions

• Hardware Implementation of NPR Techniques– Stylistic Shading

• Cartoon Shading• Pencil Sketch Shading

– Stylistic Inking• Silhouette Edge Detection• Stylized Silhouette Rendering

– Motion Lines


Stylistic Shading

• Fundamentals– Preprocess colour textures– Render as normal– Except:

• Modify lighting/shading calculations• May require texture coordinate processing

• “Painter” – Modified Shading Step– Cartoon Shading– Pencil Sketch Shading


Stylistic Shading

• Cartoon Shading– Reducing detail aids comprehension– No Smooth Shading

• Color Reduction• Hard Transitions

• Variants Simple


Stylistic Shading


ALGORITHM Cartoon Shade

Preprocess:1. Calculate the illuminated diffuse color for each material:

2. Calculate the shadowed diffuse color:

3. For each material, create a texture with two colors- u=1 is coloured Ci

- u=0 is coloured Cs

Stylistic Shading

mlmlmgi ddaaaaC

mlmgs aaaaC

Ci Vertex Color Cs Shadow Diffuse Color

al Light Ambient dl Light Diffuse

am Material Ambient dm Material Diffuse

ag Coefficient of Global Ambient Lighting


ALGORITHM Cartoon Shade

Runtime:1. Calculate the one-dimensional texture coordinate at each

vertex

2. Render the model using a standard graphics API - Lighting disabled- Enable texture mapping

Stylistic Shading

}0,{ nLMax

L normalized light vector

n unit surface normal


Stylistic Shading

• Pencil Sketch Shading– Extends cartoon shading to 2D textures– Extends cartoon shading to multiple textures– Multiple textures to represent light density

– Associate texture coordinates• Project texture from the viewport onto the model


Stylistic Shading


Stylistic Inking

• Fundamentals– Identify “important” edges

• Silhouette• Border• Crease

– Modify drawing of these edges

• “Inker” – Modified Line Drawing– Visually important edge detection– Stylized edge drawing


Stylized Inking

• Visually important edge detection– Silhouettes

• Front-Face and Back-Face share an edge

– Borders• Edge shared by two materials• Unshared polygon edge

– Creases• Large dihedral angle between polygons


Stylized Inking


Stylized InkingALGORITHM SEDPreprocess:1. Create edge-list structure with Edge Type flags

a. Create a unique edge list using a hash table. b. Edge Hash Function: sum the two vertex

indicesc. Border Flag

- Edges with only one neighboring face- Two neighboring faces with different

materialsd. Set crease flags

- Non-deformable geometry- Dihedral angle of two faces > threshold


Stylized InkingALGORITHM SEDRuntime:1. For deformable meshes:

a. Calculate face normalsb. Detect crease edges from face normalsc. Set Crease Edge flags

2. Detect silhouette edges a.


Stylized Inking

• Stylized “Important” Lines– Emphasis on these lines– Option 1: Straight Line

• Fast/Easy

– Option 2: Texture Maps• “Artistic” Style• Curvature driven “strokes”• Involves a graph driven search• This is a HACK


Stylized Inking


Motion Lines

• Gives the appearance of motion

• For fast moving objects

• Simple algorithm

• Not much overhead– One circular buffer to maintain position data


Performance

• Claim: Maintain Real-Time Speeds– 30 fps, in 2001

• Tests:– Integrated with a level of detail scheme– Altered model complexity to maintain FPS


Performance


Future Work

• Inker– Better “artistic” lines

• Painter– Pencil sketch “doesn’t look right”– Follow surface gradient perhaps

• Incorporate other NPR techniques– Those used by artists– Doesn’t say which would amenable


Questions?

• ques·tion (kw s ch n)n. – An expression of inquiry that invites or calls

for a reply.

• heck·le (h k l)tr.v. – To try to embarrass and annoy (someone

speaking or performing in public) by questions, gibes, or objections


“Computer-Generated Pen-and-Ink Illustration of Trees”

(SIGGRAPH 2000)

Oliver Deussen

Thomas Strothotte


Motivation

• The paper presents a method for creating pen-and-ink-like illustrations from detailed geometric models.

• Motivated by the need for illustrative representations of plant life in architectural and landscape renderings.

• We want to be able to produce images similar to this:


Related Work

• “Cartoon Tree” – Alvay Ray Smith– Use small disks to represent bunches of leaves

• Reeves and Blau– Use structured particle system to represent realistic trees.

• Kowalski et al. – Two step process

1. Render shaded primitives that approximate foliage

2. Overlay graftals on rendered image using Salisbury’s line drawing method


Related Work Examples

• Kowalski’s Graftal Trees:

• Reeves “particle forest”:• Smith’s “cartoon tree”:


Traditional Illustration of Trees

• The Skeleton is drawn up to the second level of branching.

• Foliage is broken up into three parts:

1. The directly lit canopy – represented by outlines and scarce inner detail

2. The half shadow – greater detail provides an overall darker grey level

3. The shaded area – achieved by thicker lines or drawing leaves in black

• Artists will general use a combination of these to draw the leaves. Rarely do they use all three.


Drawing the Skeleton

• The tree skeleton is represented by generalized cylinders.

• Analytical Silhouette Algorithms

– Markosian et al.

– Rakar and Cohen

– Depth Differences

• The skeleton is shaded with crosshatch marks. Marks could be placed using the algorithm outlined by Salisbury et al. – The authors actually use a variant of Floyd-Steinberg

diffusion that places strokes when the accumulated grey value is above a threshold.


Drawing the Foliage

• The xfrog modeling system can generate models with 200,000 leaves on a tree. We want to pair this down to 10,000 or so.

• Leaves are represented abstractly by particles: a location and an orientation vector.

• The goal is to illustrate groups of with a set of strokes.

• Idea: Render particles as primitives and use depth buffer information to determine which leaves are spatially close and outline them.


Depth Differences

• Initially, each particle is rendered with a view-facing primitive, such as a disk.

• Basic Algorithm:

– For each pixel compute the maximum positive difference between it and its neighbors

• If the distance is above the given threshold, mark as an edge.


Depth Differences Continued…

• The threshold and primitive size can be varied to achieve different effects:

Primitive size: 0.15 Primitive size: 0.70

Threshold: 1000 Threshold: 2000

• The threshold can be varied using other buffers, achieving greater detail in areas that results in a shadowing effect:


Abstract Drawing Primitives

• More realistic leaves can be achieved by using the particles’ orientation vectors to interpolate between a set of polygons that represent leaves from different views


Level-of-Abstraction

• Two Approaches1. Convert raw z-buffer values from non-linear, projected values to linear values

using formula below and vary threshold according to depth:d: depth value between 0 and 1

d0: minimal depth value (e.g. 0)

d1: maximal depth value (e.g. 65535)

z0: depth value of near clipping plane

z1: depth value of far clipping plane

2. Take advantage of the non-linearity to automatically generate more abstract representations.– Geometry farther away will be

pushed into fewer values in the z-buffer.

– A tree in the background will represented by fewer strokes than the same tree in the foreground.


Level of Abstraction Continued…

• The level-of-abstraction can be furthered by scaling the primitives so that the amount of screen space they occupy is independent of their depth.

r: scaling factor

d: desired screen space

s: actual screen space

No Scaling Scaled


Software Implementation

1. Calculate depth differences and mark pixels above threshold.

• In the interactive mode this bitmap is directly used and blended with other geometry.

2. Generate shadows in software. Large number of isolated surfaces precludes hardware implementation.

3. Vectorize bitmap using least squares fitting or primitive identification method.

• Draw lines using spline interpolation

• Apply line styles


Results


Future Work

• Apply cross-hatch shading to the leaves

• Create a continuous level-of-detail algorithm that stays performance while getting rid of artifacts that come with discrete representations.

• Apply new styles and color for cartoons.

• Questions?


Sketchy Drawings

Marc Niehaus Jürgen DöllnerUniversity of Potsdam University of

Potsdam


Sketchy Drawings

• Sketchiness shows unfinished state

• Useful for architectural and product design


Previous Work

• G-Buffers (Saito & Takahashi 1990)

• Edge enhancement (Nienhaus and Döllner 2003)

• Loose and Sketchy filter (Curtis 1998)

• Artistic Strokes (Northrup & Markosian 2000)


Algorithm Overview

• Obtain edge and shading data

• Perturb this data to get sketchy look

• Problem: It needs to be done in real time


G-Buffers

• Need to access in real time

• Depth/Color/Alpha/Stencil Buffer on graphics card

• Textures

• Current hardware allows multiple textures

• Manipulate with fragment and pixel shaders


Edge Map

• Need to find edges for most NPR algorithms

• Store three types of edges– Silhouette

– Crease

– Border

• See Edge Enhancement (Nienhaus & Döllner 2003 )for more details


Shade Map

• Render unlit 3D geometry to capture flat colors

• Store results in screen aligned texture instead of color buffer


Adding Uncertainty

• Apply uncertainty function to edge and shade maps

• Store function in texture

• Use Perlin function to get frame to frame consistency

• Multiply uncertainty values with 2x2 matrix

• Use different matrix for edge and shade maps to get a more sketchy feel


End Result

• Use shaders to combine results into texture

• Render texture on screen-aligned quad


Style Variations

• Render edges multiple times

• Roughen edge and surface colors


Issues

• Depth Information

• Shower-Door Effect


Updating Depth Info

• Depth info is lost when rendering 3D geometry to 2D textures

• Solution: Render geometry into a depth map

• Render perturbed depth sprite – use shaders to update depth buffer

• Need to account for both edge and shade map– take min value and use this for depth


Shower-Door Effect

• When model moves, edges tend to swim around

• Need to account for geometrical data

• Also want to keep uncertainty past geometry edges

• Solution: Render geometry info into another G-Buffer

• Use geometry as a parameter to the noise function


Process Summary


Questions?

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