computer graphics & visualization

Image Synthesis

NPR – Non-photorealistic rendering

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

NP vs. NPR?

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

PR vs. NPR

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Why NPR?“Science”:

Convey information more efficiently by reduction,

abstraction, accentuation

“Art”:create novel artistic,

creative imagery

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Reasons against photorealism?• Too much (unnecessary) information

maps, technical/medical illustrations

• Constraints Creativity improper for caricatures, comics, art

• Time consuming image generation (probably) not feasible for real-time applications

• No room for the fantasy of the viewer

„Photorealism, like pornography, leaves nothing to the imagination“- Cassidy Curtis

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

ApplicationsArt

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

ApplicationsComics/ Cartoons

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

ApplicationsArchitecture

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

ApplicationsMaps

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

ApplicationsMaps

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

ApplicationsMedicine

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

ApplicationsMedicine

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

ApplicationsTechnicalIllustrations

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

ApplicationsArcheology

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

BasicsObject Space Screen Space3D 2DGeometry processing Image processingObject Precision Screen Precision

Pseudo code: „for each Object do…“

Pseudo code:„for each Pixel do…“

Usually does not require recomputation on affine transformations

Recomputation every time the image changes ( no frame-to-frame coherence „Shower Door Effect“)

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

AlgorithmsToon/Cell Shading

Sketching

Halftoning

Screening

Stippling

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Toon Shading

Toon/Cell Shading:Rendering technique to create cartoon like movies

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Toon Shading

- large homogenous regions- very few colors- enhanced silhouettes - simple geometry- reduction to important elements

Motivation:Simpel and (semi-) automatic (mass-) production of comics

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Toon ShadingColor computation (object space):

precompute 2-3 shades for each color (Light-/ Shadow- color) and store them in a 1D texture

assignment of final color by 1D texture lookup

1D texture

surface

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Toon ShadingSilhouettes (Screen Space)

• generate Depth map• find discontinuities

• generate normal map• find discontinuities

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Toon ShadingCombination of both silhouette detections ideas:

Technical Illustrations:

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Sketching

Sketching:Automatic method to create computer images that look like and hand-made drawings

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Computer SketchingMotivation:• Ideal to present concepts (architecture) • Precise Depiction often looks very sterile

Methods:• Use special output device• Image space post-processing• Object space 3D model modification

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Computer SketchingExample: Object space approach:• Pseudo-Randomly change length of lines• Cut lines into multiple sub-lines• Move line center

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Halftoning

Halftoning: Reproduce (color/grayscale) images with a (very) limited amount of colors, usually only black/white

“Historically”, a necessity to print images in newspapers

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

HalftoningAlgorithm:• Input: Gray scale image• Group pixles ( loss of image resolution)• Compute average intensity• Replace group by dither matrix

2 9 4

6 1 8

7 3 5

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Halftoning

limited resolution of the human visual system

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Error Diffusion

Floyd and Steinberg

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Screening• Bild Screening:

Artistic variation of the halftoning algorithm

Replaces the simple dither matrix by one ore more complex images

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

ScreeningScreening with text

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Stippling

Stippling: Construct an image of a large number of dots

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Pointillismus

Georges Seurat - Ein Sonntagnachmittag auf der Insel La Grande Jatte

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Stippling• Motivation:

• Results in very “clean/cold” illustrations• Very time-consuming to create manually

• Applications: • Scientific and technical illustrations• Art• Print media

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

StipplingStipple points simulate both tone and texture.

Usually a uniform distribution without any recognizable pattern is used

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

StipplingObject space:• prioritized line textures

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Stippling• Other prioritized line textures

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Stippling

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Object / Image Space Stippling

computer graphics & visualization

Image Synthesis – WS 07/08Dr. Jens Krüger – Computer Graphics and Visualization Group

Object / Image Space Stippling

black-and-white stippling

Supersampling in screen spacesurface based anti-aliasing