Why is photorealism the aim? People paint! What is NPR? NPR issues NonPhotorealistic Rendering

Why photorealism? people paint! Why photorealism ? Physics provides starting point for models Photographs provide a foil to test against But people paint ! Artwork shows off important regions better Photography is just one depictive style amongst many

What is NPR? NPR is a sub-branch of Computer Graphics that studies making images that do not look photographic. NPR is said to have started around Several sub-divisions exist –paint-boxes –rendering 3D models –rendering from photographs and/or video

Paint boxes from the simple……to the complex

Rendering 3D models Breslav et al, SIGGRAPH 2007

Rendering from photographs and/or video

NPR issues NPR depends on perception and interpretation –drawing implies seeing –models are much more complex –interaction is common Mark making –What kind of mark? –Where to mark? No single foil to test against –there is no correct drawing

NPR depends on Perception and Interpretation

No single foil to test against

Mark making: what and where? What: Emulate real media pencil, oil paint, chalk, ….. physical models possible can test for quality of a given mark What: Produce new media Can the computer be used to make marks not made before? Where:Should marks be made locally, globally, or both? How to decide where to place marks? Where: How can marks be made stable in animations?

The problems Some solutions NPR from models

The problems Consider a pencil drawing, marks are used to depict: object boundaries and other contours shadows and shading texture

Apparent ridges Judd et al, SIGGRAPH 2007

For apparent ridges we need: differential geometry of surfaces projection The curvature at a surface point S(x,y) – how curved is the surface? Use the Hessian, H, which is a 2x2 matrix of 2 nd order partials of S. The EVD of H = UKU T principle directions, U = [u 1 u 2 ], principle curvature, K = [k 1 k 2 ] Ridges and valleys where k 1 is an extremum in direction u 1. Project the local surface geometry (tangents, normals n all), Look for extremum in this observed geometry.

Shadows and shading Shading lines – cross hatches etc Can be based in image space………or in object space Again, curvature can be used, this time to direct pencil lines.

Other methods use solid texture maps, that adapt to local lighting and geometry

Marks make textures too… With NPR, even the marks can be animated (look at the sea)

Where is this used? Why is this difficult? What has been done so far? NPR from images

Where is NPR from images used? Photo-booths Film special effects Adverts is related to Image Based Rendering

What has been done to date? 1990 Paint by Numbers 1994 Video Impressionism 2001 Image Analogies 2002 Gaze Directed Rendering 2003 Video Tooning 2007 Simpler shapes

Why is NPR from images difficult? Painting using STROKES –WHAT is a stroke? –WHERE to place them? Strokes are not enough

1990: Paul Haeberli Paint by numbers User interaction, place strokes over a photograph. Stroke colour from pixel Stoke direction from image gradient Stoke look from a texture map

a STROKE represents the action of a brush/pencil etc on paper/canvas etc.

There is a lot of work to model strokes; Paint brushes Palette knives Pencil Copper plate Well consider shaped dots Well place place and shape them at edges

Where to place a stroke (here a dot) Artists draw lines at object boundaries Edge detectors respond to object boundaries To get edges, typically convolve with Gaussian derivatives f(x,y) |grad(f*g)|

Shaping a dot

Edges are found using local information, but strokes are best placed using GLOBAL information

Edges do not correspond well to lines drawn by humans

distinguishing between different kinds of edges allows differential emphasis

Lets use the edge map as a kind of target. Lets place stokes near-randomly on one painting, but make MANY paintings. Keep those that match the target best. Make new painting by spawning strokes from the best fits. Continue until happy Detail is kept where needed Lost where not needed

The situation is worse still when the painting moves…

But NPR is more than strokes