why is photorealism the aim? people paint! what is npr? npr issues nonphotorealistic rendering
TRANSCRIPT
• 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 1990.
• 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 2nd order partials of S.
The EVD of H = UKUT
principle directions, U = [u1 u2], principle curvature, K = [k1 k2]
Ridges and valleys where k1 is an extremum in direction u1.
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
We’ll consider shaped dots
We’ll 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
Let’s use the “edge” mapas a kind of target.
Let’s 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