03/10/03© 2003 University of Wisconsin Last Time Tone Reproduction and Perceptual Issues Assignment 2 all done (almost)

03/10/03 © 2003 University of Wisconsin

Last Time

• Tone Reproduction and Perceptual Issues

• Assignment 2 all done (almost)


Today

• Capturing high dynamic range images

• Introduction to image-based rendering

• Assignment 3

• Projects


High Dynamic Range Imaging(Debevec and Malik, SIGGRAPH 1997)

• Problem: Limited dynamic range of film or CCDs makes it impossible to capture high dynamic range in a single image

• Solution: Take multiple images at different exposures

• Problem: How do the pieces get put back together to form a single, composite image– Made difficult because mapping from incoming radiance to pixel

values is non-linear and poorly documents

• Solution: this paper– Very influential for such a simple idea – used in lots of other papers

– Code is available


Quantities

• The output you see – pixel values – from a scanned film or digital camera, is some function of the scene irradiance:

• X is the product of irradiance and exposure time:

– Assuming the “principle of reciprocity”: double exposure and halving irradiance gives the same output, and vice versa

• Aim: recover f to allow inversion from observed values to scene irradiances– Assumption: f is monotonic (surely true, or it’s a useless imaging

device)

XfZ

tEX


Input

• A set of images, indexed by j, with known exposure times: tj

• Call the observed value in image j at pixel i Zij

• Doing some math gives us an equation involving f and Ei:

• We want the g and Ei that best represent the given data (the images)

jiij

jiij

jiij

jiij

tEZg

tEZf

tEZf

tEfZ

lnln

lnlnln 1

1


Solving

• Solve a linear least squares with the following objective:

• Terms for the function and its smoothness, plus weighting terms to give more credence to values with luminance in the mid-range of the dynamic range of the imaging system

• Gives results up to a scale, so set mid-range pixel to be unit radiance

• Don’t use all the values, just about 50 pixels (chosen by hand) and enough images to cover range

1

1

2

1 1

2max

min

lnlnZ

Zz

N

i

P

jjiijij zgzwtEZgZwO


Results – Store Input


Results – Store Mapping


Results – Store Log Plot


Results – Church Input


Results – Church Rendering(Ward’s Histogram Method)


Image-Based Rendering

• Geometry and light interaction may be difficult and expensive to model– Think of how hard radiosity is

– Imagine the complexity of modeling the exact geometry of carpet (as just one example)

• Image based rendering seeks to replace geometry and surface properties with images


Texture Mapping

• Replace small scale geometry and surface changes with an image attached to the surface

• Define a mapping from surface points into a texture image

• When the properties of that point are required, look up the pixel in the texture image

• A (mostly dead) research field in itself– Various methods of sampling the texture

– Various ways of combining the samples with direct lighting calculations

– Ways of arranging and accessing textures in memory


Environment Mapping

• Replace the world around an object with images• Generally used for fast reflection computations• Typically, define a cube around the object and associate

images with the inside of each face• When seeking reflection color, shot ray from center of

object, in reflection direction, onto cube and take corresponding image point

• Images represent a view of the world from the center of the object– Assumes that the world wouldn’t look much different if viewed

from points on the object’s surface


Plenoptic Function

• Returns the radiance:– passing through a given point, x

– in a given direction, (,)

– with given wavelength, – at a given time, t

• Many image-based rendering approaches can be cast as sampling from and reconstructing the plenoptic function

• Note, function is generally constant along segments of a line (assuming vacuum)


Image-Based Rendering

• Aim: Generate an image from a desired view using existing images from other views– May or may not know the viewing parameters for the existing

images

– Existing images may be photographs or computer generated renderings


A Plethora of Approaches

• Methods differ in many ways:– The range of new viewpoints allowed

– The density of input images

– The representation for samples (known images)

– The amount of user help required

– The amount of additional information required (such as intrinsic camera parameters)

– The method for gathering the input images


Next time

• A range of IBR techniques

• Much later, IBR for speeding up rendering of large environments


The NPR Assignment

A. Choose a paper related to non-photorealistic rendering

B. Read it thoroughly and work back through any important references

C. Prepare a 20 minute talk on the contents of the paper

D. Give your presentation in class April 7, 9, 11, 14 or 16


A. Choosing a Paper

• Your paper must have been published in an academic conference

• Good conferences include:– SIGGRAPH: most NPR papers are in the final session, 1995-– NPAR (Non-photorealistic animation and rendering): only two so far,

2000 and 2002– I3D (Symposium on Interactive 3D Graphics): a few NPAR papers

• Most papers are online now, as is some video– I have a good selection of proceedings and videos

• First person to send me the title gets the paper (no duplicates)– Choose a paper by March 18 (Monday)


B. Read the Paper

• Reading an academic paper may be new to you

• Look for the main idea:– What are they trying to do?

– What techniques are they using? How can it be summarized?

– What work does it rely upon?

• Track down any very important references– Things that are essential to know in order to understand the paper


C. Prepare a Talk

• Issues to address include:– What is the goal of the work?

– What came before?

– What is the technical contribution?

– Examples

– Future work?

• Aim for 20 minutes– PRACTICE AND TIME YOURSELF


D. Give your Talk

• Powerpoint is the easiest option– If you want to use something else, let me know

• Heuristically, one slide lasts for two minutes

• Video is possible, but tell me ahead of time

• Leave time for questions– Your prepared talk should time in at 17 minutes or so

• Scheduling will be under my control


Grading

• Approximately 20% of your grade

• Based on:– How well you convey the essence of the paper

– How well you appear to understand the contents

– Your ability to give a talk, including timing and organization

• Talk to me about your presentation before you give it

• This is intended to be a learning experience

• For those in the graphics group, this should be more formal than you’re used to


Class Projects

• Most of the grade for this class will be based upon a class project

• It must be heavily related to rendering (NOT modeling)

• You can work in pairs, or alone– Pairs should do twice as much

• An “A” project would implement something interesting (need not be novel)

• Novel things can get published, and I’m more interested in those

• Ideas? Some have been mentioned in class, but you can do whatever I approve of

• Due by Monday of exam week

Documents

03/10/03© 2003 University of Wisconsin Last Time Tone Reproduction and Perceptual Issues Assignment 2 all done (almost)