Northeastern University, Fall 2005 CSG242: Computational Photography Ramesh Raskar Mitsubishi Electric Research Labs Northeastern University September 28th, 2005 Course WebPage :

Noise in Sensors

Plan for Today First Programming AssignmentFirst Programming Assignment Camera FocusCamera Focus Second Programming AssignmentSecond Programming Assignment –Will be on the web –Due in 2 weeks

Taking Notes Use slides on the FTP siteUse slides on the FTP site Write down anecdotes and storiesWrite down anecdotes and stories Try to get what is NOT on the slideTry to get what is NOT on the slide Summarize questions and answersSummarize questions and answers

Programming Assignment Sept 14 High Dynamic RangeHigh Dynamic Range –Dataset1 –Dataset2 Take atleast 5 pictures with varying exposureTake atleast 5 pictures with varying exposure –Create HDR image Average the five images in MatlabAverage the five images in Matlab Use HDRshopUse HDRshop Write Matlab code to implement bilateral filteringWrite Matlab code to implement bilateral filtering –Submit Due Sept 28thDue Sept 28th All codeAll code For dataset1: final results (three results)For dataset1: final results (three results) For dataset2: input images and three resultsFor dataset2: input images and three results

Focus

Example of digital refocusing

Varying Focus: Extended depth-of-field Agrawala et al, Digital Photomontage, Siggraph 2004

Source imagesComputed labeling Composite

Source imagesComputed labeling Composite

How do we see the world? Let’s design a camera Idea 1: put a piece of film in front of an object Do we get a reasonable image? Slide by Steve Seitz

Pinhole camera Add a barrier to block off most of the rays This reduces blurring The opening known as the aperture How does this transform the image? Slide by Steve Seitz

Pinhole camera model Pinhole model: Captures pencil of rays – all rays through a single point The point is called Center of Projection (COP) The image is formed on the Image Plane Effective focal length f is distance from COP to Image Plane Slide by Steve Seitz

Figures © Stephen E. Palmer, 2002 Dimensionality Reduction Machine (3D to 2D) 3D world2D image What have we lost? Angles Distances (lengths)

Funny things happen…

Parallel lines aren’t… Figure by David Forsyth

Distances can’t be trusted... Figure by David Forsyth

…but humans adopt! Müller-Lyer Illusion We don’t make measurements in the image plane

Building a real camera

Camera Obscura The first camera Known to Aristotle Depth of the room is the effective focal length Camera Obscura, Gemma Frisius, 1558

Home-made pinhole camera Why so blurry?

Shrinking the aperture Why not make the aperture as small as possible? Less light gets through Diffraction effects… Less light gets through Slide by Steve Seitz

Shrinking the aperture

The reason for lenses Slide by Steve Seitz

Ideal Lens: Same projection as pinhole but gathers more light! i o Lens Formula: f is the focal length of the lens – determines the lens’s ability to bend (refract) light f different from the effective focal length f discussed before! P P’ f Image Formation using Lenses Slide by Shree Nayar

Focus

Let us review the pinhole structure Ray Origin - Image Plane All the rays collected by the camera MUST pass through a common 3D point

Focus and Defocus A lens focuses light onto the film There is a specific distance at which objects are “in focus” –other points project to a “circle of confusion” in the image How can we change focus distance? “circle of confusion” Slide by Steve Seitz

Depth Of Field

Depth of Field

Aperture controls Depth of Field Changing the aperture size affects depth of field A smaller aperture increases the range in which the object is approximately in focus But small aperture reduces amount of light – need to increase exposure

Varying the aperture f/2.8 Large apeture = small DOF f/22 Small apeture = large DOF

Nice Depth of Field effect

Field of View (Zoom)

f FOV depends of Focal Length Smaller FOV = larger Focal Length

From Zisserman & Hartley

Field of View / Focal Length Large FOV Camera close to car Small FOV Camera far from the car

Fun with Focal Length (Jim Sherwood)

Large Focal Length compresses depth © Michael Reichmann 400 mm 200 mm 100 mm 50 mm 28 mm 17 mm

Perspective vs. viewpoint Telephoto makes it easier to select background (a small change in viewpoint is a big change in background.

Focal length Principal effect: change magnification and FOV Side effect: strength of perspective (recording vs. viewing FOV) image: Kingslake 1992

Perspective vs. viewpoint Martin Scorcese, Good Fellas Moves camera as you zoom in Better known as the Hitchcock Vertigo effect

Perspective vs. viewpoint Portrait: distortion with wide angle Why? Wide angleStandardTelephoto

Elliptical Distortion image: Kingslake 1992

Elliptical Distortion image: Kingslake 1992

Focal length: pinhole optics What happens when the film is half the size? Application: Real film is 36x24mm On the 20D, the sensor is 22.5 x 15.0 mm Conversion factor on the 20D? On the SD500, it is 1/1.8 " (7.18 x 5.32 mm) What is the mm zoom on the SD500? pinhole Film/ sensor scene fd ½ s 2f