Yung-Yu Chuang Digital Visual Effects Cameras

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Cameras

Digital Visual Effects Yung-Yu Chuang

with slides by Fredo Durand, Brian Curless, Steve Seitz and Alexei Efros

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Camera trial #1

scene film

Put a piece of film in front of an object.

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Pinhole camera

scene film

Add a barrier to block off most of the rays.

• It reduces blurring

• The pinhole is known as the aperture

• The image is inverted

barrier

pinhole camera

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Shrinking the aperture

Why not making the aperture as small as possible?

• Less light gets through

• Diffraction effect

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Shrinking the aperture

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High-end commercial pinhole cameras

$200~$700

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Adding a lens

scene lens film

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Lenses

Thin lens equation:

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Thin lens formula

f D’ D

Similar triangles everywhere!

y’

y

y’/y = D’/D

Frédo Durand’s slide

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Thin lens formula

f D’ D

y’

y

y’/y = D’/D y’/y = (D’-f)/f

Similar triangles everywhere!

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Thin lens formula

f D’ D

D’ D 1 1 1

+ = f

The focal length f determines the lens’s ability to bend (refract)

light. It is a function of the shape and index of refraction of the lens.

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Adding a lens

scene lens film

“circle of confusion”

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

• Thin lens applet:

http://www.phy.ntnu.edu.tw/java/Lens/lens_e.html

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Zoom lens

Nikon 28-200mm zoom lens.

200mm

28mm

simplified zoom lens

in operation From wikipedia

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Field of view vs focal length

i o

f o

i

1 1

1  

Gaussian Lens Formula:

Scene

Sensor

f α

α = 2arctan(w/(2i)) w

Field of View:

Example: w = 30mm, f = 50mm => α ≈ 33.4º

≈ 2arctan(w/(2f))

Slides from Li Zhang

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Focal length in practice

24mm

50mm

135mm

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Distortion

• Radial distortion of the image

– Caused by imperfect lenses

– Deviations are most noticeable for rays that pass through the edge of the lens

No distortion Pin cushion Barrel

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Correcting radial distortion

from Helmut Dersch

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Vignetting

L1

L2

L3 B

A

more light from A than B !

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Vignetting

L1

L2

L3 B

A

more light from A than B !

original corrected

Goldman & Chen ICCV 2005

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Chromatic Aberration

Lens has different refractive indices for different wavelengths.

Special lens systems using two or more pieces of glass with different refractive indexes can

reduce or eliminate this problem.

http://www.dpreview.com/learn/?/Glossary/Optical/chromatic_aberration_0 1.htm

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Exposure = aperture + shutter speed

• Aperture of diameter D restricts the range of rays (aperture may be on either side of the lens)

• Shutter speed is the amount of time that light is allowed to pass through the aperture

F

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Exposure

• Two main parameters:

– Aperture (in f stop)

– Shutter speed (in fraction of a second)

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• Slower shutter speed => more light, but more motion blur

• Faster shutter speed freezes motion

Effects of shutter speeds

1/125 1/250 1/500 1/1000

Walking people Running people Car Fast train

From Photography, London et al.

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Aperture

• Aperture is the diameter of the lens opening, usually specified by f-stop, f/D, a fraction of the focal length.

– f/2.0 on a 50mm means that the aperture is 25mm – f/2.0 on a 100mm means that the aperture is 50mm

• When a change in f-stop occurs, the light is either doubled or cut in half.

• Lower f-stop, more light (larger lens opening)

• Higher f-stop, less light

(smaller lens opening)

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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

lens sensor

Point in focus

Object with texture Diaphragm

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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

lens sensor

Point in focus

Object with texture Diaphragm

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Depth of field

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Exposure

• Two main parameters:

– Aperture (in f stop)

– Shutter speed (in fraction of a second)

• Reciprocity

The same exposure is obtained with an

exposure twice as long and an aperture area half as big

– Hence square root of two progression of f stops vs. power of two progression of shutter speed – Reciprocity can fail for very long exposures

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Reciprocity

• Assume we know how much light we need

• We have the choice of an infinity of shutter speed/aperture pairs

• What will guide our choice of a shutter speed?

– Freeze motion vs. motion blur, camera shake

• What will guide our choice of an aperture?

– Depth of field, diffraction limit

• Often we must compromise

– Open more to enable faster speed (but shallow DoF)

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Exposure & metering

• The camera metering system measures how bright the scene is

• In Aperture priority mode, the photographer sets the aperture, the camera sets the shutter speed

• In Shutter-speed priority mode, photographers sets the shutter speed and the camera deduces the

aperture

• In Program mode, the camera decides both exposure and shutter speed (middle value more or less)

• In Manual mode, the user decides everything (but

can get feedback)

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Pros and cons of various modes

• Aperture priority

– Direct depth of field control

– Cons: can require impossible shutter speed (e.g. with f/1.4 for a bright scene)

• Shutter speed priority

– Direct motion blur control

– Cons: can require impossible aperture (e.g. when requesting a 1/1000 speed for a dark scene)

• Note that aperture is somewhat more restricted

• Program

– Almost no control, but no need for neurons

• Manual

– Full control, but takes more time and thinking

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Sensitivity (ISO)

• Third variable for exposure

• Linear effect (200 ISO needs half the light as 100 ISO)

• Film photography: trade sensitivity for grain

• Digital photography: trade sensitivity for noise

From dpreview.com

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Demo

See http://www.photonhead.com/simcam/

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Film camera

scene lens & film

motor

aperture

& shutter

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Digital camera

scene sensor

array lens &

motor

aperture

& shutter

• A digital camera replaces film with a sensor array

• Each cell in the array is a light-sensitive diode that converts photons to electrons

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CCD v.s. CMOS

• CCD is less susceptible to noise (special process, higher fill factor)

• CMOS is more flexible, less expensive (standard process), less power consumption

CCD CMOS

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Sensor noise

• Blooming

• Diffusion

• Dark current

• Photon shot noise

• Amplifier readout noise

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SLR (Single-Lens Reflex)

• Reflex (R in SLR) means that we see through the same lens used to take the image.

• Not the case for compact cameras

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SLR view finder

lens

Mirror

(when viewing) Mirror

(flipped for exposure)

Film/sensor Prism

Your eye

Light from scene

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Color

So far, we’ve only talked about monochrome

sensors. Color imaging has been implemented in a number of ways:

• Field sequential

• Multi-chip

• Color filter array

• X3 sensor

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Field sequential

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Field sequential

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Field sequential

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Prokudin-Gorskii (early 1900’s)

Lantern projector

http://www.loc.gov/exhibits/empire/

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Prokudin-Gorskii (early 1990’s)

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Multi-chip

wavelength

dependent

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Embedded color filters

Color filters can be manufactured directly onto

the photodetectors.

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Color filter array

Color filter arrays (CFAs)/color filter mosaics Kodak DCS620x

CMY

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Why CMY CFA might be better

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Color filter array

Color filter arrays (CFAs)/color filter mosaics

Bayer pattern

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Bayer’s pattern

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Demosaicking CFA’s

bilinear interpolation

original input linear interpolation

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Demosaicking CFA’s

Constant hue-based interpolation (Cok)

Hue:

Interpolate G first

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Demosaicking CFA’s

Median-based interpolation (Freeman)

1. Linear interpolation

2. Median filter on color

differences

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Demosaicking CFA’s

Median-based interpolation (Freeman)

original input linear interpolation

color difference (e.g. G-R)

median filter (kernel size 5)

Reconstruction (G=R+filtered difference)

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Demosaicking CFA’s

Gradient-based interpolation (LaRoche-Prescott)

1. Interpolation on G

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Demosaicking CFA’s

Gradient-based interpolation (LaRoche-Prescott)

2. Interpolation of color

differences

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Demosaicking CFA’s

bilinear Cok Freeman LaRoche

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Demosaicking CFA’s

Generally, Freeman’s is the best, especially for natural images.

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Foveon X3 sensor

• light penetrates to different depths for different wavelengths

• multilayer CMOS sensor gets 3 different spectral

sensitivities

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Color filter array

red green blue output

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X3 technology

red green blue output

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Foveon X3 sensor

X3 sensor

Bayer CFA

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Cameras with X3

Sigma SD10, SD9 Polaroid X530

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Sigma SD9 vs Canon D30

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Color processing

• After color values are recorded, more color processing usually happens:

– White balance

– Non-linearity to approximate film response or match TV monitor gamma

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White Balance

automatic white balance warmer +3

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Manual white balance

white balance with the white book

white balance with the red book

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Autofocus

• Active

– Sonar – Infrared

• Passive

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Digital camera review website

• A cool video of digital camera illustration

• http://www.dpreview.com/

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Camcorder

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Interlacing

with interlacing without interlacing

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Deinterlacing

blend weave

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Deinterlacing

Discard

(even field only or odd filed only)

Progressive scan

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Hard cases

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References

• http://www.howstuffworks.com/digital-camera.htm

• http://electronics.howstuffworks.com/autofocus.htm

• Ramanath, Snyder, Bilbro, and Sander.

Demosaicking Methods for Bayer Color Arrays, Journal of Electronic Imaging, 11(3), pp306-315.

• Rajeev Ramanath, Wesley E. Snyder, Youngjun Yoo, Mark S. Drew,

Color Image Processing Pipeline in Digital Still Cameras, IEEE Signal Processing Magazine Special Issue on Color Image Processing, vol. 22, no. 1, pp. 34-43, 2005.

• http://www.worldatwar.org/photos/whitebalance/ind ex.mhtml

• http://www.100fps.com/