Digital Cameras

Digital Cameras
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Sight

The Eye:
The Process within the Eye:
- Light enters through the cornea
- Is focused by the lens
- Stimulates sensors in the retina

Sight (cont.)

Rods:
- Stimulated by short wavelengths
- Respond quickly
- Output imacts our perception of luminance/brightness
Cones:
- \(\beta\) sensors are sensitive to wavelengths between 400 and 550 nm, but are most sensitive to a wavelength of 450 nm (what we call blue)
- \(\gamma\) sensors are sensitive to wavelengths between 420 and 660 nm, but are most sensitive to a wavelength of 540 nm (what we call green)
- \(\rho\) sensors are sensitive to wavelengths between 400 and 700 nm, but are most sensitive to a wavelength of 580 nm (what we call red)

Digitizing Visual Content

An Implication:
- When presenting visual content it makes sense to use an output device that generates/reflects red, green, and blue light
A Further Implication:
- When digitizing (and storing) visual content it makes sense to use a sensor that measures red, green, and blue light

Modeling Color

The (Linear) Color Cube

Modeling Color (cont.)

The RGB Model:
- Begin with black then add red, green, and or blue
- An additive model
- Often used for displays/monitors
The CMYK Model:
- Begin with white then remove cyan, magenta, and/or yellow
- A subtractive model
- Often used for printing

Camera Basics

The Shutter:
- Creates a hole (called the aperture) that allows light to enter
The Lens:
- Focuses the light

Sensors in a Digital Camera

Two Main Types:
- Charge-Coupled Devices (CCDs)
- Complementary Metal-Oxide-Semiconductor (CMOS) Image Sensor
Commonalities and Differences:
- Each has a rectangular grid (the size of which determines the resolution) of photosites (called picture elements, or pixels) that convert light to electricity
- CMOS sensors have an amplifier for each pixel; CCD pixels have amplifiers for entire rows of sensors

Tradeoffs

Noise:
- CMOS sensors are more susceptible to noise
Light Sensitivity:
- CMOS sensors have lower light sensitivity (because some of the light hits the amplifiers)
Power Consumption:
- CMOS sensors consume less power

Measuring Color

Using Layered Sensors (e.g., Foveon X3):
- The wavelength-dependent absoroption properties of the silicon are used to separate the light to the three layers, each of which measures a different color
Using a Beam Splitter (e.g. 3CCD):
- The beam splitter creates three paths for the light
- Each path is directed to a different sensor with a different filter (i.e., red, green, or blue)
Using a Rotating Filter:
- Red, green, and blue filters are rotated in front of a single sensor in quick succession
Using a Filter Array:
- A fixed filter grid is placed in front of a single sensor so that each photosite measures a different color

The Bayer Filter

Visualization:
Properties:
- Has as many green pixels as red and blue combined (because the eye is not equally sensitive to all three colors)
A Simple Demosaicing Algorithm:
- One color is measured exactly
- The amounts of the other two colors are obtained by interpolating the neighbors of that color

Storage

In Raw Format:
- Each pixel requires storage for its red, blue, and green components
Representation of Each Component:
- Typically one byte (256 different values)
Total Storage:
- The resolution times three (e.g., a 10 megapixel camera would require 30 megabytes for an image)

Compression

Compression (cont.)

A Simple Lossless Algorithm:
- Run Length Encoding - replace a sequence of pixels of the same color with the color and the number of times it repeats
A Simple Lossy Algorithm:
- Discard every \(n^{\text{th}}\) pixel