DIGITAL IMAGE PROCESSING

CMSC 150: Lecture 14

Conventional Cameras

Entirely chemical and mechanical processes

Film: records a chemical record of light pattern Light-sensitive grains in chemical

suspension on plastic Upon light exposure, grains undergo

reaction

Development: expose film to other chemicals Chemicals dye the layers of red, green,

blue Overlay to get full-color negative

Conventional Cameras

Digital Cameras

Sensor converts light to electrical charges 2D array of many tiny cells Light hits, converted into electrons Charge is converted into binary form

CCD: Charge Coupled Device

Analog to Digital Conversion: Sampling

Analog to Digital Conversion: Sampling

Analog to Digital Conversion: Sampling

Analog to Digital Conversion: Sampling

Analog to Digital Conversion: Sampling

Digital Image

Sensor: 2D array of values Image: "value" stored for cell in the

sensor Pixel: picture element One pixel per sensor cell

Capturing Color

Color filter placed over sensor Color at each cell determined as

"average of neighbor cells" (How Stuff Works animation)

Grayscale vs. Color

Grayscale: pixel corresponds to shade of gray Highest value: white Lowest value: black

Grayscale Images: Example

PGM: Portable Graymap Use 8-bits per pixel

256 total graylevels, 0-255 Each pixel represented by an integer

0: black 255: white

Let's play around with a few, using IrfanView

Grayscale vs. Color

Color: pixel corresponds to three color intensities Red, Green, Blue

In general, color image at least 3X footprint of grayscale

RGB: Additive Color Model

Start from no color present (black background)

Add (emit) amounts of each primary

Full intensity of each R,G,B: white Full intensity of R,G: yellow

Resolution

Image quality vs. number of pixels Each image below stretched to 200x200

pixels Fewer pixels less information stored

25x25 original625 pixels

50x50 original2500 pixels

100x100 original10000 pixels

Image Quality Vs. Color Levels

Clockwise on right: 2 levels per R,G,B 4 levels per R,G,B 10 levels per R,G,B 40 levels per R,G,B

More bits per pixel more colors larger footprint better quality