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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