Download - Image Representation
Image Representation
• Digital Cameras
• Scanned Film & Photographs
• Digitized TV Signals
• Computer Graphics
• Radar & Sonar
• Medical Imaging Devices (X-Ray, CT)
• The Internet
Digital Image Sources
• Pixel = “Picture Element”
• Image [x,y] = pixel value (number)
Images - 2D array of values = pixels
• Pixel – an element of the 2-D image array
• Pixel Value = brightness• - black = 0 • - gray = 128• - white = 255• - many shades over the 0-255 range
0 1 2 3 4 5 6 701234567
Pixels and Pixel Values
Images are digitized using a two step process:
1. sampling the continuous tone image
2. quantizing pixels
Digitizing Images
Sampling
pixel’s samples are averaged
Quantization
Quantization Example
Image Resolution
68 x 104 136 x 208 272 x 416 less detail more detail less storage more storage
Digital Cameras
• Low-Res 640x480 (TV grade)
• Medium-Res 1536x1024
• Hi-Res 2048x1536
• Very Hi-Res 2560x1920
Dynamic Range
256 levels
• The number of quantized pixel values:
16 levels 4 levels 2 levels
• Binary Images (pixel values = 0,1)
• Grayscale Images (pixel values = 0-255)
• Color Images– Each pixel has three color components
• For example, (red, green, blue) or RGB
– Each color component is 0-255
Images - 2D array of values
3 Images Overlayed
Color Images
Red
Green
Blue
• What is a histogram?
• Simple numeric example
• Histogram1 3 213
121
4
2 1 2
1 2 3 1 2 3 4 41 2 3 1 2 3 4 4
Histograms: What’s in the image?
• Histogram for each color
Color Image Histograms
RGB Additive Color ModelREDRED
bright values => high amounts of that color
dark values => low amounts of that color
GREENGREEN BLUEBLUE
BLACKBLACK
CYANCYAN MAGENTAMAGENTA
YELLOWYELLOW
CMYK Subtractive Color Model
Bright => use less of
that ink color
Dark => use lots of
that ink color
HSB: how artists perceive color properties
HSB Visual Color Model
HSB: how artists perceive color properties
HSB Visual Color Model
Select Hue
00oo
HueHue
360360oo
HSB: how artists perceive color properties
HSB Visual Color Model
SaturationSaturation
Select Hue
00oo
HueHue
360360oo
- then click in box for saturation, brightness
HSB: how artists perceive color properties
HSB Visual Color Model
Bri
ghtn
ess
Bri
ghtn
ess
SaturationSaturation
Select Hue
00oo
HueHue
360360oo
- then click in box for saturation, brightness
Storing Digital Images
• Digital images are converted to files for storage and transfer
• The file type is a special format for ordering and storing the bytes that make up the image
• File types or formats are not necessarily compatible
• You must often match the file type with the application
Storing Digital Images
• GIF (Graphic Interchange Format)– indexed color (up to 256 colors)– compressed– used in Web applications
• JPEG (Joint Photographic Experts Group)– lossy compression with variable controls– also used in Web applications
Storing Digital Images
• PNG (Portable Network Graphics)– designed for online viewing (e.g., Web)– patent-free replacement for GIF– lossless compression
• BMP– MS Windows image format
How Many Bytes to Store an Image?
• Suppose we a have an image that is 500x500 pixels in size
• That’s a total of 250,000 pixels
• Binary image (1 bit/pixel) = 31,250 bytes
• Grayscale image (8 bits/pixel) = 250,000 bytes
• Color image (24 bits/pixel) = 750,000 bytes
Indexed Color• “Indexed Color” can be used to reduce the
size of a color image file
0 1 0 1 2 1 0 1 0
0 1 2
255 255 255 255 0 255 255 255 255
0 255 0 255 255 255 0 255 0
0 0 0 0 255 0 0 0 0
= 27 bytes
= 18 bytes
Indexed Color Images
• are derived from full color images
• are smaller or more compact in storage
• are composed of pixels selected from a limited palette of colors or shades