Download Imaging-Concepts

Image Processing Concepts
Data Translation, Inc.
Basics of Image Processing
Image Processing Concepts
Types of Video Images
• Monochrome
– Image made up of varying shades of gray,
from black to white
– Number of shades depends on resolution
of frame grabber
Image Processing Concepts
Monochrome Resolutions
Bits of
Resolution
Number of
Gray Levels
Black Value
White Value
1
2
0
1
4
16
0
15
6
64
0
63
8
256
0
255
12
4096
0
4095
16
65536
0
65535
Image Processing Concepts
Types of Video Images
• RGB Color –
– Three sets of brightness signals
– One for each: RED, GREEN, BLUE
Image Processing Concepts
Capturing Images
• Terminology
– Video formats
– Frames (interlaced and non-interlaced)
– Fields (odd and even)
– Video signals
Image Processing Concepts
Standard Video Fundamentals
• Light collides with the surface of an
image sensing device (camera)
• Result: an electrical voltage level,
related to the amount of light hitting the
particular area of the surface, is
produced
Image Processing Concepts
Video Format
• This information is then arranged into a
standard format and output from the
camera
• Sync pulses are also added so that the
receiving device can recognize where
the sequence is in the frame data
Image Processing Concepts
Simplified Camera Diagram
Object
Lens
Target
Electron Beam
Video Signal
Image Processing Concepts
Pixels
• Images are broken down into horizontal
lines
• Lines are broken down into picture
elements, or pixels
Image Processing Concepts
Monochrome Pixels
Each pixel has a gray value. On 8-bit systems,
0=black and 255=white. All other values are
shades of gray.
Image Processing Concepts
Color Pixels
Each pixel contains 3 colored phosphors: RED, GREEN, and
BLUE. Each color receives a different intensity value (similar
to “gray scale” in monochrome image processing). The
resulting combinations determine which color we see.
Image Processing Concepts
Frames
• Interlaced
• Non-interlaced
Image Processing Concepts
Interlaced Image Frames
• All odd-numbered lines are read from
top to bottom, followed by all evennumbered lines
Image Processing Concepts
Interlaced Image Frames
- Diagonal lines are
active video
- Horizontal lines are
blanking (beam off)
- At the bottom of the
raster, the beam if off
and video begins its
vertical retrace
(vertical blanking)
Image Processing Concepts
Fields
• Interlacing causes the frame to be
divided into two fields: odd and even
• Each field is displayed sequentially
giving the perception that the frame is
updated twice as often as it really is
Image Processing Concepts
Fields
• Each field updated every 1/60 or 1/50 s
• Each frame updated every 1/30 or
1/25 s
• 60 (50) fields per second: 30 (25) odd
and 30 (25) even
Image Processing Concepts
Fields
• This method reduces noticeable flicker
when displaying images
• When working with graphics or thin
lines, flicker becomes extremely
noticeable
Image Processing Concepts
Fields
• To reduce flicker:
– Use horizontal lines that are wider than 1
pixel (2 lines??)
– Use long-persistence monitor
– Use non-interlaced monitor for graphics
Image Processing Concepts
Monitors
• An electron beam scans the surface of
the display tube
• A horizontal sync resets the beam to the
left-most side of the screen and then
moves it down to the next line
• When a vertical sync is detected, the
beam is reset to the top, left-most point
of the screen
Image Processing Concepts
Diagram of Monitor Essentials
Phosphors
Video Signal
Gun
Grid
Electrons
Image Processing Concepts
Video Signals
• A video signal contains a series of
analog TV lines
• Lines are separated from one another
by a sync pulse called horizontal sync
• Fields are separated by a longer sync
pulse called vertical sync
Image Processing Concepts
Typical Video Line
Blanking
Level
Active Pixel Region
Horiz.
Blanking
Horiz.
Sync
Full Scan Area
Image Processing Concepts
Video Signals
• Digital video transfers several bits
(representing pixel values)
simultaneously
• Two voltage levels, Logic 0 and 1
• Transmitted on individual TTL
(Transistor-Transistor Logic) lines or
pairs of lines in differential mode
(RS-422 standard, less noise)
Image Processing Concepts
Interfacing Input Devices with Frame
Grabbers
• Video Formats:
– RS-170 and CCIR
– RS-170 RGB and CCIR RGB
– NTSC and PAL
Image Processing Concepts
RS-170
• Specifies all timing and voltage levels
for standard commercial video signals
• Used as basis for most B&W video
equipment in the U.S.
Image Processing Concepts
RS-170
• For 60 Hz television systems (North
American standard)
• Frame consists of 525 lines and is
displayed once every 1/30 of a second
• Each field contains 262.5 lines
Image Processing Concepts
RS-170
• Each field also contains 9 sync lines (18
lines per frame) and 11 “no video” or
“blanking” lines
• A video frame consists of 485 viewable
lines:
525 – 18 (sync) – 22 (blanking) = 485
Image Processing Concepts
RS-170
• For camera compatibility, most frame
grabber manufacturers design boards
which capture 480 lines
• Therefore, lines are clipped at the top
and bottom of the image
Image Processing Concepts
RS-170 RGB
• Three RS-170 type signals, one for
each of the additive primary colors –
red, green, and blue
• Red, green and blue images are
displayed simultaneously
• Image manipulations must be
performed independently on all three
components
Image Processing Concepts
CCIR
• CCIR – International Radio Consultative
Committee
• 50 Hz equivalent to RS-170
• A frame consists of 625 lines
• Subtracting sync and blanking lines
yields 544 lines of displayable video
• Lines are clipped from top and bottom
to display 512 lines
Image Processing Concepts
NTSC
• NTSC – National Television Standards
Committee
• Standard specification for color signals
– 60 Hz
• Single line input
• Color is superimposed over the
monochrome (RS-170) signal
Image Processing Concepts
NTSC
• Color can be removed by frame grabber
using chrominance filter
• Three most popular NTSC uses:
– Broadcast television
– Cable television
– VCRs
Image Processing Concepts
PAL (Phase Alternation Line)
• 50 Hz equivalent to NTSC
• European standard
Image Processing Concepts
Summary of Standard Signals
Used
Where*
Color
Hertz
Lines In
RS-170
USA
No
60
1
RS-170
RGB
USA
Yes
60
3
CCIR
INTL
No
50
1
CCIR
RGB
INTL
Yes
50
3
NTSC
USA
Yes
60
1
PAL
INTL
Yes
50
1
*USA = US, Canada, Japan, Brazil INTL = Most other countries
Image Processing Concepts
Non-standard Video Signals
• Input device tells frame grabber when to
digitize
• Non-interlaced signal
• All lines are read in succession to
create a frame
• One type is referred to as “slow scan”
Image Processing Concepts
Non-standard Video Signals
• The following control signals must be
provided by the user
– Scan trigger
– Clock enable
– Pixel clock
– Pixel value (analog)
Image Processing Concepts
Other Definitions
Image Processing Concepts
Picture Aspect Ratio
• The relationship between the width and
height of a frame
4
3
Image Processing Concepts
Pixel Aspect Ratio
• The relationship between the width and
height of a pixel
US
5:4
1:1
INTL
3:2
1:1
Image Processing Concepts
Chrominance Filter
• Jumper-selectable circuit that removes
color information from NTSC signals.
Normally found on monochrome frame
grabbers to prevent interference with
the monochrome image
• Implemented via a notch filter
Image Processing Concepts
Look-Up Tables (LUT’s)
• Implements pixel (point) processing
• One value goes in, another comes out
0=0
60 = 75
75
75 = 19
193 = 200
222 = 222
230 = 229
19
Image Processing Concepts
Input Look-Up Tables
• Used for thresholding
• Real-time processing
• Add or multiply by a constant
Image Processing Concepts
Thresholding
• A pixel operation used to reduce the
number of gray levels displayed
• One example is binary thresholding,
resulting in either black or white
Image Processing Concepts
Binary Thresholding Example
0 - 160
0
LUT
161 - 255
255
Image Processing Concepts
Binary Thresholding Example #2
0 - 80
81 - 175
0
LUT
100
176 - 225
200
226 - 255
0
Image Processing Concepts
Contrast
• Refers to the clarity (sharpness or
dullness) of an image
• A result of the ratio of black to gray to
white
Image Processing Concepts
Histogram
• Graphic representation of contrast
Number of pixels
50
40
30
20
0 = black,
255 = white
10
0 20 40 60 80 100 120 140 160 180 200 220 240 255
Image Processing Concepts
Histogram Equalization
• Alters the histogram, thereby smoothing
the contrast
Image Processing Concepts
Zoom
• Magnification of an image
• Typical factors: 2, 4, or 8
Image Processing Concepts
Pan
• Shifts image to left or right
Image Processing Concepts
Scroll
• Shifts image up or down
Image Processing Concepts
Overlay
• Graphics or text that can be added to an
image
• Destructive and non-destructive
Image Processing Concepts
Area of Interest (AOI)
• Also known as Region of Interest (ROI)
or Active Region of Interest
• A portion of an image
• Specific rows and columns form a
rectangular section to be worked on
Image Processing Concepts
Frame Buffer
• An individual array of image data. Most
common are:
– 512 x 512 x 8 bits (256 Kb of memory)
used on older boards
– 640 x 480 x 8 bits (300 Kb of memory)
used on newer boards
Image Processing Concepts
Filtering
• A method of massaging the image’s
data
Image Processing Concepts
Types of Filtering
•
•
•
•
•
Low pass – blur
High pass – sharpen
Laplacian – enhance all edges
Horizontal edge detection/enhancement
Vertical edge detection/enhancement
Image Processing Concepts
Group Processing
• Works on a group of pixels at one time
• Used for filtering
Image Processing Concepts
Kernel
• Arithmetic grid used to perform filtering
1
0
4
-1
-1
-1
4
6
6
-1
9
-1
5
9
9
-1
-1
-1
Original Pixel
Values
Kernel
16
Resulting
Middle Pixel
Image Processing Concepts
Frame Averaging
• Adds together several frames, then
divides by the number of frames. This
produces a less noisy image
• True vs. Weighted
Image Processing Concepts
Logic Operations
• Provide a pixel-by-pixel combination of
two images
Image Processing Concepts
Logic Operations
O = False
1 = True
AND
OR
0
1
0
0
0
1
0
1
XOR
0
1
0
0
1
1
1
1
0
1
0
0
1
1
1
0
Image Processing Concepts
Hue Saturation Intensity (HSI)
• “Human view of colors”
• Rather than specifying a color as
percentages of red, green and blue,
they are specified as “dark magenta” or
“light aqua”
• Takes the same number of bits to store
an HSI image as an RGB one
Image Processing Concepts
Conclusion
Additional image processing questions?
Contact Data Translation at
(800) 525-8528