Image Processing ConceptsImage Processing Concepts
Data Translation, Inc.Basics of Image Processing
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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
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Monochrome ResolutionsBits of
ResolutionNumber of
Gray LevelsBlack 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
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Types of Video Images
• RGB Color – – Three sets of brightness signals– One for each: RED, GREEN, BLUE
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Capturing Images
• Terminology– Video formats– Frames (interlaced and non-interlaced)– Fields (odd and even)– Video signals
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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
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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
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Simplified Camera Diagram
Electron Beam
Video Signal
TargetLensObject
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Pixels
• Images are broken down into horizontal lines
• Lines are broken down into picture elements, or pixels
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Monochrome Pixels
Each pixel has a gray value. On 8-bit systems, 0=black and 255=white. All other values are shades of gray.
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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.
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Frames
• Interlaced• Non-interlaced
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Interlaced Image Frames
• All odd-numbered lines are read from top to bottom, followed by all even-numbered lines
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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)
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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
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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
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Fields
• This method reduces noticeable flicker when displaying images
• When working with graphics or thin lines, flicker becomes extremely noticeable
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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
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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
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Diagram of Monitor EssentialsPhosphors
Video Signal
Gun
Grid
Electrons
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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
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Typical Video Line
Active Pixel Region
Full Scan AreaHoriz.
Blanking
Blanking Level
Horiz. Sync
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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 ConceptsImage Processing ConceptsInterfacing Input Devices with Frame Grabbers• Video Formats:
– RS-170 and CCIR– RS-170 RGB and CCIR RGB– NTSC and PAL
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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.
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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
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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
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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
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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
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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
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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
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NTSC
• Color can be removed by frame grabber using chrominance filter
• Three most popular NTSC uses:– Broadcast television– Cable television– VCRs
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PAL (Phase Alternation Line)
• 50 Hz equivalent to NTSC• European standard
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Summary of Standard SignalsUsed
Where*Color Hertz Lines In
RS-170 USA No 60 1RS-170
RGBUSA Yes 60 3
CCIR INTL No 50 1CCIR RGB
INTL Yes 50 3
NTSC USA Yes 60 1PAL INTL Yes 50 1
*USA = US, Canada, Japan, Brazil INTL = Most other countries
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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”
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Non-standard Video Signals
• The following control signals must be provided by the user– Scan trigger– Clock enable– Pixel clock– Pixel value (analog)
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Other Definitions
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Picture Aspect Ratio
• The relationship between the width and height of a frame
4
3
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Pixel Aspect Ratio
• The relationship between the width and height of a pixel
US
INTL
5:4 1:1
1:13:2
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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
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Look-Up Tables (LUT’s)
• Implements pixel (point) processing• One value goes in, another comes out
0 = 0
60 = 75
75 = 19193 = 200
222 = 222
230 = 229
75 19
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Input Look-Up Tables
• Used for thresholding• Real-time processing• Add or multiply by a constant
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Thresholding
• A pixel operation used to reduce the number of gray levels displayed
• One example is binary thresholding, resulting in either black or white
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Binary Thresholding Example
0 - 160 0
161 - 255 255
LUT
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Binary Thresholding Example #2
0 - 80 0
226 - 255
200LUT81 - 175
176 - 225
0
100
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Contrast
• Refers to the clarity (sharpness or dullness) of an image
• A result of the ratio of black to gray to white
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Histogram
• Graphic representation of contrast
0 20 40 60 80 100 120 140 160 180 200 220 240 255
10
2030
40
50
Number of pixels
0 = black, 255 = white
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Histogram Equalization
• Alters the histogram, thereby smoothing the contrast
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Zoom
• Magnification of an image• Typical factors: 2, 4, or 8
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Pan
• Shifts image to left or right
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Scroll
• Shifts image up or down
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Overlay
• Graphics or text that can be added to an image
• Destructive and non-destructive
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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
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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
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Filtering
• A method of massaging the image’s data
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Types of Filtering
• Low pass – blur• High pass – sharpen• Laplacian – enhance all edges• Horizontal edge detection/enhancement• Vertical edge detection/enhancement
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Group Processing
• Works on a group of pixels at one time• Used for filtering
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Kernel• Arithmetic grid used to perform filtering
1 0 4
4 6 6
5 9 9
-1 -1 -1
-1 9 -1
-1 -1 -1
16
Original Pixel Values
Kernel Resulting Middle Pixel
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Frame Averaging
• Adds together several frames, then divides by the number of frames. This produces a less noisy image
• True vs. Weighted
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Logic Operations
• Provide a pixel-by-pixel combination of two images
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Logic Operations
O = False 1 = True
AND OR XOR
0 1
00 0
1 0 1
0 1
00 1
1 1 1
0 1
00 1
1 1 0
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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
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Conclusion
Additional image processing questions? Contact Data Translation at
(800) 525-8528