data translation, inc. basics of image processing
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Data Translation, Inc. Basics of Image Processing. 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. Monochrome Resolutions. Types of Video Images. RGB Color – - PowerPoint PPT PresentationTRANSCRIPT
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Image Processing ConceptsImage Processing Concepts
Data Translation, Inc.Basics of Image Processing
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Image Processing ConceptsImage 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
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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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Image Processing ConceptsImage Processing Concepts
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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Image Processing ConceptsImage 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
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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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Image Processing ConceptsImage Processing Concepts
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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Image Processing ConceptsImage 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)
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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)
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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