Data Translation, Inc. Basics of Image Processing

Image Processing ConceptsImage Processing Concepts

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 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


Types of Video Images

• RGB Color – – Three sets of brightness signals– One for each: RED, GREEN, BLUE


Capturing Images

• Terminology– Video formats– Frames (interlaced and non-interlaced)– Fields (odd and even)– Video signals


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


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


Simplified Camera Diagram

Electron Beam

Video Signal

TargetLensObject


Pixels

• Images are broken down into horizontal lines

• Lines are broken down into picture elements, or pixels


Monochrome Pixels

Each pixel has a gray value. On 8-bit systems, 0=black and 255=white. All other values are shades of gray.


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.


Frames

• Interlaced• Non-interlaced


Interlaced Image Frames

• All odd-numbered lines are read from top to bottom, followed by all even-numbered lines


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)


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


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


Fields

• This method reduces noticeable flicker when displaying images

• When working with graphics or thin lines, flicker becomes extremely noticeable


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


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


Diagram of Monitor EssentialsPhosphors

Video Signal

Gun

Grid

Electrons


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


Typical Video Line

Active Pixel Region

Full Scan AreaHoriz.

Blanking

Blanking Level

Horiz. Sync


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


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.


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


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


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


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


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


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


NTSC

• Color can be removed by frame grabber using chrominance filter

• Three most popular NTSC uses:– Broadcast television– Cable television– VCRs


PAL (Phase Alternation Line)

• 50 Hz equivalent to NTSC• European standard


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


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”


Non-standard Video Signals

• The following control signals must be provided by the user– Scan trigger– Clock enable– Pixel clock– Pixel value (analog)


Other Definitions


Picture Aspect Ratio

• The relationship between the width and height of a frame

4

3


Pixel Aspect Ratio

• The relationship between the width and height of a pixel

US

INTL

5:4 1:1

1:13:2


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


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


Input Look-Up Tables

• Used for thresholding• Real-time processing• Add or multiply by a constant


Thresholding

• A pixel operation used to reduce the number of gray levels displayed

• One example is binary thresholding, resulting in either black or white


Binary Thresholding Example

0 - 160 0

161 - 255 255

LUT


Binary Thresholding Example #2

0 - 80 0

226 - 255

200LUT81 - 175

176 - 225

0

100


Contrast

• Refers to the clarity (sharpness or dullness) of an image

• A result of the ratio of black to gray to white


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


Histogram Equalization

• Alters the histogram, thereby smoothing the contrast


Zoom

• Magnification of an image• Typical factors: 2, 4, or 8


Pan

• Shifts image to left or right


Scroll

• Shifts image up or down


Overlay

• Graphics or text that can be added to an image

• Destructive and non-destructive


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


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


Filtering

• A method of massaging the image’s data


Types of Filtering

• Low pass – blur• High pass – sharpen• Laplacian – enhance all edges• Horizontal edge detection/enhancement• Vertical edge detection/enhancement


Group Processing

• Works on a group of pixels at one time• Used for filtering


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


Frame Averaging

• Adds together several frames, then divides by the number of frames. This produces a less noisy image

• True vs. Weighted


Logic Operations

• Provide a pixel-by-pixel combination of two images


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


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


Conclusion

Additional image processing questions? Contact Data Translation at

(800) 525-8528

Documents

Data Translation, Inc. Basics of Image Processing