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 ValueWhite Value

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 Electron Beam Video Signal Target Lens Object

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 even- numbered 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 Active Pixel Region Full Scan Area Horiz. Blanking Blanking Level Horiz. Sync

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 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* ColorHertzLines In RS-170USANo601 RS-170 RGB USAYes603 CCIRINTLNo501 CCIR RGB INTLYes503 NTSCUSAYes601 PALINTLYes501 *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 INTL 5:41:1 3:2

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 = = = = =

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 LUT

Image Processing Concepts Binary Thresholding Example # LUT

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 0 = black, 255 = white

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 Original Pixel Values KernelResulting 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 = False1 = True ANDORXOR

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)