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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