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IMAGE PROGRESSING FUNDAMENTALS

Part 1: Playing with the Pixels

by James Antonakos

Start ý Imaging 101 ý RLE Compressor Tool ý Adjusting the X-Y Resolution ý Adjusting the Z-Axis Resolution ý Simple Image Operations ý Run-Length Compression ý More to Come ý Sources and PDF

IMAGING 101

When you wake up in the morning and open your eyes, your first thought is probably not, "Look at all those horizontal and vertical lines and varying shades of light and color." Your eyes and brain work together to instantly acquire, process, and extract useful information from the visual scene. As soon as you open your eyes, you see your bed, the walls of your room, a door, and possibly hundreds of other objects you instantly recognize as soon as you see them. Trillions of brain cells work in parallel to analyze the visual information, seemingly without effort. Getting a single microprocessor to do the same thing is more difficult.

What is needed to begin experimenting with image processing techniques? At a minimum, access to imaging software and a collection of sample image files is required. To capture images you need a video camera and software. Depending on the type of camera, you may also need a frame grabber, an electronic circuit that digitizes one frame of camera video and stores it in memory.

Cameras available for the personal computer, which typically plug into a USB, FireWire, or parallel port, are inexpensive and do not require a frame grabber. However, then the microprocessor in the PC must do all the work building an image from the data it receives from the camera. A frame grabber does all the work for the processor, freeing up the CPU for other activities.

Figure 1 shows a typical image capture system. Analog video from the video camera is input to a frame grabber (or I/O port) where it is digitized. Software running on the PC reads the image data and displays the image in a window.

Figure 1ýThe analog video signal from the camera feeds the input of a frame grabber, which digitizes the image under control of the PC.

Figure 2 shows a block diagram of a frame grabber. A fast A/D converter (called a flash converter) converts the analog video signal from the camera into binary pixel values. These binary values act as addresses to the input look-up table (ILUT), where they might be transformed into a different pixel value (for example, to do a negative of the image in real time). The transformed pixel values are then stored in onboard image memory.

Frame grabbers typically contain hardware to recreate the video image from the pixel data that is stored in memory. The pixels from the memory pass through the output look-up table (OLUT) where they may be further transformed via false coloring (where shades of intensity are assigned different colors) before being sent to the D/A converter.

No matter how the image is captured, you end up with a block of image memory filled with pixel values. Figure 3 illustrates the basic structure of an image. Each pixel in the image is represented by a numeric value indicating its intensity (or color in a color image). The pixels are arranged in a rectangular grid composed of a specific number of rows and columns. The X and Y resolution (the number of columns and rows), as well as the number of shades possible in a given pixel (the Z-axis resolution), are determined by the hardware inside the frame grabber (or other image capture device).

Figure 3ýIn this example, there are 256 rows and 256 columns of pixels. If each pixel requires a byte of storage space, the entire image uses 64 KB of memory.

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