Digital Image Processing
Few types of evidence are more incriminating than a photograph or videotape that places a suspect at a crime scene, whether or not it actually depicts the suspect committing a criminal act. Ideally, the image will be clear, with all persons, settings, and objects reliably identifiable. Unfortunately, though, that is not always the case, and the photograph or video image may be grainy, blurry, of poor contrast, or even damaged in some way.
In such cases, investigators may rely on computerized technology that enables digital processing and enhancement of an image. The U.S. government, and in particular, the military, the FBI, and the National Aeronautics and Space Agency (NASA), and more recently, private technology firms, have developed advanced computer software that can dramatically improve the clarity of and amount of detail visible in still and video images. NASA, for example, used digital processing to analyze the video of the Challenger incident.
The first step in digital image processing is to transfer an image to a computer, digitizing the image and turning it into a computer image file that can be stored in a computer's memory or on a storage medium such as a hard disk or CD-ROM. Digitization involves translating the image into a numerical code that can be understood by a computer. It can be accomplished using a scanner or a video camera linked to a frame grabber board in the computer.
The computer breaks down the image into thousands of pixels. Pixels are the smallest component of an image. They are the small dots in the horizontal lines across a television screen. Each pixel is converted into a number that represents the brightness of the dot. For a black-and-white image, the pixel represents different shades between total black and full white. The computer can then adjust the pixels to enhance image quality.
The three main categories of digital image processing are image compression, image enhancement and restoration, and measurement extraction. Image compression is a mathematical technique used to reduce the amount of computer memory needed to store a digital image. The computer discards some information, while retaining sufficient information to make the image pleasing to the human eye. Enhancement of a compressed image may reveal artifacts of the compression process. Evidence that information has been discarded from the image may limit its usefulness in a criminal investigation.
Image enhancement techniques can be used to modify the brightness and contrast of an image, to remove blurriness, and to filter out some of the noise. Using mathematical equations called algorithms, the computer applies each change to either the whole image or targets a particular portion of the image. For example, global contrast enhancement would affect the entire image, whereas local contrast enhancement would improve the contrast of small details, such as a face or a license plate on a vehicle. Some algorithms can remove background noise without disrupting key components of the image. Following image enhancement, measurement extraction is used to gather useful information from an enhanced image.
- Alan Tietjen, Research Scientist, Innovative Sciences and Technology Experimentation Facility, Kennedy Space Center, Florida.
- A Short Introduction to Digital Image Processing
For other techniques used in Forensic Files, click here.