Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!husc6!rutgers!sri-spam!ames!elroy!jplgodo!wlbr!etn-rad!jru From: jru@etn-rad.UUCP (John Unekis) Newsgroups: comp.graphics Subject: Re: Computerized Enhancement of Pictures Message-ID: <250@etn-rad.UUCP> Date: Thu, 13-Aug-87 15:25:54 EDT Article-I.D.: etn-rad.250 Posted: Thu Aug 13 15:25:54 1987 Date-Received: Sat, 15-Aug-87 11:47:32 EDT References: <5013@ihlpa.ATT.COM> Reply-To: jru@etn-rad.UUCP (0000-John Unekis) Organization: Eaton Inc. IMSD, Westlake Village, CA Lines: 102 Keywords: How? In article <5013@ihlpa.ATT.COM> hhm@ihlpa.ATT.COM (Mayo) writes: >Where can I find information about the computerized enhancement of >black and white pictures to recover images not apparent on the original >print. How is it done? Is it applicable to film? What reference sources >are available? > The computerized enhancement of black and white pictures is a very broad and mature field and there exists thousands of volumes on the techniques involved. A brief summary would probably include the following steps - Image capture : The film or other source for the image is placed in front of a video camera or CCD array scanner and the image is reduced to an array of dots. The number of dots depends on the resolution of the device used, typical sizes are 640x480 (near TV quality picture), 512x512, or on higher resolution systems 1024x1024 or even 2048x2048. The higher the resolution, the more dots, and the better the image will look on the screen, but it will take correspondingly longer to process. The capture device feeds the intensity value for each dot to a device called an analog to digital converter, which assigns a numeric value to the intensity. The typical range for intensities is from 0 to 255 (Which happens to fit nicely in an 8-bit computer byte). The digitized image is then stored on computer disk or in computer memory as a two dimensional array of values. Image enhacement : Several types of image enhancement are possible. The most common, and most visually effective, is histogram equalization. If the original image is washed-out looking, where the black isn't very black, or the white isn't very white, or everything is just sort of gray, then if you look at the intensity values of the digital image they will use only a small portion of the available range. Out of the possible 0 to 255 , they may only use say, 80 to 120. One way to correct this is to calculate a histogram of the intensity values of the image. This would be an array with 256 locations, where each location would hold a count of how many dots in the image had that particular intensity value. Histogram equalization can be done by replacing each dot in the image with a value corresponding to the percentage of dot s in the image that are lower in value than this dot. Lets say that in our image a dot had the value 81, and that one percent of the dots in the image had values less than 81, we would the 81 with one percent of our available intensity, or about a 2. If another dot had a value of 120, and 99 percent of the dots in the image had values less than 120, then we would replace the dot with 99 percent of our possible intensity , or about 253. This process makes the black areas get very black, the white areas get very white, and in between areas have more visible contrast. An out of focus image can be made to look more in focus by a technique called edge enhancement. To do this we look at all the closest neighbors of a dot in a 3x3 area within the image , with the dot in question at the center. By replacing the dot in the center with 9 times itself, minus one times each of its eight neighbors, and by repeating this for each dot in the image, the picture will suddenly look sharper and more focused. One drawback is that this will exaggerate any graininess in the picture. There are dozens of other usefull techniques to enhace the digital version of images, far to many to go into here. Image display: A CRT display of the image that is being processed can be created using a display controller which converts digital intensity values for each dot of the image into analog intensities that can be shown on a display tube. Image Hardcopy: A new physical copy of the picture that was enhanced can be produced by sending the digital values for each dot in the image to a device such as a film recorder or laser printer which can produce dots of varying intensity on paper and thereby reproduce the intensity values of the image as a physical picture. More detailed documentation on all this can be obtained from text books on digital image processing which are available at any large university book store in the conputer science section. If you would like to actually buy equipment to do this sort of processing a simple low cost system for the IBM PC is available from Media Cybernetics by the title of 'IMAGE PRO'. More expensive, but higher speed and higher resolution systems are available from companies like VICOM of San Jose, CA. and Matrox of Ottawa Canada. If you have more specific questions, or would like to take a look at the image processing system that my company produces, email to me seismo!cbosgd!ihnp4!wlbr!etn-rad!jru or physical mail at John Unekis ms208 EATON corp. 31717 La Tienda Dr. Westlake Village, CA. 91359