b w algebraic menu

Digital images are actually just big arrays of numbers, where the numbers tell what the light intensity is at each point in the image. When people say that they add two such images together, what they mean is that they create a new image in which each pixel's value is the sum of the values of the corresponding pixels in the two original images. Similarly, you can subtract two images, multiply them, divide them, even take the logarithm of an image.

Such algebraic operations are often useful. People sometimes subtract two images of the same scene, acquired at different times, to enhance everything that has changed in the scene during that time.

In this menu, you can manipulate up to four images at a time. When you first enter the menu, your "main" image (the one you have in memory) is called Image A. You can then specify the name of a second image (which will be called Image B) and click on the READ IMAGE B menu field. The system will read this image into memory, too. Now that you have two images in memory, you can do algebraic operations with them. If you click on the A+B=>C menu field, for example, the system will add the two images and deposit the result in a new image called Image C, which you can then display by clicking on the DISPLAY C menu field. Another set of menu fields are for moving the images around, making them change places. If you click on C=>A, for example, Image C will be moved into the area in memory where Image A was. Image A will be destroyed in this process (only in memory, not on disk).

You can manipulate the images around as long as you want. When you want to save a particular result to disk, or you want to run some filter on it, be sure to move it to Image A before you leave the menu. Image A is the only one you can "take with you", and use in the other processing menus.

A final note; when you do algebraic operations, the result image must be rescaled to make its intensity values fit in the 0 to 255 range. The Clip percentage menu field lets you specify what percentage of image pixels will be "clipped" to black and to white during this rescaling procedure.

You can restrict the algebraic operation to work on only a portion of the image by specifying an Area of Interest (AOI). The AOI can be either rectangular or polygonal. To specify a rectangular AOI, first display the image, then click on USE RECTANGULAR AREA. The system will prompt you to indicate the rectangular area on the image. Do so with two clicks. When you then apply the algebraic operation, only the pixels in the rectangular area will be affected. Similarly, you can make an irregular AOI by first clicking on USE POLYGONAL AREA and then outlining the AOI with a series of clicks on mouse button 1. Terminate the drawing of the AOI by clicking with button 2. If you have restricted processing to an AOI and you want to make it apply to the entire image again, click on USE ENTIRE IMAGE.