b w stretch menu

Stretching an image means modifying its contrast. You do this by designing a mapping function that tells the system how to change one grey shade in the image into another grey shade.

The procedure for this is:

You click on DISPLAY IMAGE to display the image you are stretching.
You click on the DISPLAY HISTOGRAM menu field to display the image histogram. The histogram shows how many pixels there are of each different grey shade. Above the histogram, a mapping function will appear. Initially, the mapping function is a straight line, and it has no effect on the image contrast.
You modify the mapping function, by clicking on the line, and clicking again where you want the line to go. The image display will reflect the result of the stretch.
When you are satisfied with the image, click on the STRETCH field to make the system apply the stretch to the image. If you don't do this, and you leave the stretch menu, the image will remain unstretched.
When you have STRETCHed the image, click on the EXIT field to get back to the B&W Processing menu (it is usually covered up at this point).

You can see where particular image pixels fall in the histogram by moving the mouse to the portion of the image you are interested in, and clicking on button 2. The system will put a small arrow on the histogram to indicate the intensity of that pixel. This is called backpicking.

If you click on the EQUALIZE HISTOGRAM menu field, the system will maximize contrast across the entire image. This sometimes produces a result that is pleasing to the eye. The Clipping level menu field lets you prevent the equalization procedure from placing too much emphasis on regions of low contrast. A good typical value for this field is 0.07%. Setting it higher will give the equalization process more of a free hand.

There is a toggle on the menu called Update Result Histogram?. If this field is set to Yes, then you will get a display that shows the effect of the stretch on the image's histogram. If set to No, no such display is produced.

You can save a mapping function to disk by clicking on the SAVE STRETCH FILE field. Such a saved mapping function can be recalled and applied to other images by clicking on the RECALL STRETCH FILE field. You can also use such a mapping function to digitize bitmap images directly in the Digitize Bitmap Image menu.

You can restrict the stretch 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 stretch, only the pixels in the rectangular area will be affected. 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.