The histogram module visually displays the pixel intensity information for the selected image. The top histogram shows the un-stretched image data from 0 - 65535. The bottom histogram displays the image data based upon the black and white point sliders. By adjusting the black and white arrows, the stretch on the selected image is changed. This is displayed on the Stretched Histogram as well stretching the on screen display of the image. It's important to note that performing a histogram stretch in this fashion does not alter the original image in any way. This is only for display purposes.