The Histogram Display

If you click the Show Histogram button in the display parameters area, a window with an image histogram will pop up (see Fig 5.4). The histogram essentially tells you how many pixels within a given intensity range are present in the image. By clicking into the histogram plot, you can set the minimum (click left mouse button) and maximum (click right mouse button) data value to be displayed. The values set are also indicated by green (min) and red (max) vertical lines.

If you have a few (or even only one) outlying pixel in your image, the histogram will make it very easy to adjust the min and max for good contrast. Just click on the left and right edge of the bulk of the pixels.

Below the histogram plot, you can manually type absolute min and max values and go to full scale, which are exactly the same functions as in the SM_GUI main window. Additionally, you can give a fractional minimum and maximum, which means "throw out the brightest $x$ percent and the darkest $y$ percent of all pixels." This is called Histogram clipping.

You can switch the histogram plot between linear and logarithmic vertical scaling. While the logarithmic scaling makes it easier to see which brightness values are present in the image at all, the linear scaling shows more truly (and dramatically) how many pixels we are talking about.

All changes in min and max are reflected in the actual image in real time. The SM_GUI main window will be insensitive while you have the histogram window open.

Figure 5.4: Histogram display in SM_GUI