2.3 Obtaining frequency information

The histogram plot gives a good overview of the frequency distribution of data values in an image selection. However, you can also obtain more detailed numerical information about the histogram from the Bilko status bar.

The frequency of a single data value

By placing your cursor at a particular position along the histogram x-axis, you can obtain information about the number of pixels that have a particular data value. This may be done in three different ways; you can

1. select a data value position by clicking on the position of a particular frequency bar with your mouse (useful for checking on the data value represented by a peak or trough in the histogram),
2. use the left and right arrow keys to move the cursor in increments of 10 along the x-axis, or
3. hold down the control key and use the left of right arrow keys to move the cursor in increments of 1.

Frequency information related to the chosen data value will be displayed on the status bare (see figure (4K)). From left to right the panels on the status bar reveal the proportion (in %) of all valid pixels that have the selected data value, the lowest data value of the range represented by a histogram bar, the highest data value in the range (in this case they are both the same as one histogram bar represents a single DN), and the exact number of pixels that have data values within this range.

Often you will want to use a combination of the three ways of moving the cursor. For instance, if you want to check on the data value of a peak, you select the peak by eye and click on it (1), then use the control and arrow keys (3) to find the exact position of the peak (i.e. the value that has the highest number of pixels as revealed in the fourth panel on the status bar).

The frequency of a range of data values

You can also select a range of data values in the histogram. This may be done in three different ways:

1. By eye, using the mouse: Move the mouse pointer to the lower end of the range, depress the left mouse button and drag the mouse along until you reach the top end of the range, then let go of the mouse button.
2. Using the keyboard to select a range in increments of 10: Use the arrow keys to move the cursor to the start of the range. Hold down the [SHIFT] while moving the cursor with the right arrow key to the upper end of the range.
3. Using the keyboard to select a range in increments of 1: Hold down the control key and use the arrow key to move the cursor to the start of the range. Then hold down both [SHIFT] and [CTRL] keys simultaneously while using the right arrow key to move the cursor to the end of the range.

The range you have selected will be highlighted in the histogram. As before frequency information about the range will be displayed on the status bar. From left to right: The percentage of the total represented by the selected range of values, the lowest value in the selected range, the highest value in the selected range, and the tota number of pixels with values falling within the range..