Shapes and ROIs (Regions of Interest)

Region Of Interest (ROI) Definition Window

When looking at satellite data image, you might be interested in a special region inside the image. This region of interest (ROI) does not have to be contiguous area of pixels; a ROI is allowed to be any number of independent pixels inside the data.

VISAT lets you define four different criteria for the definition of the ROI. Use can use a geometric shapes which you can simply draw on the image, include the pixels within a value range, covered by a bitmask and finally the pixels selected by the currently defined pins. The combination of one or more of these criteria offers a great flexibility to select pixels that show the features you are interested in.

All that can be done in one dialog, the ROI Definition Window.

To illustrate the available options in this dialog, they have all been activated. Let's have a little closer look on this example.
This ROI will be assembled from four criteria

1. a geometric shape (which has to be created before, see below),
2. a value range (i.e. use values between 0.0 and 0.45), and
3. a condition (a click on the button will bring up the Expression Editor).
4. a the pixels under the current pins

Undo ROI assignment

If you want to start over defining a ROI, click on the icon. This will reset the dialog box to default values: all criteria deselected, value range (disabled) from 0.0 to 1.0, bitmask criterion (disabled) erased.

Multiple Assignment to other Bands

You can assign the current ROI definition to multiple other visible bands of the selected product by clicking the icon. This will open a band chooser dialog where the bands can be selected. By pressing the OK button the current ROI will be assigned to all selected bands.

Zoom To ROI

A click on the icon in the centers the image view on the selected ROI.

Creation of ROIs from geometric shapes

The creation of ROIs from shapes in VISAT includes two steps:

1. import a transect or draw the shape
2. convert the shape into a ROI

In VISAT you define geometric areas by drawing them into your image. For this purpose, several drawing tools are available.
The following figures show the necessary steps to create a ROI graphically:

Once the ROI is created, you will be able to gather information about the affected pixels in various dialogs. The Statistics, Histogram, and Scatter Plot dialogs will have an active 'Compute for ROI' button then.

Graphical tools

Selection Tool

This tool is enabled by default; you use it to point into the image and to move the panning or zoom windows.

Drawing tools

The following tools help to draw geometric shapes.

Note: Tf you want to add a shape to an existing shape, hold down the Shift key.
To subtract a shape from an existing one, hold down the Control key.
If you don't press one of these keys while drawing a shape, the new shape will replace any existing one.

Draw Line Tool

With this tool you draw a single line.
Click for the start and for the end point.

Draw Rectangle Tool

Click for the start point, drag to the end point and release the mouse button.

Draw Ellipse Tool

The handling is similar to the rectangle tool: click for the start, drag to the end point of the boundary rectangle and release the mouse button.

Draw Polyline Tool

With this tool you are able to draw a polyline, i.e. consecutive lines, each starting at the end point of the previous line. Click for the start and for the end point, the end point will be the start point of the following line. Double-click to terminate the polyline.

Draw Polygon Tool

The handling of the polygon tool is somewhat similar to the polyline tool; except that double-clicking will close the shape (a line from the end point of the last line will be drawn to the start point of the first line). In effect, the shape defines a closed area.