The Process > Show Point Features options show point features of the area visible in the Main window. When you show point features, the points in the view will be temporarily colored according to values based on the option you select. You can show point features for:
- Point frequency
- Surface normal
- Input file source ID
- Laser return number
The Show Point Features options, described following, are useful tools for debugging when you are developing custom feature extraction algorithms with the ENVI API. When you select an option, ENVI LiDAR processes the points that are visible in the Main window, then updates the display to show them colored by the selected feature.
Frequency
Frequency is a measure of how smooth or flat a surface is. When you show point features by frequency, ENVI LiDAR analyzes neighboring points and colors them darker based on how many are in the same plane. The more points in a plane, the darker the color. For example, points related to a smooth surface, such as a road, will display as darker points, and a rough or noisy surface, such as a vegetation cluster, will display as white.
When you select By Frequency, the Color By Parameters dialog appears. Use this dialog to set thresholds for analyzing the neighboring points. The parameters are:
- Radius: Enter a radius, in meters, to define the area of surrounding points to analyze. The default is a minimum setting, based on the density of the data. The maximum setting is 5 meters.
- Refine by Neighbors: Enable this check box to refine the results when the data is noisy. When enabled, ENVI LiDAR will perform an iteration after processing to use good matches to refine the noisy neighboring points for a match and color them accordingly.
Normal
A surface normal is a unit vector perpendicular to the surface. For example, a flat table top will have a vertical normal that points straight up or down. Both of these normal vectors are perpendicular to the table top surface.
When you select By Normal, ENVI LiDAR shows the orientation of the surface the point belongs to. The relationship between the normals of neighboring points also gives an indication of the smoothness or roughness of the surface. If neighboring normals are the same, the surface is smooth. Where the normals of neighboring points differ, there is a change in the surface orientation. Random normal variation indicates surface roughness, such as the leaves of a bush, whereas a consistent, well-defined, localized normal variation indicates a change in surface orientation.
When you select By Normal, the Color By Parameters dialog appears. Use this dialog to set thresholds for analyzing the neighboring points. The parameters are:
- Radius: Enter a radius, in meters, to define the area of surrounding points to analyze. The default is a minimum setting, based on the density of the data. The maximum setting is 5 meters.
- Refine by Neighbors: Enable this check box to refine the results when the data is noisy. When enabled, ENVI LiDAR will perform an iteration after processing to use good matches to refine the noisy neighboring points for a match and color them accordingly.
Source ID
Source ID is a value marked for each file when files are imported into a project. When multiple input files are imported, a different source ID is used for each file. If only one input file is imported, the source ID is not used. To show point features by source ID, the project must contain two or more input files that contain multiple, distinct source ID values.
When you select By Source ID, ENVI LiDAR displays each source ID as a separate color.
Return Number
Return Number is the number of times a laser pulse is reflected off an object and returns to the system receiver. A single LiDAR point might have multiple returns. Typically, multiple returns occur in vegetation. Using a tree as an example, when the laser beam hits the top of the tree canopy, some of the beam is reflected as the first return. As the original beam continues, some of it is reflected off the tree branches as the second return. The third return is created when the original beam continues and is reflected off the ground.
Return Number is supported by LAS, NITF-wrapped LAS, and MRSID file types. You can determine if your data has multiple returns by viewing the header file. If multiple returns exist, the header information will look similar to the following:
Num points by return [1]: 1,843,260
Num points by return [2]: 298,934
Num points by return [3]: 6,906
Num points by return [4]: 0
When you select By Return Number, ENVI LiDAR colors the points based on the number of returns. Points with one return display as black and points with the most returns are white. Points with a number of returns between the upper and lower range of returns are gray.