# Tip 83: Using 3D Light Rays for Photogrammetric Project Assessment

PhotoModeler has an interesting photogrammetry visualization tool for assessing the geometry and the structure of a 3D solution. This tool is called “Rays” – short for ‘visualized light rays’. A light ray is shown in the 3D View as a solid white line and portrays the vector that light would have traveled from the point in the scene to the camera that imaged it. In this way, the light ray in the 3D View shows how the points in the 3D model and the individual camera positions (where photographs were taken) are related.

The camera station symbol in the 3D View (blue block with green lens) depicts the position and orientation of the camera when each photograph was taken. The Rays can be shown from an individual point back to all the photographs/stations that image it, or Rays can be shown for all points imaged by one photo. These are great ways to show relationships. As well multiple points or camera stations can be selected at once to show all their Rays – this makes it easier to see issues with overlap.  Rays are also a handy way to identify low angle points (points formed by rays with small angle between them alone tend to be less accurate).

To display rays in the 3D View, first Camera Stations need to be made visible on the “Points, Vectors, Cameras” tab of the 3D Viewer Options dialog, or on the Visibility ‘fly-out’ menu at left. Next, on the “Axes, Exaggeration and Rays” tab of 3D View Options, the “Shows rays on selection of point or camera station” check box needs to be checked. Now, when a camera station is selected, the rays connecting the camera station and all points marked on the associated photo are drawn; and when one or more points are selected, the rays connecting the point(s) to all photos they are imaged on are shown.

The rays feature can be used to:

1. See the points used to calculate the camera’s position.
2. Identify the number and spread of points on a photo.
3. Determine the number of photos a point is marked on.
4. Visualize the angle between light rays, and identify weak ones.

With points or camera stations selected, the Open Photos Showing Selected right click menu item can be used to view the photo of interest and see the points and features marked on that photo.

Showing Rays can be a helpful way to visually assess a model’s geometry, and this information can be used to improve its quality and accuracy.

Watch the YouTube video for further detail on the use of 3D Ray display: