Creating Multiple Levels of Detail

on this page: steps | example | performance LOD | ranges | modes | selecting | adding a level | removing a level

A Level of Detail (LOD) grouping contains any number of child objects, referred to as levels. For a Distance LOD, you specify the distance at which each level is displayed. For a Performance LOD, you allow the browser to determine which level to display for optimal performance.

Find it: Click the Level of Detail Editor button on the Action palette:

Steps for Creating an LOD Grouping

Follow these steps to create a simple LOD grouping:

  1. Create a cube, a sphere, and a cone. Select them in that order.

  2. Press Create LOD Grouping Over All Selections.

  3. The first level (the cube; level 0) is now displayed. To cycle through the other levels, click in the range bar to move the blue highlight. When you select the middle range, the sphere (level 1) is displayed. When you select the last range, the cone (level 2) is displayed. You can also use the Current Level arrows to cycle through the levels, or type a level into the Current Level text field.

  4. To see the LOD grouping in action, click the Active LOD box. Now dolly in and out of the scene and watch the levels change as the distance changes.

  5. Next, try adjusting the ranges so that the levels change at new distances. To adjust a range, click and drag one of the tick marks on the range bar to the left or right. Be sure the Active LOD box is checked, and then try dollying in and out of the scene again to see the levels change.

  6. When an object is very far away, you often want it to disappear from the scene entirely. To create this effect, press the Append Empty Level button. The scene now contains a fourth level (level 3), which displays nothing.

  7. To equalize the ranges, deselect and reselect the LOD group in the main window. The ranges adjust to a more even spacing.

Example

Now that you have a basic feel for how an LOD grouping works, open the file /usr/share/Insight/library/SGI_bookshelves/Help/books/CosmoWorlds_UG/Models/brazier.wrl.

This LOD has a textured brazier for the first level, a brown cube for the second level, and an empty third level, as shown below. Its structure is typical of a useful LOD grouping, which usually contains a least one detailed object for close viewing, one stripped-down version, and one empty group for very far distances. You could also create another intermediate-level version of the first object using the Polygon Reduction Editor.

Select the object and open the Level of Detail Editor to examine the settings. Here's what you'll see:

Performance LOD

Click the Performance box to convert an LOD to a Performance LOD, which allows the browser to select the appropriate level based on rendering performance, not distance. When you choose the Performance LOD option, the range bar shows "Best" at the left and "Fastest" at the right. The browser selects the level with the lowest index that it can render while still maintaining an acceptable frame rate. Some browsers may ignore this option.

When you've selected the Performance option, the Distances Computed from Point and Range Limit text boxes are grayed out.

How Ranges Work

For Distance LODs, distances are measured from the user's viewpoint to the center point of the object. You specify this center point by typing values into the x, y, and z text boxes shown under the label Distances Computed from Point in the Level of Detail Editor. The default center point is the origin (0 0 0). The brazier example shown earlier has a center of (740 127.007 -1187.5).

The Range Limit text field shows the range limit for any range but the last one. It reflects the limit of the currently selected range (in blue on the range bar). When you type a new value into this field and press Enter, the new value is shown on the range bar.

Two Modes: Testing and Editing

The LOD Editor has two modes, one for testing LOD groupings (Active LOD) and one for editing the levels (Keep Current Level). Use the Active LOD mode when you want to test the transitions between one level and the next. As you zoom in and out of the scene, the transition between levels should feel gradual; you don't want to see a sudden "pop" as levels switch. Use the Keep Current Level option to stay with one level so that you can perform detailed editing on it.

You can click and drag the range bar tick marks in either mode.

Selecting an LOD Grouping

If the LOD grouping is selected, switching between levels doesn't change the current selection. If a level or part of a level is selected, switching between levels does change the current selection.

When you save the file, the Active LOD mode should be active. Note, though, that if you save, then select the LOD grouping, and then select an individual level, the mode switches automatically to Keep Current Level. This change enables you to make changes to the selected level. When you deselect the level and then select the entire LOD grouping, the mode returns to Active LOD.

Adding a Level

Adding a level to an LOD grouping is the same as adding a child to any group. See Selecting and Grouping Objects for details.

Removing a Level from an LOD Grouping

Removing a level from an LOD grouping is the same as removing a child from any group. Select the level and then cut it or detach it from the group.

Jump to: Level of Detail Editor Quick Reference