The release of Unity 5.1 brought some nice changes to how Unity handles SpeedTree LODs. The biggest of these changes involves the option to animate crossfades in the SpeedTree LOD group. Previously, Unity handled the LODs based on the distance from the model. This meant that as a tree moved to the billboard the crossfade could be stuck somewhere between the LOD2 and the billboard where the dithering lent it a checkerboard appearance. By adding the option to animate the crossfade the transitions are now based on a timed animation rather than simply distance, eliminating the unsightly dithered trees.
Another new addition is the ability to choose a fade mode for each SpeedTree LOD group. The Options are SpeedTree (default), Crossfade, or none. To get a better idea of what this means lets take a look at the difference between crossfade and SpeedTree’s SmoothLOD.
SpeedTree has lots of different techniques to smooth the transition between geometry LODs. The branches that are going away shrink down into their spines rather than simply disappearing. Leaves also shrink as other leaves grow larger to take their place and maintain their full appearance. This can all be tweaked within our SpeedTree modeler to better suit your needs but is tuned by our artists to try to accommodate as many different purposes as needed. Radial and length segments can be reduced, as well as entirely different meshes used for leaves and fronds. SpeedTree maintains the normals, texcoords, and AO between LODs to minimize any noticeable pops. You’ll notice this same effect in many AAA titles that use SpeedTree. Note that when “animate crossfade” is checked, the SmoothLOD still bases it’s transition on distance to avoid too much of a pop.
Crossfade is an option to transition LODs between the different 3d models but is necessary for the final transition between model and billboard. Because the billboard is a discrete picture of the tree it can’t shrink away in the same manner as the 3d models. Dithering is used to minimize the pop of the billboard. Below is an example of a tree in transition displaying the checkerboard effect:
If you’re not happy with the way the timed animations work you can always revert to the previous method by unchecking “animate crossfade” and choosing the transition width and speed manually.
Animate Crossfade: A new importer setting “Animate Cross-fading” is added to SpeedTree assets. With this option on (by default), the dithering LOD transition between mesh/billboard and billboard/culled will be animated by time. This should make the trees look better in most of the scenarios.
LOD Fade mode: You can now choose to use Crossfade, SpeedTree, or neither for SpeedTree models. Note that SpeedTree will still use a crossfade on the final LOD transition to billboard
LOD Shadows: Tree casts shadow when in cross-fading to billboard
Wind: Wind animation will not be interrupted if the tree gets outside of the view