Billboards are impostors used in place of tree geometry when rendered far away, just a picture of a tree. Because they can be batched together easily, billboards allow for very large forests to be rendered very quickly. Smooth LOD down to a billboard is required to maintain the illusion that billboards are actual trees in the distance. SpeedTree® handles much of this for you, but there are a number of settings you can change for different situations.
The faded trees in the background of the picture below display where billboards are being used. By using normal mapping and Effect LOD, billboards can match the geometry tree extremely well, even when in motion due to wind.
The Tree object has a number of settings for dealing with its billboards. Most notably, this includes the number of billboards to make (“Billboards:Number” property). In order for a tree to match the billboard from a number of different directions, multiple billboards need to be created for it. These billboards are evenly spaced all the way around the tree. Typical usage sees 6-10 billboards created per tree.
If a horizontal billboard is wanted, that is a billboard that is viewable from above the tree, then the “Billboards:Horizontal:Include” property will enable the creation of one. The “Billboards:Horizontal:Height” property specifies where in the tree, in percentage, to place this horizontal billboard.
The size of the individual billboard textures in the atlas are based on the size of the tree when compared to all the other trees in the atlas. To further affect this size, the “Billboards:Size scalar” property can be used to either make them bigger or smaller in relation to its original size.
Occasionally in low light or very ambiently lit scenarios, billboards won't match the geometry tree as well. This is because the billboard textures are predominantly rendered with diffuse lighting. To allow some ambient light to seep into the billboard texture, the “Billboards:Ambient contribution” property can be used. The actual color of ambient light to be used is set up per-material in the SpeedTree® Modeler.
The image below shows the effects of ambient contribution, left to right.
The Atlas object has a number of properties that deal with billboards. When billboards are not being merged into the texture atlas, the size of the billboard atlas is specified with the “Layout:Billboards:Atlas size” property. The other layout properties are explained in the documentation section on atlas layout.
The property button “Layout:Billboards:Reset tree scalars” is a convenience to reset all of this atlas's trees billboard size scalar to 1.0. This will reset all of the billboard sizes to be relative to the differences of the tree sizes.
The “Layout:Billboards:Use extra space” property enables smaller billboards to get larger if there is enough space for them to do so. This is an iterative process, so it may make atlas layout take a good bit longer, but it will ensure that there is very little wasted space in the billboard atlas texture (or the texture atlas if they are merged).
Billboards are able to receive global motion wind effects, which greatly improves the realism of billboards. If a tree is setup to have wind at the lowest LOD in the Wind LOD dialog, then the billboard will have global motion. The SRT writer is intelligent enough to fade out other wind effects down to just global motion, if necessary. If the tree does not have wind at the lowest LOD, then the billboard will not have wind either.